摘要:

z Authors or Speakers. OF FOUNDED 1903. TO PROMOTE THE STUDY OF ~LRCTROCHEMISTRY, LLECTROMETALLUR~Y, CHEMICAL PHYSICS, METALLOQRAPHY, AND KINDRED 8UBdECTS. VOL. 111. JULY, 1907. PART I. THE APPLICATION OF THE ELECTRON THEORY TO ELECTROLYSIS. BY E. E. FOURNIER-DALBE. ( A Paper read before the Faraday Society on Tuesday, January 15, 1907, SIR JOSEPH SWAN, F.R.S., PAST-PRESIDENT, in the Chair.) The electron theory of electricity and magnetism may be fitly described as an extension of the ionic theory of electrochemistry to solids, gases, and a vacuum, inasmuch as it postulates material carriers of all electric charges, and reduces electric phenomena to the configuration and motion of these carriers. The application of the electron theory to electrolysis means, therefore, no revolutionary change in electrochemical conceptions, but simply an attempt to bring our extended knowledge of the properties of the elementary carriers of electricity to bear upon well-known phenomena, and thus to gain a clearer insight into the actual mechanism of electrolytic conduction and dissociation.One of the most valuable achievements of the electron theory is the complete harmonisation of the processes of metallic and electrolytic conduc- tion. Both processes depend upon the existence of minute charged bodies capable of threading their way through a mass of other bodies which are either uncharged or less mobile. If a body contains no such charged bodies -ions in the wider tense-or if its ions are fixed in position, it is incapable of conveying an electric current of any description.A current consists in the displacement of ions, The conductivity of a body is accurately and precisely defined by the number of ions it contains in unit volume and by their average mobility ; this mobility, in its usual acceptation, excluding any measurable free path. The definition and physical interpretation of conductivity is thus the same in liquids and solids. It is only when we consider the individual carriers themselves that a fundamental difference appears between metallic and electrolytic conduction. Compare a cm.-cube of copper with a cm.-cube of dilute hydrochloric acid,2 THE APPLICATION OF each of them conveying a current of one ampere. The copper conveys its current almost solely by its free electrons, some 400 trillion in number, which possess a mobility some 100 niillion times greater than that of a hydrogen ion.The hydrochloric acid conveys the current by means of its hydrogen and chlorine ions, which, in a millinormal solution, amount to about 2 trillion. Since the current is the same in each case, the electrons in the copper must move some 200 times more slowly than in the electrolyte. But this is not usually the case, for in a copper wire I mm. in diameter conveying one ampere the speed of the electrons is of the order of I cm. per second. The greatly superior mobility of the free electrons brings it about that a much lower voltage is required to maintain an ampere in a copper conductor than in a liquid conductor of the same dimensions. Whence this difference between solids and liquids? At first sight it seems strange that free electrons should be observable in a vacuum and traceable in a metal, but that there should be none in a liquid, The elucidation of this question would be of fundamental importance for the theory of electrolysis, and would constitute a valuable contribution from the general theory to the special field to which, as a matter of fact, it owes its origin.It cannot be said that this question is anywhere near a complete solution. But since the whole range of electromagnetic phenomena has become unified under the sway of the electron theory, much additional material has become available upon which to base our judgment. The existence of free electrons in metals is made possible by the close packing of their atoms, which brings about a frequent exchange of electrons between one atom and another.During the short period occupied by its change of allegiance-a period amounting to something like I-5,oooth of its average time of attachment to a metallic atoni-the electron is free to fall along the potential gradient, and thus to constitute an electric current. When the metal expands by heat, this process of exchange naturally becomes rarer, and the metallic resistance increases. The positive atoms and neutral atoms are so tightly packed that they contribute nothing perceptible to the conductivity. In an insulating solid or liquid the electrons are effectively bound up with atoms or molecular groups, and these are arranged in some structure, usually crystalline in the case of solids, which precludes the formation of mobile charged groups and the liberation of electrons.In an electrolyte, on the other hand, such charged groups are formed, and they are sufficiently mobile to follow the E.M.F. That free electrons are not produced is capable,sf a simple explanation. Two copper atoms have equal attractions for an electron, but atoms of, say, hydrogen and chlorine havc not. Hydrogen loses one of its electrons more easily than chlorine, and when the two atoms of a molecule of hydrochloric acid separate, the chlorine takes one of the normal hydrogen electrons away with it. The hydrogen atom thus reniains positively charged, and the chlorine atom negatively. The two ions so produced act as condensation nuclei, like any other small charged particles.The study of electric condensation nuclei will, I believe, shed a flood of light on electrolytic problems. The fundamental researches of C. T. R. Wilson * have proved that in vapours, at all events, ions act as condensation nuclei, and that negative ions are slightly more effective than positive ions. Of this phenomenon J. J. Thomson -/- has furnished a simple explanation. When a charged drop evaporates its electric charge remains, and its potential increases as its diameter diminishes. More energy is therefore * C. T. H. Wilson, Plzil. Trans. 189, p. 265, 1897. t Coiitlirction of Electricity tkroziglt Gases, 2nd ed., p. 179.THE ELECTRON THEORY TO ELECTROLYSIS 3 required to evaporate a charged drop than an uncharged drop. Condensa- tion on the drop involves an expenditure of available electric energy, and thus condensation is facilitated.This fact has an important bearing on the theory of electrolysis. It explains the low mobility of the ions, the drop of potential at the electrodes,.and the liberation of uncharged products at the latter. The different mobilities of the ions can, I think, only be attributed to their different degrees of normal hydration. That hydroxyl should have a mobility over four times that of lithium, points to the lithium atom as a par- ticularly efficient condensation nucleus when deprived of one electron. It moves so slowly through the water under the influence of the potential gradient because it has to drag a number of water molecules (probably not more than half a dozen) along with it.The number must be the same for each ion at a given temperature, but cannot be very large, to judge from the actual mobility (347 x I O ~ ) compared with that of the electron in copper (about 5 x 104). When these hydrated ions reach the electrodes the E.M.F. tends to drive them into the metal of the electrodes. But the atoms of the latter are so closely packed, as a general rule, that no other atoms can force themselves between them. There arises a deadlock, which is only released when electrons are able to pass from the solution into the metal, or vice vers2. The mobility of electrons, though comparatively great inside a metal, is very small at its surface, where the close packing of the metallic atoms loses its effect. But when the E.M.F. exceeds a certain minimum an electron may pass, say, into a hydrogen ion, and neutralise its positive charge.The immediate consequence will be that the hydrogen loses its condensing power. The link between it and the water molecules is broken, and the hydrogen is liberated as an uncharged gas. The converse process goes on at the anode. The chlorine atom passes its superfluous electron into the anode, thereby becoming neutral and free from its aqueous encumbrance. In the further theoretical investigation of electrolytic action on the basis of the electron theory the study of mobilities will have to play a prominent part. Bredig’s laws of mobility are of special interest in this connection. We require a quantitive determination of the hydration of the ions. This determination should not be very difficult, since a number of data, such as heats of solution, melting- and boiling-points, are ready to hand.It should be possible to determine the diameter and bulk of the ions, and their shape from stereochemical considerations. The fact, noted, by Bredig, that the strongly electropositive and electronegative metals have a high mobility points to a superior facility of reaction. A high mobility implies a low degree of hydration of the ion. This low degree of hydration enables such ions, when of opposite signs, to combine more freely and rapidly than is the case in ions having a larger bulk. The heats of reaction, the heats of ionisation, and the electro-affinities or electrode potentials of the majority of elements are fairly well known, and afford valuable data for determining the sizes of the ions.We already know that isomeric and metameric ions have the same mobility ; that the mobility of an ion of a given sign is the smaller, the greater the number of atoms it contains; and that the effect of any change in the constitution of an ion is the greater, the smaller the number of atoms contained in the ion. These facts leave the road clear for a determination of the actual constitu- tion of a single ion on kinetic principles. When an ion contains 50 or 60 atoms it appears to have a minimuin mobility which is not preceptibly diminished by a further addition of atoms. Such an ion must have a diameter of about a micromillimetre, and therefore comes within the range of4 THE APPLICATION OF Szigmondy’s ultramicroscope.There is, therefore, a possibility that the motion of such heavy ions may be studied by actual observation, and that they may be watched as they cover a distance of I cm. in an hour and a half under the influence of a potential gradient of I volt per cm. It then becomes a comparatively simple calculation to find the diameter of such an approxi- mately spherical ion moving through a viscous liquid, or, in the last resort, threading its way among molecules of perhaps half its diameter. Such calculations have been made in connection with metallic conduction, and have led to the conclusion that the carriers of the current in a wire are almost solely the free electrons. A difficulty is presented by the circumstance that ions of the higher valencies exhibit mobilities which are neither “ independent ” nor constant for different concentrations.” The extraordinarily high mobility of lead is not easily harmonised with our notions of the physical constitution of the ions.But it is interesting to note that the influence of hydration increases with the valency and the concentration, which is what we should expect. The future development of electrolytic theory will lie, I think, in the direction of statistical analysis on kinetic principles. Such analyses have yielded most suggestive results in gas discharges, more especially the electric arc, in the vacuum tube, the Zeeman phenomenon, and, quite recently, in metallic conduction, Thomson’s famous ‘( counting experiment ” must be followed up. He produced ions in a moist gas, and watched them as they settled down under the influence of gravity.We must follow their motions along the potential gradient in a liquid, and folIow the fate of the electrons they convey through the liquid into one electrode, and back into the liquid through the other. The complete determination of the energy absorbed or emitted at the various stages of this transmission means the complete mastery of the problems of electrolysis. z Authors or Speakers. OF FOUNDED 1903. TO PROMOTE THE STUDY OF ~LRCTROCHEMISTRY, LLECTROMETALLUR~Y, CHEMICAL PHYSICS, METALLOQRAPHY, AND KINDRED 8UBdECTS. VOL. 111. JULY, 1907. PART I. THE APPLICATION OF THE ELECTRON THEORY TO ELECTROLYSIS. BY E. E. FOURNIER-DALBE. ( A Paper read before the Faraday Society on Tuesday, January 15, 1907, SIR JOSEPH SWAN, F.R.S., PAST-PRESIDENT, in the Chair.) The electron theory of electricity and magnetism may be fitly described as an extension of the ionic theory of electrochemistry to solids, gases, and a vacuum, inasmuch as it postulates material carriers of all electric charges, and reduces electric phenomena to the configuration and motion of these carriers.The application of the electron theory to electrolysis means, therefore, no revolutionary change in electrochemical conceptions, but simply an attempt to bring our extended knowledge of the properties of the elementary carriers of electricity to bear upon well-known phenomena, and thus to gain a clearer insight into the actual mechanism of electrolytic conduction and dissociation.One of the most valuable achievements of the electron theory is the complete harmonisation of the processes of metallic and electrolytic conduc- tion. Both processes depend upon the existence of minute charged bodies capable of threading their way through a mass of other bodies which are either uncharged or less mobile. If a body contains no such charged bodies -ions in the wider tense-or if its ions are fixed in position, it is incapable of conveying an electric current of any description. A current consists in the displacement of ions, The conductivity of a body is accurately and precisely defined by the number of ions it contains in unit volume and by their average mobility ; this mobility, in its usual acceptation, excluding any measurable free path. The definition and physical interpretation of conductivity is thus the same in liquids and solids.It is only when we consider the individual carriers themselves that a fundamental difference appears between metallic and electrolytic conduction. Compare a cm.-cube of copper with a cm.-cube of dilute hydrochloric acid,2 THE APPLICATION OF each of them conveying a current of one ampere. The copper conveys its current almost solely by its free electrons, some 400 trillion in number, which possess a mobility some 100 niillion times greater than that of a hydrogen ion. The hydrochloric acid conveys the current by means of its hydrogen and chlorine ions, which, in a millinormal solution, amount to about 2 trillion. Since the current is the same in each case, the electrons in the copper must move some 200 times more slowly than in the electrolyte.But this is not usually the case, for in a copper wire I mm. in diameter conveying one ampere the speed of the electrons is of the order of I cm. per second. The greatly superior mobility of the free electrons brings it about that a much lower voltage is required to maintain an ampere in a copper conductor than in a liquid conductor of the same dimensions. Whence this difference between solids and liquids? At first sight it seems strange that free electrons should be observable in a vacuum and traceable in a metal, but that there should be none in a liquid, The elucidation of this question would be of fundamental importance for the theory of electrolysis, and would constitute a valuable contribution from the general theory to the special field to which, as a matter of fact, it owes its origin.It cannot be said that this question is anywhere near a complete solution. But since the whole range of electromagnetic phenomena has become unified under the sway of the electron theory, much additional material has become available upon which to base our judgment. The existence of free electrons in metals is made possible by the close packing of their atoms, which brings about a frequent exchange of electrons between one atom and another. During the short period occupied by its change of allegiance-a period amounting to something like I-5,oooth of its average time of attachment to a metallic atoni-the electron is free to fall along the potential gradient, and thus to constitute an electric current.When the metal expands by heat, this process of exchange naturally becomes rarer, and the metallic resistance increases. The positive atoms and neutral atoms are so tightly packed that they contribute nothing perceptible to the conductivity. In an insulating solid or liquid the electrons are effectively bound up with atoms or molecular groups, and these are arranged in some structure, usually crystalline in the case of solids, which precludes the formation of mobile charged groups and the liberation of electrons. In an electrolyte, on the other hand, such charged groups are formed, and they are sufficiently mobile to follow the E.M.F. That free electrons are not produced is capable,sf a simple explanation. Two copper atoms have equal attractions for an electron, but atoms of, say, hydrogen and chlorine havc not.Hydrogen loses one of its electrons more easily than chlorine, and when the two atoms of a molecule of hydrochloric acid separate, the chlorine takes one of the normal hydrogen electrons away with it. The hydrogen atom thus reniains positively charged, and the chlorine atom negatively. The two ions so produced act as condensation nuclei, like any other small charged particles. The study of electric condensation nuclei will, I believe, shed a flood of light on electrolytic problems. The fundamental researches of C. T. R. Wilson * have proved that in vapours, at all events, ions act as condensation nuclei, and that negative ions are slightly more effective than positive ions.Of this phenomenon J. J. Thomson -/- has furnished a simple explanation. When a charged drop evaporates its electric charge remains, and its potential increases as its diameter diminishes. More energy is therefore * C. T. H. Wilson, Plzil. Trans. 189, p. 265, 1897. t Coiitlirction of Electricity tkroziglt Gases, 2nd ed., p. 179.THE ELECTRON THEORY TO ELECTROLYSIS 3 required to evaporate a charged drop than an uncharged drop. Condensa- tion on the drop involves an expenditure of available electric energy, and thus condensation is facilitated. This fact has an important bearing on the theory of electrolysis. It explains the low mobility of the ions, the drop of potential at the electrodes,.and the liberation of uncharged products at the latter. The different mobilities of the ions can, I think, only be attributed to their different degrees of normal hydration.That hydroxyl should have a mobility over four times that of lithium, points to the lithium atom as a par- ticularly efficient condensation nucleus when deprived of one electron. It moves so slowly through the water under the influence of the potential gradient because it has to drag a number of water molecules (probably not more than half a dozen) along with it. The number must be the same for each ion at a given temperature, but cannot be very large, to judge from the actual mobility (347 x I O ~ ) compared with that of the electron in copper (about 5 x 104). When these hydrated ions reach the electrodes the E.M.F. tends to drive them into the metal of the electrodes.But the atoms of the latter are so closely packed, as a general rule, that no other atoms can force themselves between them. There arises a deadlock, which is only released when electrons are able to pass from the solution into the metal, or vice vers2. The mobility of electrons, though comparatively great inside a metal, is very small at its surface, where the close packing of the metallic atoms loses its effect. But when the E.M.F. exceeds a certain minimum an electron may pass, say, into a hydrogen ion, and neutralise its positive charge. The immediate consequence will be that the hydrogen loses its condensing power. The link between it and the water molecules is broken, and the hydrogen is liberated as an uncharged gas. The converse process goes on at the anode.The chlorine atom passes its superfluous electron into the anode, thereby becoming neutral and free from its aqueous encumbrance. In the further theoretical investigation of electrolytic action on the basis of the electron theory the study of mobilities will have to play a prominent part. Bredig’s laws of mobility are of special interest in this connection. We require a quantitive determination of the hydration of the ions. This determination should not be very difficult, since a number of data, such as heats of solution, melting- and boiling-points, are ready to hand. It should be possible to determine the diameter and bulk of the ions, and their shape from stereochemical considerations. The fact, noted, by Bredig, that the strongly electropositive and electronegative metals have a high mobility points to a superior facility of reaction. A high mobility implies a low degree of hydration of the ion.This low degree of hydration enables such ions, when of opposite signs, to combine more freely and rapidly than is the case in ions having a larger bulk. The heats of reaction, the heats of ionisation, and the electro-affinities or electrode potentials of the majority of elements are fairly well known, and afford valuable data for determining the sizes of the ions. We already know that isomeric and metameric ions have the same mobility ; that the mobility of an ion of a given sign is the smaller, the greater the number of atoms it contains; and that the effect of any change in the constitution of an ion is the greater, the smaller the number of atoms contained in the ion.These facts leave the road clear for a determination of the actual constitu- tion of a single ion on kinetic principles. When an ion contains 50 or 60 atoms it appears to have a minimuin mobility which is not preceptibly diminished by a further addition of atoms. Such an ion must have a diameter of about a micromillimetre, and therefore comes within the range of4 THE APPLICATION OF Szigmondy’s ultramicroscope. There is, therefore, a possibility that the motion of such heavy ions may be studied by actual observation, and that they may be watched as they cover a distance of I cm. in an hour and a half under the influence of a potential gradient of I volt per cm. It then becomes a comparatively simple calculation to find the diameter of such an approxi- mately spherical ion moving through a viscous liquid, or, in the last resort, threading its way among molecules of perhaps half its diameter.Such calculations have been made in connection with metallic conduction, and have led to the conclusion that the carriers of the current in a wire are almost solely the free electrons. A difficulty is presented by the circumstance that ions of the higher valencies exhibit mobilities which are neither “ independent ” nor constant for different concentrations.” The extraordinarily high mobility of lead is not easily harmonised with our notions of the physical constitution of the ions. But it is interesting to note that the influence of hydration increases with the valency and the concentration, which is what we should expect. The future development of electrolytic theory will lie, I think, in the direction of statistical analysis on kinetic principles. Such analyses have yielded most suggestive results in gas discharges, more especially the electric arc, in the vacuum tube, the Zeeman phenomenon, and, quite recently, in metallic conduction, Thomson’s famous ‘( counting experiment ” must be followed up. He produced ions in a moist gas, and watched them as they settled down under the influence of gravity. We must follow their motions along the potential gradient in a liquid, and folIow the fate of the electrons they convey through the liquid into one electrode, and back into the liquid through the other. The complete determination of the energy absorbed or emitted at the various stages of this transmission means the complete mastery of the problems of electrolysis.

ISSN:0014-7672    

DOI:10.1039/TF9070300001

出版商:RSC    

年代:1907

数据来源: RSC

摘要:

4 THE APPLICATION OF DISC USSIO N. Mr. John G. A. Rhodin : I have read this Paper with great interest, and the subject is one of great fascination to me. I must, however, con- fess that, although I have tried to follow the resuscitation of the electron theory since the discovery of the Zeeman effect, my mind fails to understand what the actual gain has been to our science with regard to the interpretation of the more ordinary phenomena. Like most chemical theories, it seems to me to be a reversion to the ancient method of the quadrature of closed figures. The same applies to the atomic theory. To my mind, the atomic weights are nothing else but numerical solutions of differential co- efficients, relating to acting masses and gravitation during a chemical reaction. Whether they are circular or elliptical functions, as has been sometimes suggested, is not known.To make matters simple, Dalton con- ceived a quadrature with the value for hydrogen as unity. This is in itself a permissible operation, but I doubt whether any scientific man is convinced about the indivisibility of the atoms, except as an image to elucidate the constancy of a ratio between two masses. Infinite divisibility is the necessary supposition for the proof of most problems in the calculus, if our reason is going to be satisfied. Similarly, this must be the case with matter in general. Take a solution of HCl. We can easily conceive that the * See Kohlrausch and Gruneisen, Sitzungsber. Berlin Akad., 40, July 28, 1904.THE ELECTRON THEORY TO ELECTROLYSIS 5 centres of motion in the form of HCl molecules may differ as to mass, as long as H : C1= constant.In other words, the atomic weights do not exisf, except where a reaction takes pzace. The greatest supporter of Dalton’s theory -J. J. Berzelius-must have had a similar idea, as he doggedly stuck to the term equivalent, which no doubt illustrates the only actual knowledge which we posscss on this subject. In the same manner all chemists in actual practice cease to bother about atoms very soon after leaving the university, simply retaining the well-established numerical ratios as a guide. If we now turn to what I should like to call “metaphysical chemistry,” or “meta- chemical physics,” we shall find that our knowledge is confined to experiment- ally determined numbers and mathematical relationships.The explanatory hypotheses are simply more or less crude images, generally encouraged by the simplicity of numerical calculations in limiting cases. The well-known rule, lim a’ = za, may thus be said to be the basis of the electrolytic dissociation theory. Hence the eternal search for constants for infinite dilution. This has, however, become a fashion. With regard to electrical and electromagnetic phenomena, it was necessary to suppose a medium unaffected by gravitation, or one very slightly so, but capable of immense velocity of motion. In this way we got the ether and the electrons. Both are simple conceptions to explain action at a distance. Newton’s light corpuscles, which emanated from a luminous body, were the prototype of the electrons.If some theory of harmonic whirls could have been easily elaborated to explain interference and polarisa- tion phenomena, we might not have had the ether. Transverse vibrations in a kind of solid ether were more easily treated mathematically. Hence, exit emanation, enter undulation and ether. The genius of men like Gauss, Weber, Maxwell, Helmholtz, and others, gave the ether a long lease of life, which I presume has not run out as yet. Certain other phenomena, studied in the sixties, and a short decennium ago the Zeeman effect, could be explained by corpuscles of immense mobility, hence enter electrons ! Now, with regard to electrolysis, I contend that most phenomena are very easily treated by the fairly simple theory of ions as a fundamental principle.What we require is a tabulated knowledge of phenomena in concentrated solutions and with finite current densities. The nature of the charges carried is of so much less consequence, as we are never likely to have a look at them. Why make experiments to support a favourite notion which can never be proved to the satisfaction of our senses ? Even the ions are some- what too excessive a dose of credulity to be medicinal. Why fill them with a similar conception ? The answer is near at hand. To discover new, valuable scientific facts and laws is rather difficult, hence unconsidered trifles have to be magnified. Even the realm of ghosts has been invaded by scientific men in all earnest. Theories are evolved on the most slender basis of facts, and just now they are so many that I am willing to issue a challenge that no man in the world can define which are the accepted ones. That the great liking for the outrd which characterises our time has something to with this is certain, as somc of the most well-known facts are left without an explanation.How do you, for instance, reconcile the vapour density and valence of FeC4 with the theory of indivisible atoms? Why does hard-drawn pure copper wire gain only 2 per cent. in conductivity by annealing, when hard-drawn silver gains 4 per cent. (approximately) ? Why does an annealed, soft, pure copper wire lose density by hard drawing ? Why do we reckon copper as not being a hydrogen-displacing metal when, as a matter of fact, copper dissolves fairly rapidly in 10 N HCl at Boo C. with evolution of hydrogen ? I have personally6 THE APPLICATION OF verified this with pure electrolytic copper, and Mr.Bertram Blount, who doubted it, did the same to his own satisfaction. That volumes could be filled with such queries is evident to most chemists who do not take chemistry as the Athanasian Creed, We do not at present want any more theories-we want a generalisation of existing theories of such a nature that one notion does not contradict another. The rest would not suffer by being noted as observed facts. An international committee for this purpose would probably do more good than a certain other one, which compounded a terminology of organic chemistry which nobody knows or accepts. We practical men have a certain claim that our science should be so systematised as to be available to the average intelligence without unduly taxing one’s memory, which after all has to be used for strictly utilitarian purposes.Dr. H. Borns had no doubt that the author could have given them a great deal more if time had permitted, but he regretted that the author had not found it possible to indicate the lines on which he conceived metallic con- duction by electrons. He did not understand in which sense he was to regard the electron theory as an extension of the ionic theory of electro- chemistry. In its modern conception the electron theory might be dated from J. J. Thomson’s deduction of 1887, that an electrically charged body in motion should behave as if its mass were increased by a certain quantity. He could not quite follow the argument concerning mobility of electrons in copper and in hydrochloric acid, and why there should be electrons in a vacuum and in a metal, but not in a liquid, What became of the electrons when a metal was fused? Perhaps the author would also explain the con- densation taking place on a charged drop a little further.The author had referred to Drude, Schuster, and J. J. Thomson. He might have added the names of Kaufmann, Riecke, Lorentz and Abraham, and others; to the latter two we owed complete theories. The author had not referred to the difficulties of the electron theory, to the Hall effect, nor to the fact that metallic conduction seemed to become badly involved when we assumed positive in addition to negative electrons. Had not Kaufmann’s recent experiments, as discussed by the Naturforscher-Versamndung at Stuttgart, left the two theories mentioned rather under a cloud, moreover ? Dr.T. M. Lowry thanked the author for his fascinating and suggestive Paper. The numerical data given in reference to the number, size, and velocity of the electrons were relnarkably definite, and he would be glad if the author could refer him to the original papers in which these figures had been deduced. Mr. Fournier had stated that electrolytes had positive temperature coefficients. In many cases the conductivity reached a maximum, and at higher temperatures the c6efficient was negative. The question had been fully discussed in a recent paper (Bousfield and Lowry, Proc. Roy. SOC., 1902, 71. 42). The author would perhaps be interested to know that quantitative determinations of the hydrations of the ions were being made by a number of investigators, including Biltz, H.C. Jones, and Bousfield. The latter had arrived at the conclusion that the potassium and chlorine ions were combined with about 5 molecules of water, but the more sluggish lithium ion with about 20 molecules. The bearing of ionic hydration on the theory of electrolysis had already been discussed before the Society (Tram. July, 1905). He would like to know the authority for the statement that ‘‘ when an ion contains 50 or 60 atoms it appears to have a minimum mobility which is not In conclusion, what I mean is this. This was by no means universally true,THE ELECTRON THEORY TO ELECTROLYSIS 7 perceptibly diminished by a further addition of atoms.” The only law of which he was aware was in reference to the temperature coefficients of mobility, which in the case of the more massive ions reached a maximum limit identical with the temperature coefficient of the fluidity of water.Mr. N. T. M. Wilsmore (partly commudcaled) : I must beg to differ from some of the previous speakers, as I consider, with Mr. Fournier-d’Albe, that the electron theory helps to simplify our views of the mechanism of electrolytic phenomena. This is more especially the case on account of the fact that so far oiily l r negative ” electrons appear to be capable of existence in the free state, and that, therefore, most electrochemical reactions may be looked upon as being, to a great extent, merely interchanges of these negative electrons.If it could be proved that ‘(positive” electrons were incapable of existence, the name “electron” might be used as an abbreviation of ‘(negative electron”; but, as long as that proof is wanting, one might, perhaps, be allowed to condense the latter expression into ‘( negatron.’ ‘k On the electron theory of J. J. Thomson, Drude, &c., the electrolytic potentials of the elements are believed to be due to what may be called the dissociation pressures of the free electrons-according to J. J. Thomson, negative electrons-which they contain, This hypothesis is supported by the observation of J. J. Thomson t that the strongly “ electro-positive ” elements spontaneously give off negatrons, and by the results obtained by Ramsay and Spencer $ from a study of the action of ultra-violet light on various elements, the rates at which these give off negatrons being found proportional to their positions in the electrochemical series.It is not necessary to assume that the interchange of electrons at an electrode is always between atoms of the same kind. For instance, the action on a (‘ reduction ” electrode, with, say, a ferrous salt as electrolyte, is most simply represented thus- a ferrous ion becoming a ferric by giving up a negatron to the (platinum) electrode. Similarly the reaction on a typical ( I oxidation ” electrode may be shown thus- MnO: + 8H * + ge =Mn - * + qH,O the electrolyte, permanganate + dilute acid, taking five negatrons from the (platinum) electrode, with formation of manganous salt and water. The direct reaction in solution between permanganate and ferrous iron would be- Mn0,’+8Hm+gFe**= Mno*+4H,0+gFe’” the corresponding interchange of negatrons being here between the ferrous ions and the hydrions.Or ferrous iron may be supposed to react with nascent oxygen in acid solution as follows- 2 F e - . + H,O+ O=2Fe0**+20H’ 20H’ + 2H = aH,O * This word has little claim to acceptance on aesthetic or etymological grounds : but electrical science appears to be saddled permanently with the terms “ positive ” and negative,’’ although their cqFnotation has lately suffered reversal, ‘‘ positive ” now meaning “ excess of matter instead of ‘‘ excess of electricity.” t Phil. Mag. [6] x 584 (190j). 1 Ibid. xii. 397 (I@).8 THE APPLICATION OF hydroxidions being the carriers of the negatrons between the ferrous ions and the hydrions.The net reaction is of course- 2Fe * * + 2H * + 0 = 2Fe - * * + H,O. For example, according to Bodlander and Eberlein * a solution of silver in potassium cyanide contains the silver in the form of a complex anion Ag(CN),”. According to the views till lately in fashion the reactions of this complex at an electrode would take place in two stages- Ag(CN):’ + Ag + 3CN’ Ag + 8 + Ag (metal), viz. (taking the reactions from left to right), first a dissociation of the complex, giving Ag- ion, followed by the discharge and deposition of this ion by a negatron from the cathode. Haber -f has lately shown, however, that the probable concentration of the Ag’ ions (about IO-,~) is far too small to allow of their being the carriers of an appreciable current into or out of the solution.The reactions at an electrode must therefore be represented thus- Ag(CN),” + 8 # 3CN’ + Ag (metal), viz., a direct interchange of negatrons between the CN‘ ions and the electrode. As before, the reaction from left to right represents the deposition of silver on the cathode. A specially interesting case is that of the “positive” plate of the lead accumulator. Two hypotheses to explain the reversible reactions on this electrode have found favour, viz., that of Liebenow, which may be shown Electrode reactions of complex ions may also be viewed in this way. thus- PbO, + 20 (I PbO:‘ PbO,“ + 4H 0- Pb + 2H,O, and that of Le Blanc- PbO, + 4H * + Pb * + 2H,O Pb.... + ze,Pbo*. According to Liebenow the formation of PbO:’ ions, according to Le Blanc that of Pb ions is an essential step in the carrying of the current into or out of the solution.Now the concentration of either of these ions works out to something of the order of 10-9, so that an application of Haber’s reasoning shows that neither of the above hypotheses gives a satis- factory account of what takes place. The reversible reaction on the ‘( positive ” electrode is probably simply- PbO, + 4H * + 2e + Pb- + 2H,O, viz., a direct interchange of negatrons between the electrode and the hydrions of the electrolyte. The difference in the mechanism of conduction in elements and com- pounds must depend on more than the relative closeness of packing of the atoms. In the fused salts, for instance, where the closeness of packing is not greatly less than in metals, conduction must be carried on entirely by ions, seeing that fused salts are transparent and that Faraday’s law is obeyed strictly at the electrodes.The negatrons, which pass from the “electro- positive ” to the “ electro-negative ” atoms in the process of combination * 2. AIZOIZ. Ch. 39. 197 (1904). t 2. Elektroclt. 10. 433 (1904).THE ELECTRON THEORY TO ELECTROLYSIS 9 between two elements, appear to be too firmly retained by the electro- negative atoms to allow of the same kind of dissociation as in metals. The great need in electrochemistry is a clear idea of the mechanism by which ions are produced in an electrolyte. At present we are almost wholly in the dark here ; and it will be a great triumph for the electron theory if it can bridge this gap.In conclusion, I think the thanks of the Society are due to Mr. Fournier- d’Albe for the highly suggestive Paper which he has brought before it. Mr. F. Kaye remarked that if it were a fact that the various mobilities of the ions were the result of different hydrations caused by the different condensation powers of the positive and negative ions, it was a result of great interest and importance. This theory, however, appeared to him to present considerable difficulties for its clear acceptance. The initial cause of the condensation, and therefore of the hydration, was purported to be the electrical charge on the ions. But in a solution the charges in the cations and anions were equal and opposite.What determined the degree of hydration-the nature of the ion or the sigil, and amount, of its charge ? The mass of the atoms was evidently not always the determining factor in the mobility of its ion, for in the two groups of atoms Li, Na, and Kon the one hand, and Ca, Sr, and Ba 011 the other, the heavier ions were the more mobile. Should not the structure of the atom, or of an ionic group, and therefore the space its hydrated ion occupied, be taken into consideration ? Again, with rising temperatures the mobilities of the ions tend to become the same. Can we say that the power of condensation, due to the electrified nucleus, is weakened at higher temperatures, or that the structural functions are less able to exert themselves ? He would like to ask Mr. Fournier whether, when a liquid was evaporat- ing, there was a movement of electrons into the vapour, and on the other hand, what happened when the vapour was condensed.Mr. F. S. Spiers (commurzicated) : In view of the present practical importance of conductors of the solid electrolytic type, it is of interest to note how easily the electron theory explains this form of conduction, and, in particular, the remarkable fact that at high temperatures probably every insulator (unless an element like sulphur, which cannot dissociate) conducts electrolytically. At normal temperatures no free electrons will exist in such bodies, but the effect of raising the temperature will be to cause chemical dissociation of the molecule into two parts, of which one has a greater attraction for the electron than the other.The dissociated parts of the molecule thus become positive and negative ions, and under the influence of an electric field will, like a conducting solution, conduct electrolytically. It is evident that dissociation is always the necessary preliminary to electrolytic conduction, whether in a solid or liquid. The electron theory, however, does not seem to help explain in what manner this preliminary dissociation takes place when, for example, a salt is dissolved in water. Dr. J. A. Harker asked the author whether he conceived of atoms as made up entirely of electrons, or whether he regarded them as consisting of a central nucleus with surrounding electrons, something like the solar system. If the former, he thought that there should be a number of intermediate atoms in between the limited number of stable ones known to us, unless it could be proved that certain groupings were more stable than casual grouping would be.He had heard it from a well-known German physicist that HelmholtzI 0 THE APPLICATION OF once asked Lord Kelvin whether he really believed in the objective truth of the vortex atom theory, and Lord Kelvin answered, ‘‘ Es war nur ein Traum.” Did mathematicians similarly regard the electron theory as merely one out of a number of possible explanations, and did they believe it to be really true ? It was really, he thought, most important that the fundamental con- ceptions of the theory should be perfectly clear and definite, in order that a complete system could be built up, after the fashion of Euclid, on the basis of a few simple axioms and postulates. The Chairman : I know that I express the unanimous sentiment of this meeting when I say that we feel deeply indebted to Mr.Fournier-d’Albe for the Paper he has communicated to the Society, and for the illuminating oral explanations he has added this evening. It would be impossible, even if it weredesirable, to discharge the obligation his lriiidness imposes upon us by his coming from Dublin to attend this meeting, but we have made him the best return in our power by subjecting his Paper to trenchant criticism-by doing our best to pull it to pieces ; and he has had the satisfaction of seeing how well it has withstood the trying ordeal. Mr. Fournier-d’Albe could not have chosen a more important or a more interesting subject than the application of the electron theory to explain the phenomena of electrolysis.There is a widespread feeling of the inadequacy of the old ideas to account completely for all the phenomena of electric conduction, electrolytic and other ; and yet I confess it is not easy (in the case of a hardened sinner like myself) to get rid of old ideas, and their dis- placement is a necessary step towards the reception of new-the ideas SO vividly expounded by Mr. Fournier-dAlbe. The discussion of this evening is a great help, and when we can ponder the rapidly spoken words more deliberately, as I hope we may be able to do by means of a printed report, much that may on delivery have seemed obscure or wrong wit1 doubtless become clear and free from the seeming error.Mr. E. E. Fournier d’Albe, in reply, said that he had not been dis- appointed in his expectations of hearing an interesting discussion. He admired Mr. Rhodin for defending Berzelius against Dalton, but he did not think that the few considerations put forward were sufficient to over- throw the electron theory. Mr. Rhodin would, from his particular standpoint, define Jupiter as a conception to explain action at a distance, but the same reasoning and lines of argument used to cxplain the mass of Jupiter were employed in determining the mass and charge of the electron. In reply to Dr. Borns, the theory was an extension of the ionic theory in the sense that in the latter electricity was regarded as something in the nature of discrete particles, or ions, attached to matericzZ particles, in contrast to an etheric theory.The facilitating of condensation by a charge of electricity was in accordance with a well-known principle of energetics. The Hall effect was explained by supposing that the free paths of the elec- tronswere screwed round in a magnetic field. The reverse swing mentioned by Dr. Borns was not necessarily due to positive electrons, but might be simply caused by positive ions with great mobility. Only a short time previously there was another alleged discovery of positive electrons by Jean Becquerel, but in this case also the existence of positive electrons was not necessarily implied. With regard to Kaufmann’s very delicate expcrirnents, these merely tested the relative accuracy of the various electron theories : that is to say, they decided the relative claims of the hard and rigid electron (Abraham), the electron which is flying through space becomes an ellipsoid of revolution (Stoney, Lorenz), and the electron which, though flattened, retains its originalTHE ELECTRON THEORY TO ELECTROLYSIS 11 volume (Bucherer), in order to account for the negative results of the Michelson-Morley experiment, and to escape from the dilemma caused thereby.Kaufmann’s experiments tended to support the theory of Abraham, but the theory itself certainly did not stand or fall by these experiments. Answering Dr. Harker, not only did the atom, according to Thomson, contain a central positive nucleus, but the latter formed the overwhelming proportion of its total mass, so the analogy to the solar system was a very close one.That there existed not an innumerabIe number of intermediate atoms was probably due to the fact that only certain configurations were stable, just as in the floating magnet experiments of Professor J. J. Thomson. He could not agree with Dr. Harker’s last remarks. The theory was too closely in touch with experimental facts-that was its strength-for the application of rigid methods of deduction from a few simple axioms and postulates. As regards metallic conduction, he would refer to an article by Professor Schuster in the Philosoplzical Magazine (February, 1904). There was these evaluated the number of electrons conveying a current through a metal. Referring to the temperature coefficients of the conductivity of electrolytes, it would most likely be found that continuity existed between metals, metalloids, and electrolytes.He was obliged for the references to work on hydration of ions, and glad that the subject had been so far worked out. It was self-evident that free electrons could not exist in liquids; their mobility was so great that they would give rise to a conductivity far beyond that actually observed. Dr. Borns asked what happens to the electrons when you liquefy a conducting solid body. Probably some kind of polymerisation always occurs, due to the greater molecular freedom of the liquid, and therefore ionic groups form. Mercury, for example, has the same order of conductivity in the liquid as in the gaseous state; it was known that the mercury arc owed its conductivity to the free electrons contained in mercury vapour.With regard to Mr. Wilsmore’s suggested term (‘ negatron,” it would be inadvisable to stereotype the unfortunate ‘‘ negative ” character of what we know to be the actual electric current in metals. The word “ electron ” had been coined by Dr. Johnstone Stoney, and had, with rare unanimity, been adopted by practically the whole electrical world. In reply to Mr. Kaye, the negative ion was the more mobile, probably because it was smaller. The question of evaporation raised interesting points. A liquid surface, according to osmotic theory, was practically a semi-permeable membrane, and it would behave in a like manner to electrons, which did not evaporate with the vapour. 4 THE APPLICATION OF DISC USSIO N.Mr. John G. A. Rhodin : I have read this Paper with great interest, and the subject is one of great fascination to me. I must, however, con- fess that, although I have tried to follow the resuscitation of the electron theory since the discovery of the Zeeman effect, my mind fails to understand what the actual gain has been to our science with regard to the interpretation of the more ordinary phenomena. Like most chemical theories, it seems to me to be a reversion to the ancient method of the quadrature of closed figures. The same applies to the atomic theory. To my mind, the atomic weights are nothing else but numerical solutions of differential co- efficients, relating to acting masses and gravitation during a chemical reaction.Whether they are circular or elliptical functions, as has been sometimes suggested, is not known. To make matters simple, Dalton con- ceived a quadrature with the value for hydrogen as unity. This is in itself a permissible operation, but I doubt whether any scientific man is convinced about the indivisibility of the atoms, except as an image to elucidate the constancy of a ratio between two masses. Infinite divisibility is the necessary supposition for the proof of most problems in the calculus, if our reason is going to be satisfied. Similarly, this must be the case with matter in general. Take a solution of HCl. We can easily conceive that the * See Kohlrausch and Gruneisen, Sitzungsber. Berlin Akad., 40, July 28, 1904.THE ELECTRON THEORY TO ELECTROLYSIS 5 centres of motion in the form of HCl molecules may differ as to mass, as long as H : C1= constant.In other words, the atomic weights do not exisf, except where a reaction takes pzace. The greatest supporter of Dalton’s theory -J. J. Berzelius-must have had a similar idea, as he doggedly stuck to the term equivalent, which no doubt illustrates the only actual knowledge which we posscss on this subject. In the same manner all chemists in actual practice cease to bother about atoms very soon after leaving the university, simply retaining the well-established numerical ratios as a guide. If we now turn to what I should like to call “metaphysical chemistry,” or “meta- chemical physics,” we shall find that our knowledge is confined to experiment- ally determined numbers and mathematical relationships.The explanatory hypotheses are simply more or less crude images, generally encouraged by the simplicity of numerical calculations in limiting cases. The well-known rule, lim a’ = za, may thus be said to be the basis of the electrolytic dissociation theory. Hence the eternal search for constants for infinite dilution. This has, however, become a fashion. With regard to electrical and electromagnetic phenomena, it was necessary to suppose a medium unaffected by gravitation, or one very slightly so, but capable of immense velocity of motion. In this way we got the ether and the electrons. Both are simple conceptions to explain action at a distance. Newton’s light corpuscles, which emanated from a luminous body, were the prototype of the electrons. If some theory of harmonic whirls could have been easily elaborated to explain interference and polarisa- tion phenomena, we might not have had the ether.Transverse vibrations in a kind of solid ether were more easily treated mathematically. Hence, exit emanation, enter undulation and ether. The genius of men like Gauss, Weber, Maxwell, Helmholtz, and others, gave the ether a long lease of life, which I presume has not run out as yet. Certain other phenomena, studied in the sixties, and a short decennium ago the Zeeman effect, could be explained by corpuscles of immense mobility, hence enter electrons ! Now, with regard to electrolysis, I contend that most phenomena are very easily treated by the fairly simple theory of ions as a fundamental principle.What we require is a tabulated knowledge of phenomena in concentrated solutions and with finite current densities. The nature of the charges carried is of so much less consequence, as we are never likely to have a look at them. Why make experiments to support a favourite notion which can never be proved to the satisfaction of our senses ? Even the ions are some- what too excessive a dose of credulity to be medicinal. Why fill them with a similar conception ? The answer is near at hand. To discover new, valuable scientific facts and laws is rather difficult, hence unconsidered trifles have to be magnified. Even the realm of ghosts has been invaded by scientific men in all earnest. Theories are evolved on the most slender basis of facts, and just now they are so many that I am willing to issue a challenge that no man in the world can define which are the accepted ones.That the great liking for the outrd which characterises our time has something to with this is certain, as somc of the most well-known facts are left without an explanation. How do you, for instance, reconcile the vapour density and valence of FeC4 with the theory of indivisible atoms? Why does hard-drawn pure copper wire gain only 2 per cent. in conductivity by annealing, when hard-drawn silver gains 4 per cent. (approximately) ? Why does an annealed, soft, pure copper wire lose density by hard drawing ? Why do we reckon copper as not being a hydrogen-displacing metal when, as a matter of fact, copper dissolves fairly rapidly in 10 N HCl at Boo C.with evolution of hydrogen ? I have personally6 THE APPLICATION OF verified this with pure electrolytic copper, and Mr. Bertram Blount, who doubted it, did the same to his own satisfaction. That volumes could be filled with such queries is evident to most chemists who do not take chemistry as the Athanasian Creed, We do not at present want any more theories-we want a generalisation of existing theories of such a nature that one notion does not contradict another. The rest would not suffer by being noted as observed facts. An international committee for this purpose would probably do more good than a certain other one, which compounded a terminology of organic chemistry which nobody knows or accepts. We practical men have a certain claim that our science should be so systematised as to be available to the average intelligence without unduly taxing one’s memory, which after all has to be used for strictly utilitarian purposes.Dr. H. Borns had no doubt that the author could have given them a great deal more if time had permitted, but he regretted that the author had not found it possible to indicate the lines on which he conceived metallic con- duction by electrons. He did not understand in which sense he was to regard the electron theory as an extension of the ionic theory of electro- chemistry. In its modern conception the electron theory might be dated from J. J. Thomson’s deduction of 1887, that an electrically charged body in motion should behave as if its mass were increased by a certain quantity. He could not quite follow the argument concerning mobility of electrons in copper and in hydrochloric acid, and why there should be electrons in a vacuum and in a metal, but not in a liquid, What became of the electrons when a metal was fused? Perhaps the author would also explain the con- densation taking place on a charged drop a little further.The author had referred to Drude, Schuster, and J. J. Thomson. He might have added the names of Kaufmann, Riecke, Lorentz and Abraham, and others; to the latter two we owed complete theories. The author had not referred to the difficulties of the electron theory, to the Hall effect, nor to the fact that metallic conduction seemed to become badly involved when we assumed positive in addition to negative electrons. Had not Kaufmann’s recent experiments, as discussed by the Naturforscher-Versamndung at Stuttgart, left the two theories mentioned rather under a cloud, moreover ? Dr.T. M. Lowry thanked the author for his fascinating and suggestive Paper. The numerical data given in reference to the number, size, and velocity of the electrons were relnarkably definite, and he would be glad if the author could refer him to the original papers in which these figures had been deduced. Mr. Fournier had stated that electrolytes had positive temperature coefficients. In many cases the conductivity reached a maximum, and at higher temperatures the c6efficient was negative. The question had been fully discussed in a recent paper (Bousfield and Lowry, Proc. Roy. SOC., 1902, 71. 42). The author would perhaps be interested to know that quantitative determinations of the hydrations of the ions were being made by a number of investigators, including Biltz, H.C. Jones, and Bousfield. The latter had arrived at the conclusion that the potassium and chlorine ions were combined with about 5 molecules of water, but the more sluggish lithium ion with about 20 molecules. The bearing of ionic hydration on the theory of electrolysis had already been discussed before the Society (Tram. July, 1905). He would like to know the authority for the statement that ‘‘ when an ion contains 50 or 60 atoms it appears to have a minimum mobility which is not In conclusion, what I mean is this. This was by no means universally true,THE ELECTRON THEORY TO ELECTROLYSIS 7 perceptibly diminished by a further addition of atoms.” The only law of which he was aware was in reference to the temperature coefficients of mobility, which in the case of the more massive ions reached a maximum limit identical with the temperature coefficient of the fluidity of water.Mr. N. T. M. Wilsmore (partly commudcaled) : I must beg to differ from some of the previous speakers, as I consider, with Mr. Fournier-d’Albe, that the electron theory helps to simplify our views of the mechanism of electrolytic phenomena. This is more especially the case on account of the fact that so far oiily l r negative ” electrons appear to be capable of existence in the free state, and that, therefore, most electrochemical reactions may be looked upon as being, to a great extent, merely interchanges of these negative electrons.If it could be proved that ‘(positive” electrons were incapable of existence, the name “electron” might be used as an abbreviation of ‘(negative electron”; but, as long as that proof is wanting, one might, perhaps, be allowed to condense the latter expression into ‘( negatron.’ ‘k On the electron theory of J. J. Thomson, Drude, &c., the electrolytic potentials of the elements are believed to be due to what may be called the dissociation pressures of the free electrons-according to J. J. Thomson, negative electrons-which they contain, This hypothesis is supported by the observation of J. J. Thomson t that the strongly “ electro-positive ” elements spontaneously give off negatrons, and by the results obtained by Ramsay and Spencer $ from a study of the action of ultra-violet light on various elements, the rates at which these give off negatrons being found proportional to their positions in the electrochemical series.It is not necessary to assume that the interchange of electrons at an electrode is always between atoms of the same kind. For instance, the action on a (‘ reduction ” electrode, with, say, a ferrous salt as electrolyte, is most simply represented thus- a ferrous ion becoming a ferric by giving up a negatron to the (platinum) electrode. Similarly the reaction on a typical ( I oxidation ” electrode may be shown thus- MnO: + 8H * + ge =Mn - * + qH,O the electrolyte, permanganate + dilute acid, taking five negatrons from the (platinum) electrode, with formation of manganous salt and water.The direct reaction in solution between permanganate and ferrous iron would be- Mn0,’+8Hm+gFe**= Mno*+4H,0+gFe’” the corresponding interchange of negatrons being here between the ferrous ions and the hydrions. Or ferrous iron may be supposed to react with nascent oxygen in acid solution as follows- 2 F e - . + H,O+ O=2Fe0**+20H’ 20H’ + 2H = aH,O * This word has little claim to acceptance on aesthetic or etymological grounds : but electrical science appears to be saddled permanently with the terms “ positive ” and negative,’’ although their cqFnotation has lately suffered reversal, ‘‘ positive ” now meaning “ excess of matter instead of ‘‘ excess of electricity.” t Phil. Mag. [6] x 584 (190j). 1 Ibid. xii. 397 (I@).8 THE APPLICATION OF hydroxidions being the carriers of the negatrons between the ferrous ions and the hydrions. The net reaction is of course- 2Fe * * + 2H * + 0 = 2Fe - * * + H,O.For example, according to Bodlander and Eberlein * a solution of silver in potassium cyanide contains the silver in the form of a complex anion Ag(CN),”. According to the views till lately in fashion the reactions of this complex at an electrode would take place in two stages- Ag(CN):’ + Ag + 3CN’ Ag + 8 + Ag (metal), viz. (taking the reactions from left to right), first a dissociation of the complex, giving Ag- ion, followed by the discharge and deposition of this ion by a negatron from the cathode. Haber -f has lately shown, however, that the probable concentration of the Ag’ ions (about IO-,~) is far too small to allow of their being the carriers of an appreciable current into or out of the solution. The reactions at an electrode must therefore be represented thus- Ag(CN),” + 8 # 3CN’ + Ag (metal), viz., a direct interchange of negatrons between the CN‘ ions and the electrode.As before, the reaction from left to right represents the deposition of silver on the cathode. A specially interesting case is that of the “positive” plate of the lead accumulator. Two hypotheses to explain the reversible reactions on this electrode have found favour, viz., that of Liebenow, which may be shown Electrode reactions of complex ions may also be viewed in this way. thus- PbO, + 20 (I PbO:‘ PbO,“ + 4H 0- Pb + 2H,O, and that of Le Blanc- PbO, + 4H * + Pb * + 2H,O Pb.... + ze,Pbo*.According to Liebenow the formation of PbO:’ ions, according to Le Blanc that of Pb ions is an essential step in the carrying of the current into or out of the solution. Now the concentration of either of these ions works out to something of the order of 10-9, so that an application of Haber’s reasoning shows that neither of the above hypotheses gives a satis- factory account of what takes place. The reversible reaction on the ‘( positive ” electrode is probably simply- PbO, + 4H * + 2e + Pb- + 2H,O, viz., a direct interchange of negatrons between the electrode and the hydrions of the electrolyte. The difference in the mechanism of conduction in elements and com- pounds must depend on more than the relative closeness of packing of the atoms. In the fused salts, for instance, where the closeness of packing is not greatly less than in metals, conduction must be carried on entirely by ions, seeing that fused salts are transparent and that Faraday’s law is obeyed strictly at the electrodes.The negatrons, which pass from the “electro- positive ” to the “ electro-negative ” atoms in the process of combination * 2. AIZOIZ. Ch. 39. 197 (1904). t 2. Elektroclt. 10. 433 (1904).THE ELECTRON THEORY TO ELECTROLYSIS 9 between two elements, appear to be too firmly retained by the electro- negative atoms to allow of the same kind of dissociation as in metals. The great need in electrochemistry is a clear idea of the mechanism by which ions are produced in an electrolyte. At present we are almost wholly in the dark here ; and it will be a great triumph for the electron theory if it can bridge this gap.In conclusion, I think the thanks of the Society are due to Mr. Fournier- d’Albe for the highly suggestive Paper which he has brought before it. Mr. F. Kaye remarked that if it were a fact that the various mobilities of the ions were the result of different hydrations caused by the different condensation powers of the positive and negative ions, it was a result of great interest and importance. This theory, however, appeared to him to present considerable difficulties for its clear acceptance. The initial cause of the condensation, and therefore of the hydration, was purported to be the electrical charge on the ions. But in a solution the charges in the cations and anions were equal and opposite.What determined the degree of hydration-the nature of the ion or the sigil, and amount, of its charge ? The mass of the atoms was evidently not always the determining factor in the mobility of its ion, for in the two groups of atoms Li, Na, and Kon the one hand, and Ca, Sr, and Ba 011 the other, the heavier ions were the more mobile. Should not the structure of the atom, or of an ionic group, and therefore the space its hydrated ion occupied, be taken into consideration ? Again, with rising temperatures the mobilities of the ions tend to become the same. Can we say that the power of condensation, due to the electrified nucleus, is weakened at higher temperatures, or that the structural functions are less able to exert themselves ? He would like to ask Mr.Fournier whether, when a liquid was evaporat- ing, there was a movement of electrons into the vapour, and on the other hand, what happened when the vapour was condensed. Mr. F. S. Spiers (commurzicated) : In view of the present practical importance of conductors of the solid electrolytic type, it is of interest to note how easily the electron theory explains this form of conduction, and, in particular, the remarkable fact that at high temperatures probably every insulator (unless an element like sulphur, which cannot dissociate) conducts electrolytically. At normal temperatures no free electrons will exist in such bodies, but the effect of raising the temperature will be to cause chemical dissociation of the molecule into two parts, of which one has a greater attraction for the electron than the other.The dissociated parts of the molecule thus become positive and negative ions, and under the influence of an electric field will, like a conducting solution, conduct electrolytically. It is evident that dissociation is always the necessary preliminary to electrolytic conduction, whether in a solid or liquid. The electron theory, however, does not seem to help explain in what manner this preliminary dissociation takes place when, for example, a salt is dissolved in water. Dr. J. A. Harker asked the author whether he conceived of atoms as made up entirely of electrons, or whether he regarded them as consisting of a central nucleus with surrounding electrons, something like the solar system. If the former, he thought that there should be a number of intermediate atoms in between the limited number of stable ones known to us, unless it could be proved that certain groupings were more stable than casual grouping would be.He had heard it from a well-known German physicist that HelmholtzI 0 THE APPLICATION OF once asked Lord Kelvin whether he really believed in the objective truth of the vortex atom theory, and Lord Kelvin answered, ‘‘ Es war nur ein Traum.” Did mathematicians similarly regard the electron theory as merely one out of a number of possible explanations, and did they believe it to be really true ? It was really, he thought, most important that the fundamental con- ceptions of the theory should be perfectly clear and definite, in order that a complete system could be built up, after the fashion of Euclid, on the basis of a few simple axioms and postulates.The Chairman : I know that I express the unanimous sentiment of this meeting when I say that we feel deeply indebted to Mr. Fournier-d’Albe for the Paper he has communicated to the Society, and for the illuminating oral explanations he has added this evening. It would be impossible, even if it weredesirable, to discharge the obligation his lriiidness imposes upon us by his coming from Dublin to attend this meeting, but we have made him the best return in our power by subjecting his Paper to trenchant criticism-by doing our best to pull it to pieces ; and he has had the satisfaction of seeing how well it has withstood the trying ordeal. Mr. Fournier-d’Albe could not have chosen a more important or a more interesting subject than the application of the electron theory to explain the phenomena of electrolysis.There is a widespread feeling of the inadequacy of the old ideas to account completely for all the phenomena of electric conduction, electrolytic and other ; and yet I confess it is not easy (in the case of a hardened sinner like myself) to get rid of old ideas, and their dis- placement is a necessary step towards the reception of new-the ideas SO vividly expounded by Mr. Fournier-dAlbe. The discussion of this evening is a great help, and when we can ponder the rapidly spoken words more deliberately, as I hope we may be able to do by means of a printed report, much that may on delivery have seemed obscure or wrong wit1 doubtless become clear and free from the seeming error.Mr. E. E. Fournier d’Albe, in reply, said that he had not been dis- appointed in his expectations of hearing an interesting discussion. He admired Mr. Rhodin for defending Berzelius against Dalton, but he did not think that the few considerations put forward were sufficient to over- throw the electron theory. Mr. Rhodin would, from his particular standpoint, define Jupiter as a conception to explain action at a distance, but the same reasoning and lines of argument used to cxplain the mass of Jupiter were employed in determining the mass and charge of the electron. In reply to Dr. Borns, the theory was an extension of the ionic theory in the sense that in the latter electricity was regarded as something in the nature of discrete particles, or ions, attached to matericzZ particles, in contrast to an etheric theory. The facilitating of condensation by a charge of electricity was in accordance with a well-known principle of energetics.The Hall effect was explained by supposing that the free paths of the elec- tronswere screwed round in a magnetic field. The reverse swing mentioned by Dr. Borns was not necessarily due to positive electrons, but might be simply caused by positive ions with great mobility. Only a short time previously there was another alleged discovery of positive electrons by Jean Becquerel, but in this case also the existence of positive electrons was not necessarily implied. With regard to Kaufmann’s very delicate expcrirnents, these merely tested the relative accuracy of the various electron theories : that is to say, they decided the relative claims of the hard and rigid electron (Abraham), the electron which is flying through space becomes an ellipsoid of revolution (Stoney, Lorenz), and the electron which, though flattened, retains its originalTHE ELECTRON THEORY TO ELECTROLYSIS 11 volume (Bucherer), in order to account for the negative results of the Michelson-Morley experiment, and to escape from the dilemma caused thereby.Kaufmann’s experiments tended to support the theory of Abraham, but the theory itself certainly did not stand or fall by these experiments. Answering Dr. Harker, not only did the atom, according to Thomson, contain a central positive nucleus, but the latter formed the overwhelming proportion of its total mass, so the analogy to the solar system was a very close one. That there existed not an innumerabIe number of intermediate atoms was probably due to the fact that only certain configurations were stable, just as in the floating magnet experiments of Professor J. J. Thomson. He could not agree with Dr. Harker’s last remarks. The theory was too closely in touch with experimental facts-that was its strength-for the application of rigid methods of deduction from a few simple axioms and postulates. As regards metallic conduction, he would refer to an article by Professor Schuster in the Philosoplzical Magazine (February, 1904). There was these evaluated the number of electrons conveying a current through a metal. Referring to the temperature coefficients of the conductivity of electrolytes, it would most likely be found that continuity existed between metals, metalloids, and electrolytes. He was obliged for the references to work on hydration of ions, and glad that the subject had been so far worked out. It was self-evident that free electrons could not exist in liquids; their mobility was so great that they would give rise to a conductivity far beyond that actually observed. Dr. Borns asked what happens to the electrons when you liquefy a conducting solid body. Probably some kind of polymerisation always occurs, due to the greater molecular freedom of the liquid, and therefore ionic groups form. Mercury, for example, has the same order of conductivity in the liquid as in the gaseous state; it was known that the mercury arc owed its conductivity to the free electrons contained in mercury vapour. With regard to Mr. Wilsmore’s suggested term (‘ negatron,” it would be inadvisable to stereotype the unfortunate ‘‘ negative ” character of what we know to be the actual electric current in metals. The word “ electron ” had been coined by Dr. Johnstone Stoney, and had, with rare unanimity, been adopted by practically the whole electrical world. In reply to Mr. Kaye, the negative ion was the more mobile, probably because it was smaller. The question of evaporation raised interesting points. A liquid surface, according to osmotic theory, was practically a semi-permeable membrane, and it would behave in a like manner to electrons, which did not evaporate with the vapour.

ISSN:0014-7672    

DOI:10.1039/TF9070300004

出版商:RSC    

年代:1907

数据来源: RSC

摘要:

GENERAL DISCUSSION ON OSMOTIC PRESSURE. At the Meeting of the Society held on Tuesday, January 29, 1907, a General Discussion on Osmotic Pressure took place. Professor H. E. Armstrong, F.K.S., was in the chair. The discussion was opened by the Earl of Berkeley, who exhibited and described his Apparatus for the Direct Measure- ment of Osmotic Pressure. The ordinary direct method of measuring osmotic pressures is to obtain equilibrium on the two sides of the semi-permeable membrane by means of the pressure of a head of liquid. The method devised by the author and Mr. E. G. J. Hartley substitutes mechanical pressure, which is put straight on to the solution, and equilibrium thus obtained. Reversing the usual arrangement, the solution is put outside the semi-permeable tube, in an outer containing tube of gun-metal, and the solvent inside, suitable mechanical arrangements being provided for rendering the joints leak-tight.The pressure is obtained from a plunger worked with a pile of weights and delivered through a mercury U-tube to the solution outside the septum. Equilibrium is obtained or calculated by watching through a telescope the motion of the solvent in a gauge. The measurements may be made at any desired temperature. Corrections are applied for the imperfect semi- permeability of the membrane by analysing the solvent to see if any solution has passed into it.::: A vapour pressure method for measuring osmotic pressure was also described.+ This is a modification of the Ostwald and Walker dynamical method. Air is sucked through a train of glass vessels containing first sulphuric acid, to dry it, then the solution from which it gets saturated up to the vapour pressure of the latter, then the solveiit, of which it here takes up more, and finally sulphuric acid again. There are two trains of vessels arranged to oscillate about a central axis, so that the air does not actually bubble through the liquids, which is unsatisfactory, but merely passes over them, small platinum tubes being placed in the tubes to increase the wetted surface.The whole apparatus can be placed in a bath and worked at any desired temperature. Arrhenius’ relation modified for concentrated solu- tions was used for calculating the osmotic pressures from the lowered vapour pressures, and very concordant results were shown to have been obtained between measurements made by the direct-pressure and the vapour-pressure methods, The author is of opinion that the latter method will chiefly have to be relied upon to furnish accurate data for the further study of osmotics. Both of these methods are described in full in the Papers referred to below. Mr.W. C. Dampier Whetham, F.R.S., spoke on ‘‘ Indirect Methods of Measuring Osmotic Pressure.” I should like, first of all, to express my admiration of the apparatus which Lord Berkeley has shown to us, and to say that although we have seen the * See also Trans. Roy. Sot., vol. 106, I@, p. 481. t See also PYOC. Roy. Soc., vol. 77, 1906, p. 156. I2OSMOTIC PRESSURE apparatus which has led to such extremely interesting results, we have not seen all the many forms of apparatus which failed, but led, by slow develop- ment, to this great success.The work has really been going on for years, and only one who has followed, as I have, the different vicissitudes of it can appreciate fully the enormous amount of labour and skill which has been shown by Lord Berkeley and Mr. Hartley. I have been asked to say something to-night about indirect methods of measuring osmotic pressure. One of them has been described to us already, this extraordinarily accurate vapour pressure method ; and I believe, with Lord Berkeley, that it is the only way to which we can look for a really thorough investigation of osmotic phenomena. Therefore, it becomes of extreme interest to consider the theory of it as fully as we are able.The formula which Lord Berkeley has developed to connect osmotic and vapour Dressures is- where p and px are the vapour pressures of the solvent and solution respec- tively, P the osmotic pressure, a, the density of the vapour reckoned at the standard atmospheric pressure A, and v the volume of unit mass of the solvent. This expression is not quite the same as that obtained by the consideration of a thermo-dynamic cycle of operations, and the cause of the discrepancy has been worked out by Mr. Spens. The thermo-dynamic formula which is reached by the ordinary van’t Hoff cycle may be written in the same form, but the quantity v, instead of denoting the volume of unit mass of the solvent, now signifies the change of the volume of the solution when unit mass of the solvent enters through a semi-permeable wall.The two results become identical, in cases where there is no change in volume of the solvent as it enters the solution. In solutions of sugar, any change in volume is small, and experiments on such solutions cannot decide between the two formula?. Consequently, it becomes of great interest to develop further such observations as the one which Lord Berkeley has described with calcium ferrocyanide, to see whether with solutions in which a volume change does occur, the thermo-dynamic result is verified. If in reality we are measuring the ordinary thermo-dynamic osmotic pressure, that is to say, the pressure at which a solution is in equilibrium with its solvent through an ideal semi- permeable wall, the thermo-dynamic cycle must, I think, lead to the right result; but, from an experimental point of view, it is most important to carry the observations further, and see which of the two formulze agrees with the facts.Now that Lord Berkeley and Mr. Hartley have developed the vapour pressure method to such a state of perfection, we may claim to possess the means of determining indirectly the osmotic pressures of concentrated solu- tions. Turning to the other end of the scale, we are still dependent on Mr. Griffiths’ partly pubiished results on freezing-points for a knowledge of the osmotic pressures of very dilute solutions. Mr. Bedford has been continuing the experiments, and his results are awaited with interest. GENERAL DISCUSSION ON OSMOTIC PRESSURE. At the Meeting of the Society held on Tuesday, January 29, 1907, a General Discussion on Osmotic Pressure took place.Professor H. E. Armstrong, F.K.S., was in the chair. The discussion was opened by the Earl of Berkeley, who exhibited and described his Apparatus for the Direct Measure- ment of Osmotic Pressure. The ordinary direct method of measuring osmotic pressures is to obtain equilibrium on the two sides of the semi-permeable membrane by means of the pressure of a head of liquid. The method devised by the author and Mr. E. G. J. Hartley substitutes mechanical pressure, which is put straight on to the solution, and equilibrium thus obtained. Reversing the usual arrangement, the solution is put outside the semi-permeable tube, in an outer containing tube of gun-metal, and the solvent inside, suitable mechanical arrangements being provided for rendering the joints leak-tight.The pressure is obtained from a plunger worked with a pile of weights and delivered through a mercury U-tube to the solution outside the septum. Equilibrium is obtained or calculated by watching through a telescope the motion of the solvent in a gauge. The measurements may be made at any desired temperature. Corrections are applied for the imperfect semi- permeability of the membrane by analysing the solvent to see if any solution has passed into it.::: A vapour pressure method for measuring osmotic pressure was also described.+ This is a modification of the Ostwald and Walker dynamical method. Air is sucked through a train of glass vessels containing first sulphuric acid, to dry it, then the solution from which it gets saturated up to the vapour pressure of the latter, then the solveiit, of which it here takes up more, and finally sulphuric acid again.There are two trains of vessels arranged to oscillate about a central axis, so that the air does not actually bubble through the liquids, which is unsatisfactory, but merely passes over them, small platinum tubes being placed in the tubes to increase the wetted surface. The whole apparatus can be placed in a bath and worked at any desired temperature. Arrhenius’ relation modified for concentrated solu- tions was used for calculating the osmotic pressures from the lowered vapour pressures, and very concordant results were shown to have been obtained between measurements made by the direct-pressure and the vapour-pressure methods, The author is of opinion that the latter method will chiefly have to be relied upon to furnish accurate data for the further study of osmotics.Both of these methods are described in full in the Papers referred to below. Mr. W. C. Dampier Whetham, F.R.S., spoke on ‘‘ Indirect Methods of Measuring Osmotic Pressure.” I should like, first of all, to express my admiration of the apparatus which Lord Berkeley has shown to us, and to say that although we have seen the * See also Trans. Roy. Sot., vol. 106, I@, p. 481. t See also PYOC. Roy. Soc., vol. 77, 1906, p. 156. I2OSMOTIC PRESSURE apparatus which has led to such extremely interesting results, we have not seen all the many forms of apparatus which failed, but led, by slow develop- ment, to this great success.The work has really been going on for years, and only one who has followed, as I have, the different vicissitudes of it can appreciate fully the enormous amount of labour and skill which has been shown by Lord Berkeley and Mr. Hartley. I have been asked to say something to-night about indirect methods of measuring osmotic pressure. One of them has been described to us already, this extraordinarily accurate vapour pressure method ; and I believe, with Lord Berkeley, that it is the only way to which we can look for a really thorough investigation of osmotic phenomena. Therefore, it becomes of extreme interest to consider the theory of it as fully as we are able. The formula which Lord Berkeley has developed to connect osmotic and vapour Dressures is- where p and px are the vapour pressures of the solvent and solution respec- tively, P the osmotic pressure, a, the density of the vapour reckoned at the standard atmospheric pressure A, and v the volume of unit mass of the solvent.This expression is not quite the same as that obtained by the consideration of a thermo-dynamic cycle of operations, and the cause of the discrepancy has been worked out by Mr. Spens. The thermo-dynamic formula which is reached by the ordinary van’t Hoff cycle may be written in the same form, but the quantity v, instead of denoting the volume of unit mass of the solvent, now signifies the change of the volume of the solution when unit mass of the solvent enters through a semi-permeable wall.The two results become identical, in cases where there is no change in volume of the solvent as it enters the solution. In solutions of sugar, any change in volume is small, and experiments on such solutions cannot decide between the two formula?. Consequently, it becomes of great interest to develop further such observations as the one which Lord Berkeley has described with calcium ferrocyanide, to see whether with solutions in which a volume change does occur, the thermo-dynamic result is verified. If in reality we are measuring the ordinary thermo-dynamic osmotic pressure, that is to say, the pressure at which a solution is in equilibrium with its solvent through an ideal semi- permeable wall, the thermo-dynamic cycle must, I think, lead to the right result; but, from an experimental point of view, it is most important to carry the observations further, and see which of the two formulze agrees with the facts. Now that Lord Berkeley and Mr. Hartley have developed the vapour pressure method to such a state of perfection, we may claim to possess the means of determining indirectly the osmotic pressures of concentrated solu- tions. Turning to the other end of the scale, we are still dependent on Mr. Griffiths’ partly pubiished results on freezing-points for a knowledge of the osmotic pressures of very dilute solutions. Mr. Bedford has been continuing the experiments, and his results are awaited with interest.

ISSN:0014-7672    

DOI:10.1039/TF9070300012

出版商:RSC    

年代:1907

数据来源: RSC

摘要:

OSMOTIC PRESSURE FROM THE STANDPOINT OF THE KINETIC THEORY. Dr. T. Martin Lowry contributed a Paper entitled "Osmotic Pressure from the Standpoint of the Kinetic Theory." KINETIC THEORY OF GASES. The incessant motion of the particles, which forms the fundamental hypothesis of the kinetic theory, leads in the case of simple gases to two chief results : (I) the gas is able to expand to any extent, and (2) it exerts ,a pressure whenever this expansion is resisted by enclosing the gas. DIFFUSION IN GASES. When several gases are enclosed together the motion of the particles leads to a process of diffusion or mixing by which each gas gradually penetrates until its particles are distributed throughout the whole of the available space in much the same way as if no other gas were present.The ultimate distribution is, as a rule, uniform, but in the atmosphere where there is a very large pressure gradient the process of diffusion results in an accumulation of the less compressible gases in the higher levels, whilst the more compressible gases predominate in the lower levels (Dewar, Presi- dential Address, British Association, Belfast meeting, 1903, pp. 39-46). A similar inequality of concentration may be set up when gases of unequal compressibility are submitted to the action of centrifugal forces (V. Calzavara, " The Separatore Mazza," Padua, 1903). OSMOTIC PRESSURE IN GASES. When the process of diffusion in a composite gas is hindered the motion of the particles may manifest itself in the form of a pressure precisely analogous to that which is produced by simple gases when prevented from expanding.The simplest example of such a pressure is found in the common experiment in which a porous pot containing air is surrounded by an atmosphere of hydrogen; the hydrogen diffuses into the pot more rapidly than the air is able to escape, and a transient pressure is produced in the pot. In order to produce a permanent measured pressure it is necessary not merely to delay the mixing, but to prevent it altogether. This is usually done by means of a semi-permeable membrane, through which only one of the constituents can pass. In the case under considera- tion this might be made from platinum or palladium foil as suggested by Arrhenius (Zeit. phys. Chem., 1889, 3, 119). Ramsay has shown (Phil. Mag., 1894, 38, 206) that, when a palladium membrane heated to 6oo"is used to separate equal volumes of hydrogen and nitrogen, the nitrogen is retained on one side of the membrane, whilst the hydrogen passes through until it is equally distributed on either side, In the final condition of equilibrium there is an excess of pressure on the side of the nitrogen exactly equal to that which it exerted before the advent of the diluent hydrogen.It should I4OSMOTIC PRESSURE be noted that in this case the excess of pressure, which forms an exact analogue of osmotic pressure in liquids, is actually produced by the bom- bardment of the walls of the vessel by the nitrogen molecules and is independent of the concentration of the hydrogen, and indeed-if a suitable membrane be provided-of the nature of the diluent gas.KINETIC THEORY OF LIQUIDS. In liquids the free motion of the particles is held in check by their mutual attraction. In the interior of the liquid this attraction acts equally in all directions, and the particles are able to move with no other restriction than that imposed by their closer packing and greatly reduced range of free motion, At the surface, however, the attraction acts entirely in one direction, and produces a force which manifests itself in the surface tension and latent heat of evaporation of the liquid; this force prevents the escape of all particles except those which reach the surface with a velocity sufficiently great to break away from the attraction of their fellows. LIQUID DIFFUSION. When two liquids are mixed they may either (I) amalgarnatc completely, as in the case of gases, or (2) form two distinct layers or phases.The latter possibility is in itself sufficient to indicate that the conditions prevailing in a liquid are very different from those which exist in gases, and suggests that considerable caution is necessary in applying to liquids laws that have been deduced from experiments made with gases. In liquids of the former class the motion of the particles again leads to a process of diffusion, though this proceeds much more slowly than in gases, owing to the great reduction in the length of the mean free path of the particles. Ultimately, however, a condition of equilibrium is reached in which, almost invariably, the constituents are uniformly distributed through- out the mass? In the second case the mutual attractions of like particles are so much greater than those of the unlike particles that only a few particles of each kind are able to break away from their fellows and mingle with the particles of the other phase or layer.The conditions of equilibrium are then very similar to those which prevail between liquid and vapour, the chief difference being that there is an interchange of two kinds of particle instead of one. In this case also the motion of the particles leads to a uniform distribution of the constituents in each phase, though the relative concentrations in the two phases may differ widely. In solids the motion of the particles is mainly oscillatory, and although a tendency to diffuse may exist, the actual process of diffusion is in the majority of cases too slow to be detected.OSMOTIC PRESSURE IN LIQUIDS. The application of the equation PV = RT to the osmotic pressure of gases could be predicted on general theoretical grounds, but the conditions pre- vailing in liquids are so much more complex, that there is no h priori reason for supposing that the osmotic pressure developed by a solution would be governed by the same law, or would bear any relationship to the gas pressure produced by a gas of equal molecular concentration. The idea that the osmotic pressure of a dilute solution can be calculated from the same * If the solvent and solution are unequally compressible, varialions of pressure would produce a corresponding gradient of concentration. This, however is usually negligible.16 OSMOTIC PRESSURE FROM THE STANDPOINT formula that is used to predict the pressure exerted by a gas has been arrived at, not by intuition, but by experiment.Owing to the difficulty of discovering efficient semi-permeable membranes, accurate measurements of osmotic pressure have been confined almost exclusively to aqueous solutions of the sugars separated from the pure solvent by a membrane of copper ferro- cyanide, The experiments usually quoted are those of Pfeffer (Osmotische Untersuchurzgen, Leipzig, 1877), but the recent accurate measurements of Morse and Fraser have entirely confirmed the conclusions already arrived at from the earlier experiments, since they have shown that within the limits of M/IO to M the actual osmotic pressure produced by a cane-sugar solution agrees with that calculated from the gas-formula with an average deviation of less than 4 per cent.Indirect methods of measuring osmotic pressure by lowering of vapour pressure, elevation of boiling-point, and depression of freezing-point, have extended the observations to a very wide range of substances, and in some cases a very high degree of accuracy has been attained. In particular, Griffiths has shown that the osmotic pressure of a cane-sugar solution deduced from measurements of the freezing-point agrees with that calculated from the gas-formula with an error not exceeding 0.01 per cent. It is, however, important to notice that the formula A = -- , by which the molecular eleva- tion of the boiling-point or depression of the freezing-point can be derived from the absolute temperature T of boiling or freezing, and the latent heat q involved in the change of state is based upon the assumptions ( I ) that the change of temperature is determined by the work done against the osmotic pressure when the solution is concentrated by evaporating ,or freezing out the solvent, and (2) that this osmotic pressure can be calculated from the gas- formula ; it follows, therefore, that the validity of the gas-laws as applied to solutions is proved, not merely by a few direct measurements of the osmotic pressure of sugar solutions, iior even by isolated experiments such as those of Griffiths, but also by the countless experiments in which the Beckmann apparatus has been successfully employed in determining the molecular weights of dissolved substances.The proof of the numerical identity of osmotic pressure with gas pressure has been discussed somewhat fully because it appears to be of fundamental importance, and has been SD fully verified by experiment that it must be accepted as a fact in any consideration of the origin and mechanism of osmotic pressure. RTa Q THE MECHANISM OF OSMOTIC PRESSURE. There can at the present time be no doubt that osmotic pressure de- pends essentially on the phenomena of selective solubility. The palladium membrane acts in virtue of its ability to absorb or dissolve hydrogen, but not nitrogen ; the amount of hydrogen absorbed depends on the pressure, and equilibrium is attained when the partial pressure of the hydrogen inside the vessel is equal to its total pressure outside.A water-membrane has been used by Nernst to develop an osmotic pressure between ether and an ethereal solution of benzene, the former being soluble and the latter insoluble in water. Copper ferrocyanide, which absorbs or dissolves water and certain salts blit not sugars, forms an efficient semi-permeable membrane for aqueous sugar solutions, but not for salt solutions. The presence of the sugar diminishes the solubility of the water in the membrane, and a flow of liquid is set up because the membrane when saturated with regard to the water on one side is supersaturated with regard to the solution on the other side.OF THE KINETIC THEORY The amount of water taken up by the membrane can, however, be increased by compressing the liquid, and it is thus possible to counterbalance the decrease of solubility due to the sugar, and by equalising the solubility on the two sides of the membrane to stop the flow of solvent into the solution.The pressure required to equalise the solubilities and stop the flow is the so-called (‘ osmotic pressure,” and it may again be urged that the conditions are so far different from those prevailing in a gas, that the gas-analogy, though s u s e s - tive, would be insufficient to justify the application of the gas laws to osmotic pressure unless these could be verified experimentally or could be established on an independent theoretical basis. Pickering’s theory (Ber., 1891, 24, 3639), that the action of the semi- permeable membrane depends on the relative size of the molecules of solvent and solute, has not been confirmed ; the very similar mechanical theory of Sutherland (Phil.Mag., 1897 (v) 4, 493-498), that the membrane consists of “ meshes ” through which water but not sugar can pass, appears to be equally untenable, and need not be discussed here; Traube’s theory is referred to later. VAN’T HOFF’S GAS THEORY OF OSMOTIC PRESSURE. When the discovery was made that osmotic pressure obeyed the same laws as gas pressure it was generally assumed that the two phenomena must be essentially similar in character. Thus van’t Hoff, in his classical paper, (‘ Die Rolle des osmotischen Druckes in der Analogie zwischen Losungen und Gasen” (2eit.phys. Chem, 1887, I. 481-508), whilst recognising in general terms the attrac- tive influence of the solution for the solvent (die wasseranziekende Wirkurzg der Liisultg), attributed the osmotic pressure to the gas-like bombardment of the membrane by the molecules of the solute, the solvent being regarded as practically inert.* Three years later, at the Leeds meeting of the British Association (Report, 1890, p.356) he suggested that “the action on a semi-permeable diaphragm is due partly to the shock of the dissolved molecules, partly to the difference of forces acting upon them from the solvent on one side and from the solution on the other,” but added that ‘‘ in very dilute solutions the shock is alone the origin of pressure, as it is in gases.” In support of this view he also quoted (Zeif. jhys. Chem., 1890,5, 174-176) the case of gaseous osmotic pressure suggested by Arrhenius and subsequently verified by Ramsay as described above.van’t Hoff’s theory had at least the merit of giving a simple and quantita- tive explanation of the osmotic phenomena, and was not unreasonable when applied to the osmotic pressure of gases dissolved in liquids. The concep- tion was, however, not easy to apply to the more ordinary cases of osmotic pressure, such as those afforded by aqueous sugar solutions. In the majority of cases the solute molecules appear to have a very stiiall mobility, and in the absence of the solvent they are unable to produce any hydrostatic pressure whatever upon the walls of the containing vessel. I t is, therefore, scarcely reasonable to attribute the whole action of the solution to the relatively inert solute, whilst neglecting entirely the very active part played by the solvent (Fitzgerald, B.A. Report, 18g0, p. 327). POYNTI NG’S ‘r H EORY. Much interest was aroused by the publication in 1896 of a paper by Professor Poynting (Phil. Mag. (v) 42, 289-~oo), in which an alternative * Es handelt sich im ersten Falle um die Stosse der Gas-molekiile an die Gefasswand, im letzteren um diejenigen der Molekiile vom gelosten Korper an die halbdurchl5ssige Membran, da ja die des beiderseitig anwesenden Losungsmittels als hindurchgehend, nicht in Betracht kommen (ibid., p. 482).18 OSMOTIC PRESSURE FROM THE STANDPOINT explanation was given of the osmotic pressure of liquids. In many respects the view he advocated is very similar to that described below, but the two theories differ in one essential point, and consequently lead to widely different explanations of the origin of osmotic pressure.In particular, Poynting was led to assume that osmotic pressure was due to the formation of labile hydrates, and it became necessary, as Whetham pointed out (Nature, Oct. 15, I&$), to assume that all substances which gave a normal osmotic pressure were monohydrated, whilst the double osmotic pressure of binary salts might be ascribed to the formation of a dihydrate or of two monohydrated ions." The invariable formation of loose monohydrates was so improbable, and is so far in contradiction to recent work on the hydrates present in solution, that Poynting's theory has failed to secure general acceptance as an explanation of the osmotic phenomena. OSMOTIC PRESSURE AS A KINETIC PHENOMENON.The theory of osmotic pressure now described formed the subject of a paper read before the Chemical Society of the Central Technical College as long ago as May, 1896, but it was only recently that it was recognised as being sufficiently novel to warrant further publicity. The starting-point of the theory is a consideration of the conditions prevailing at the surface of separation of the solution and the semi-permeable membrane, which may be either a layer of copper ferrocyanide or merely the boundary between liquid and vapour or liquid and ice. The simplest of these cases is undoubtedly that which involves the equilibrium between liquid and vapour. In this case the kinetic theory postulates a continual process of evaporation, whereby rapidly moving particles are constantly escaping from the surface of the liquid into the vapour space.This is balanced by the con- densation of practicatly all the molecules of the vapour that impinge on the liquid surface. When the vapour reaches a certain concentration the rate of condensation becomes equal to the rate of evaporation, and a condition of equilibrium is attained, not because evaporation has ceased, but because it is neutralised by an equal and opposite process of condensation. In the case of a non-volatile liquid the mobility of the relatively heavy molecules or mole- cular complexes is so small that very few are able to escape, and the maximum vapour pressure of such liquids is inappreciable.If now a solution be prepared by mixing a volatile and a non-volatile liquid, or by dissolving a non-volatile solid in a volatile liquid, the surface will contain both kinds of molecules. If one of the solvent molecules be struck by a rapidly moving molecule from the interior of the liquid, it will be projected into the vapour space. If, however, one of the non-volatile molecules be struck, it will be unable to escape, and the solvent particle will rebound in much the same way as if it had struck the wall of the containing vessel. The rate of evaporation is therefore reduced by the addition to the solvent of a non-volatile solute. On the other hand, it is probable that the presertce of the raora-volatile molecules would not interfere with the rate of coltdensation of the vapour.This point is of fundamental importance, as the opposite view was advocated by Poynting, who supposed that con- densation and evaporation would be checked to an equal extent, just as if the surface had been covered by a plate of perforated zinc. It must be remem- bered, however, that a considerable upward velocity is required before a molecule can escape from the surface of a liquid, and that a molecule descending with ever. the smallest downward velocity would have little * -4 similar deduction was drawn by I. Traube ( A m . Pltys. Cltena., 1897, ii. 62, 490-506).OF THE KINETIC THEORY 19 chance of escaping when once it came within the range of attraction of the liquid. Even if, on reaching the liquid surface, a vapour molecule should strike against a non-volatile molecule of the solute the attraction of the neighbouring molecules of the solvent would be sufficient to hold it, and thus ensure its condensation.I t need scarcely be pointed out that similar conditions would prevail at the surface separating liquid and ice or at the surface of one of the more conventional semi-permeable membranes. In the former case the presence of a non-isomorphous solute might prevent the adhesion to the ice of a molecule of solvent moving towards it but separated from it by a molecule of solute. On the other hand, it would not prevent the melting off or dissolution of an ice molecule if the average kinetic energy (k, temperature) of the ice were raised by the latent heat of crystallisation of other molecules passing from thc liquid to the solid state.In the case of a membrane such as copper ferrocyanide the solute would check the escape of solvent mole- cules from the solution into the membrane, but would not oppose the return of wanderers migrating from the membrane back into the solution. This would lead to a disturbance of the equilibrium between the solvent and the liquids on either side, and would be sufficient to produce an osmotic flow and a consequent osmotic pressure. DEDUCTION OF THE GAS-FORRIULX FROM THE KINETIC THEORY OF OSMOTIC PRESSURE. Of the theories of osmotic pressure that have hitherto been put forward, that of van’t Hoff is the one that leads most directly to a simple quantitative explanation of the phenomena. Poynting, in developing his theory quantita- tively, considered that solute molecules and stable compounds of solvent and solute would merely alter the effective surface of the liquid without changing the relative rates of evaporation and condensation ; only in the case of labile compounds was it considered possible that the rate of evaporation might be checked without altering the rate of condensation ; on this basis a quantitative explanation was only possible on the impracticable assumption that all ordinary solutes are loosely monohydrated in aqueous solutions.The more recent theory that osmotic pressure depends on a disturbance in the equilibrium between simple and polymerised solvent molecules, e.g., H,O,H,,O, (Armstrong, Proc. Roy. Soc., 1906, A. 78, &4-271), has also, as yet, failed to yield a quantitative interpretation of the pressures produced.The quantitative interpretation of the kinetic theory of osmotic pressure follows at once from Nernst’s proof of the relationship between osmotic pressure and vapour pressure (TIzeoreiicaZ Clzemisfry, pp. ~ q - ~ z g ) . Thus if N be the number of gram-molecules of solvent, and n the number of gram- molecules of solute in unit volume of the solution the validity of the gas- formula for osmotic pressure is readily deducible from the equation- N A’ N + n-p’ where p is the vapour pressure of the solvent, and p’ that of the solution.* If the simple assuniption is made that the molecules of solvent are uniformly distributed in the surface layer, and that the spacing or packing is the same as in the pure solvent, it follows a t once that the rates of evaporation from unit surface of solvent and solution will be in the ratio N + n to N, and that this will also be the ratio of the vapour pressures, as required by the above * The value of N is determined by the molecular weight of the solvent as it exists in the vapour, and not by its molecular weight in the liquid state.20 OSMOTIC PRESSURE FROM THE STANDPOINT formula.The argument is, of course, identical with that used by Poynting to show that each molecule of solute must destroy the mobility of a molecule of solvent, but, whereas he was led to assume the regular formation of labile monohydrates, the theory given above merely postulates that the mobility of a solvent molecule is destroyed when its place in the surface of the liquid is occupied by a molecule of solute.SURFACE STRUCTURE OF LIQUIDS. It will at once be noticed that in its simplest form the kinetic theory of osmotic pressure would indicate .that the pressures calculated from the gas- formula might be subject to a small correction for the volume changes accompanying dissolution. Whether such corrections are necessary can only be determined by experiment, but the evidence now available points to a very close agreement between the values observed and calculated for dilute solutions. If this identity should be confirmed, it will be possible to deduce from observations of osmotic pressure some information in reference to the surface structure of liquids, since if the agreement is exact there must be an equally exact replacement of solvent by solute in the surface of the liquid.Thus in view of the different iiiasses of the molecules that may be dis- solved in the same solute and yield identical osmotic pressures, there must be a considerable spacing between the actual molecules in the surface. Again, it may be noted that this exact replacement does not take place in the interior of the liquid, where the molecular volumes of different solvents differ widely, and when calculated in the conventional way may even have a negative value. It is, however, by no means improbable that the marshalling of the molecules 011 the frontiers of the liquid may be governed by a stricter discipline than that which prevails in the interior, and that the surface molecules may even be forced to conform to the exact regulations which govern the replacement of molecules in solid solutions or isomorphous mixtures. FORhfATION OF COMPLEXES.The formation in the solution of loose complexes of solvent with solute or of solvent molecules with one another has not yet been referred to, but presents no difficulty in the development of the theory. Such complexes are usually formed without any large change of volume, and under the stricter condi- tions prevailing at the surface no alteration in the area of surface occupied need result from the linking up of the ‘‘ residual affinities ” of the molecules. Neither need it be supposed that the rate of evaporation would be affected otherwise than by a general reduction of mobility::: due to the chemical attraction of the molecules and producing equal effects in solvent and in dilute solution.Thus, as there is 110 change of energy involved in the replace- ment of one solvent molecule in a complex by another from outside, a rapidly moving particle impinging on a complex might drive a combined solvent moleculc into the vapour space and itself occupy the vacant position in the complex, the effects produced being much the same as if no complex existed. A similar statement would apply to molecules of solvent attached to the solute in the form of labile hydrates or compounds; a free solvent molecule impinging on a combined molecule in the surface of the liquid might drive it out and take its place, but if it should impinge on the nuclear solute molecule it would be repulsed and driven back into the interior, just as it would be by an uncombined molecule of solute.cohesion in a gas as represented by the quantity * Compare the reduction of mobility caused by liquid in van der Waals’ equation, 2jaOF T H E KINETIC THEORY 21 CONCENTRATED SOLUTIONS. No attempt has been made i n the above to account quantitatively for the osmotic pressure of concentrated solutions. Even in the case of gases, the equation PV = RT only applies strictly to a material gas within narrow limits of pressure, but Morse and Fraser’s experiments indicate a much wider applicability when the formula is applied to osmotic pressures, provided only that for concentrated solutions V is interpreted as the volume of solvent used to dissolve the solute and not the total volume of the solution.It need scarcely be pointed out that this modification is very similar in type to the co-volume correction in van der WBals’ equation. OSMOTIC PRESSURE AND SURFACE TENSION. In view of the close relationship that has been indicated between osmotic pressure and surface structure it would not be surprising that a relation- ship should exist between surface tension and osmotic pressure. Such a relationship has been postulated theoretically by Traube, who supposes that osmotic pressure depends on a tendency to equalise the surface tension of the two liquids, and has been confirmed experimentally by Batelli and Stephanini (Atli. R. Acad. Lincei, 1905, (v), 14, ii. pp. 3-14), who find that solutions of equal surface tension have equal osmotic pressures, even in cases in which the solutions are not nominally equimolecular. In conclusion it may be pointed out that whilst the vicws advocated above were comparatively novel ten years ago, the idea that osmotic pressure depends on the activity of the solvent rather than on that of the solute has now become widely accepted, and has been advocated, not only by Poynting, but also by Armstrong ( E m .Brit., 26. 739), Beilby, (B. A. Report, South Africa, 1905, 361), and others. At the present time, therefore, the only points for which any degree of novelty can be claimed are in reference to the mechanism by which the activity of the solvent at the surface of the liquid is reduced by the ‘‘ blocking action ” of the solute operating in one direction only, and to the possibility of deducing from the osmotic phenomena information as to the surface structure of liquids.130, HORSEFERRY ROAD, WESTMINSTER, S.W. OSMOTIC PRESSURE FROM THE STANDPOINT OF THE KINETIC THEORY. Dr. T. Martin Lowry contributed a Paper entitled "Osmotic Pressure from the Standpoint of the Kinetic Theory." KINETIC THEORY OF GASES. The incessant motion of the particles, which forms the fundamental hypothesis of the kinetic theory, leads in the case of simple gases to two chief results : (I) the gas is able to expand to any extent, and (2) it exerts ,a pressure whenever this expansion is resisted by enclosing the gas. DIFFUSION IN GASES. When several gases are enclosed together the motion of the particles leads to a process of diffusion or mixing by which each gas gradually penetrates until its particles are distributed throughout the whole of the available space in much the same way as if no other gas were present.The ultimate distribution is, as a rule, uniform, but in the atmosphere where there is a very large pressure gradient the process of diffusion results in an accumulation of the less compressible gases in the higher levels, whilst the more compressible gases predominate in the lower levels (Dewar, Presi- dential Address, British Association, Belfast meeting, 1903, pp. 39-46). A similar inequality of concentration may be set up when gases of unequal compressibility are submitted to the action of centrifugal forces (V. Calzavara, " The Separatore Mazza," Padua, 1903). OSMOTIC PRESSURE IN GASES. When the process of diffusion in a composite gas is hindered the motion of the particles may manifest itself in the form of a pressure precisely analogous to that which is produced by simple gases when prevented from expanding.The simplest example of such a pressure is found in the common experiment in which a porous pot containing air is surrounded by an atmosphere of hydrogen; the hydrogen diffuses into the pot more rapidly than the air is able to escape, and a transient pressure is produced in the pot. In order to produce a permanent measured pressure it is necessary not merely to delay the mixing, but to prevent it altogether. This is usually done by means of a semi-permeable membrane, through which only one of the constituents can pass. In the case under considera- tion this might be made from platinum or palladium foil as suggested by Arrhenius (Zeit.phys. Chem., 1889, 3, 119). Ramsay has shown (Phil. Mag., 1894, 38, 206) that, when a palladium membrane heated to 6oo"is used to separate equal volumes of hydrogen and nitrogen, the nitrogen is retained on one side of the membrane, whilst the hydrogen passes through until it is equally distributed on either side, In the final condition of equilibrium there is an excess of pressure on the side of the nitrogen exactly equal to that which it exerted before the advent of the diluent hydrogen. It should I4OSMOTIC PRESSURE be noted that in this case the excess of pressure, which forms an exact analogue of osmotic pressure in liquids, is actually produced by the bom- bardment of the walls of the vessel by the nitrogen molecules and is independent of the concentration of the hydrogen, and indeed-if a suitable membrane be provided-of the nature of the diluent gas.KINETIC THEORY OF LIQUIDS. In liquids the free motion of the particles is held in check by their mutual attraction. In the interior of the liquid this attraction acts equally in all directions, and the particles are able to move with no other restriction than that imposed by their closer packing and greatly reduced range of free motion, At the surface, however, the attraction acts entirely in one direction, and produces a force which manifests itself in the surface tension and latent heat of evaporation of the liquid; this force prevents the escape of all particles except those which reach the surface with a velocity sufficiently great to break away from the attraction of their fellows.LIQUID DIFFUSION. When two liquids are mixed they may either (I) amalgarnatc completely, as in the case of gases, or (2) form two distinct layers or phases. The latter possibility is in itself sufficient to indicate that the conditions prevailing in a liquid are very different from those which exist in gases, and suggests that considerable caution is necessary in applying to liquids laws that have been deduced from experiments made with gases. In liquids of the former class the motion of the particles again leads to a process of diffusion, though this proceeds much more slowly than in gases, owing to the great reduction in the length of the mean free path of the particles.Ultimately, however, a condition of equilibrium is reached in which, almost invariably, the constituents are uniformly distributed through- out the mass? In the second case the mutual attractions of like particles are so much greater than those of the unlike particles that only a few particles of each kind are able to break away from their fellows and mingle with the particles of the other phase or layer. The conditions of equilibrium are then very similar to those which prevail between liquid and vapour, the chief difference being that there is an interchange of two kinds of particle instead of one. In this case also the motion of the particles leads to a uniform distribution of the constituents in each phase, though the relative concentrations in the two phases may differ widely. In solids the motion of the particles is mainly oscillatory, and although a tendency to diffuse may exist, the actual process of diffusion is in the majority of cases too slow to be detected.OSMOTIC PRESSURE IN LIQUIDS. The application of the equation PV = RT to the osmotic pressure of gases could be predicted on general theoretical grounds, but the conditions pre- vailing in liquids are so much more complex, that there is no h priori reason for supposing that the osmotic pressure developed by a solution would be governed by the same law, or would bear any relationship to the gas pressure produced by a gas of equal molecular concentration. The idea that the osmotic pressure of a dilute solution can be calculated from the same * If the solvent and solution are unequally compressible, varialions of pressure would produce a corresponding gradient of concentration.This, however is usually negligible.16 OSMOTIC PRESSURE FROM THE STANDPOINT formula that is used to predict the pressure exerted by a gas has been arrived at, not by intuition, but by experiment. Owing to the difficulty of discovering efficient semi-permeable membranes, accurate measurements of osmotic pressure have been confined almost exclusively to aqueous solutions of the sugars separated from the pure solvent by a membrane of copper ferro- cyanide, The experiments usually quoted are those of Pfeffer (Osmotische Untersuchurzgen, Leipzig, 1877), but the recent accurate measurements of Morse and Fraser have entirely confirmed the conclusions already arrived at from the earlier experiments, since they have shown that within the limits of M/IO to M the actual osmotic pressure produced by a cane-sugar solution agrees with that calculated from the gas-formula with an average deviation of less than 4 per cent.Indirect methods of measuring osmotic pressure by lowering of vapour pressure, elevation of boiling-point, and depression of freezing-point, have extended the observations to a very wide range of substances, and in some cases a very high degree of accuracy has been attained. In particular, Griffiths has shown that the osmotic pressure of a cane-sugar solution deduced from measurements of the freezing-point agrees with that calculated from the gas-formula with an error not exceeding 0.01 per cent.It is, however, important to notice that the formula A = -- , by which the molecular eleva- tion of the boiling-point or depression of the freezing-point can be derived from the absolute temperature T of boiling or freezing, and the latent heat q involved in the change of state is based upon the assumptions ( I ) that the change of temperature is determined by the work done against the osmotic pressure when the solution is concentrated by evaporating ,or freezing out the solvent, and (2) that this osmotic pressure can be calculated from the gas- formula ; it follows, therefore, that the validity of the gas-laws as applied to solutions is proved, not merely by a few direct measurements of the osmotic pressure of sugar solutions, iior even by isolated experiments such as those of Griffiths, but also by the countless experiments in which the Beckmann apparatus has been successfully employed in determining the molecular weights of dissolved substances.The proof of the numerical identity of osmotic pressure with gas pressure has been discussed somewhat fully because it appears to be of fundamental importance, and has been SD fully verified by experiment that it must be accepted as a fact in any consideration of the origin and mechanism of osmotic pressure. RTa Q THE MECHANISM OF OSMOTIC PRESSURE. There can at the present time be no doubt that osmotic pressure de- pends essentially on the phenomena of selective solubility. The palladium membrane acts in virtue of its ability to absorb or dissolve hydrogen, but not nitrogen ; the amount of hydrogen absorbed depends on the pressure, and equilibrium is attained when the partial pressure of the hydrogen inside the vessel is equal to its total pressure outside.A water-membrane has been used by Nernst to develop an osmotic pressure between ether and an ethereal solution of benzene, the former being soluble and the latter insoluble in water. Copper ferrocyanide, which absorbs or dissolves water and certain salts blit not sugars, forms an efficient semi-permeable membrane for aqueous sugar solutions, but not for salt solutions. The presence of the sugar diminishes the solubility of the water in the membrane, and a flow of liquid is set up because the membrane when saturated with regard to the water on one side is supersaturated with regard to the solution on the other side.OF THE KINETIC THEORY The amount of water taken up by the membrane can, however, be increased by compressing the liquid, and it is thus possible to counterbalance the decrease of solubility due to the sugar, and by equalising the solubility on the two sides of the membrane to stop the flow of solvent into the solution.The pressure required to equalise the solubilities and stop the flow is the so-called (‘ osmotic pressure,” and it may again be urged that the conditions are so far different from those prevailing in a gas, that the gas-analogy, though s u s e s - tive, would be insufficient to justify the application of the gas laws to osmotic pressure unless these could be verified experimentally or could be established on an independent theoretical basis.Pickering’s theory (Ber., 1891, 24, 3639), that the action of the semi- permeable membrane depends on the relative size of the molecules of solvent and solute, has not been confirmed ; the very similar mechanical theory of Sutherland (Phil. Mag., 1897 (v) 4, 493-498), that the membrane consists of “ meshes ” through which water but not sugar can pass, appears to be equally untenable, and need not be discussed here; Traube’s theory is referred to later. VAN’T HOFF’S GAS THEORY OF OSMOTIC PRESSURE. When the discovery was made that osmotic pressure obeyed the same laws as gas pressure it was generally assumed that the two phenomena must be essentially similar in character. Thus van’t Hoff, in his classical paper, (‘ Die Rolle des osmotischen Druckes in der Analogie zwischen Losungen und Gasen” (2eit.phys. Chem, 1887, I.481-508), whilst recognising in general terms the attrac- tive influence of the solution for the solvent (die wasseranziekende Wirkurzg der Liisultg), attributed the osmotic pressure to the gas-like bombardment of the membrane by the molecules of the solute, the solvent being regarded as practically inert.* Three years later, at the Leeds meeting of the British Association (Report, 1890, p. 356) he suggested that “the action on a semi-permeable diaphragm is due partly to the shock of the dissolved molecules, partly to the difference of forces acting upon them from the solvent on one side and from the solution on the other,” but added that ‘‘ in very dilute solutions the shock is alone the origin of pressure, as it is in gases.” In support of this view he also quoted (Zeif.jhys. Chem., 1890,5, 174-176) the case of gaseous osmotic pressure suggested by Arrhenius and subsequently verified by Ramsay as described above. van’t Hoff’s theory had at least the merit of giving a simple and quantita- tive explanation of the osmotic phenomena, and was not unreasonable when applied to the osmotic pressure of gases dissolved in liquids. The concep- tion was, however, not easy to apply to the more ordinary cases of osmotic pressure, such as those afforded by aqueous sugar solutions. In the majority of cases the solute molecules appear to have a very stiiall mobility, and in the absence of the solvent they are unable to produce any hydrostatic pressure whatever upon the walls of the containing vessel.I t is, therefore, scarcely reasonable to attribute the whole action of the solution to the relatively inert solute, whilst neglecting entirely the very active part played by the solvent (Fitzgerald, B. A. Report, 18g0, p. 327). POYNTI NG’S ‘r H EORY. Much interest was aroused by the publication in 1896 of a paper by Professor Poynting (Phil. Mag. (v) 42, 289-~oo), in which an alternative * Es handelt sich im ersten Falle um die Stosse der Gas-molekiile an die Gefasswand, im letzteren um diejenigen der Molekiile vom gelosten Korper an die halbdurchl5ssige Membran, da ja die des beiderseitig anwesenden Losungsmittels als hindurchgehend, nicht in Betracht kommen (ibid., p. 482).18 OSMOTIC PRESSURE FROM THE STANDPOINT explanation was given of the osmotic pressure of liquids.In many respects the view he advocated is very similar to that described below, but the two theories differ in one essential point, and consequently lead to widely different explanations of the origin of osmotic pressure. In particular, Poynting was led to assume that osmotic pressure was due to the formation of labile hydrates, and it became necessary, as Whetham pointed out (Nature, Oct. 15, I&$), to assume that all substances which gave a normal osmotic pressure were monohydrated, whilst the double osmotic pressure of binary salts might be ascribed to the formation of a dihydrate or of two monohydrated ions." The invariable formation of loose monohydrates was so improbable, and is so far in contradiction to recent work on the hydrates present in solution, that Poynting's theory has failed to secure general acceptance as an explanation of the osmotic phenomena. OSMOTIC PRESSURE AS A KINETIC PHENOMENON.The theory of osmotic pressure now described formed the subject of a paper read before the Chemical Society of the Central Technical College as long ago as May, 1896, but it was only recently that it was recognised as being sufficiently novel to warrant further publicity. The starting-point of the theory is a consideration of the conditions prevailing at the surface of separation of the solution and the semi-permeable membrane, which may be either a layer of copper ferrocyanide or merely the boundary between liquid and vapour or liquid and ice.The simplest of these cases is undoubtedly that which involves the equilibrium between liquid and vapour. In this case the kinetic theory postulates a continual process of evaporation, whereby rapidly moving particles are constantly escaping from the surface of the liquid into the vapour space. This is balanced by the con- densation of practicatly all the molecules of the vapour that impinge on the liquid surface. When the vapour reaches a certain concentration the rate of condensation becomes equal to the rate of evaporation, and a condition of equilibrium is attained, not because evaporation has ceased, but because it is neutralised by an equal and opposite process of condensation. In the case of a non-volatile liquid the mobility of the relatively heavy molecules or mole- cular complexes is so small that very few are able to escape, and the maximum vapour pressure of such liquids is inappreciable.If now a solution be prepared by mixing a volatile and a non-volatile liquid, or by dissolving a non-volatile solid in a volatile liquid, the surface will contain both kinds of molecules. If one of the solvent molecules be struck by a rapidly moving molecule from the interior of the liquid, it will be projected into the vapour space. If, however, one of the non-volatile molecules be struck, it will be unable to escape, and the solvent particle will rebound in much the same way as if it had struck the wall of the containing vessel. The rate of evaporation is therefore reduced by the addition to the solvent of a non-volatile solute.On the other hand, it is probable that the presertce of the raora-volatile molecules would not interfere with the rate of coltdensation of the vapour. This point is of fundamental importance, as the opposite view was advocated by Poynting, who supposed that con- densation and evaporation would be checked to an equal extent, just as if the surface had been covered by a plate of perforated zinc. It must be remem- bered, however, that a considerable upward velocity is required before a molecule can escape from the surface of a liquid, and that a molecule descending with ever. the smallest downward velocity would have little * -4 similar deduction was drawn by I. Traube ( A m . Pltys. Cltena., 1897, ii. 62, 490-506).OF THE KINETIC THEORY 19 chance of escaping when once it came within the range of attraction of the liquid.Even if, on reaching the liquid surface, a vapour molecule should strike against a non-volatile molecule of the solute the attraction of the neighbouring molecules of the solvent would be sufficient to hold it, and thus ensure its condensation. I t need scarcely be pointed out that similar conditions would prevail at the surface separating liquid and ice or at the surface of one of the more conventional semi-permeable membranes. In the former case the presence of a non-isomorphous solute might prevent the adhesion to the ice of a molecule of solvent moving towards it but separated from it by a molecule of solute. On the other hand, it would not prevent the melting off or dissolution of an ice molecule if the average kinetic energy (k, temperature) of the ice were raised by the latent heat of crystallisation of other molecules passing from thc liquid to the solid state.In the case of a membrane such as copper ferrocyanide the solute would check the escape of solvent mole- cules from the solution into the membrane, but would not oppose the return of wanderers migrating from the membrane back into the solution. This would lead to a disturbance of the equilibrium between the solvent and the liquids on either side, and would be sufficient to produce an osmotic flow and a consequent osmotic pressure. DEDUCTION OF THE GAS-FORRIULX FROM THE KINETIC THEORY OF OSMOTIC PRESSURE. Of the theories of osmotic pressure that have hitherto been put forward, that of van’t Hoff is the one that leads most directly to a simple quantitative explanation of the phenomena.Poynting, in developing his theory quantita- tively, considered that solute molecules and stable compounds of solvent and solute would merely alter the effective surface of the liquid without changing the relative rates of evaporation and condensation ; only in the case of labile compounds was it considered possible that the rate of evaporation might be checked without altering the rate of condensation ; on this basis a quantitative explanation was only possible on the impracticable assumption that all ordinary solutes are loosely monohydrated in aqueous solutions. The more recent theory that osmotic pressure depends on a disturbance in the equilibrium between simple and polymerised solvent molecules, e.g., H,O,H,,O, (Armstrong, Proc. Roy.Soc., 1906, A. 78, &4-271), has also, as yet, failed to yield a quantitative interpretation of the pressures produced. The quantitative interpretation of the kinetic theory of osmotic pressure follows at once from Nernst’s proof of the relationship between osmotic pressure and vapour pressure (TIzeoreiicaZ Clzemisfry, pp. ~ q - ~ z g ) . Thus if N be the number of gram-molecules of solvent, and n the number of gram- molecules of solute in unit volume of the solution the validity of the gas- formula for osmotic pressure is readily deducible from the equation- N A’ N + n-p’ where p is the vapour pressure of the solvent, and p’ that of the solution.* If the simple assuniption is made that the molecules of solvent are uniformly distributed in the surface layer, and that the spacing or packing is the same as in the pure solvent, it follows a t once that the rates of evaporation from unit surface of solvent and solution will be in the ratio N + n to N, and that this will also be the ratio of the vapour pressures, as required by the above * The value of N is determined by the molecular weight of the solvent as it exists in the vapour, and not by its molecular weight in the liquid state.20 OSMOTIC PRESSURE FROM THE STANDPOINT formula.The argument is, of course, identical with that used by Poynting to show that each molecule of solute must destroy the mobility of a molecule of solvent, but, whereas he was led to assume the regular formation of labile monohydrates, the theory given above merely postulates that the mobility of a solvent molecule is destroyed when its place in the surface of the liquid is occupied by a molecule of solute.SURFACE STRUCTURE OF LIQUIDS. It will at once be noticed that in its simplest form the kinetic theory of osmotic pressure would indicate .that the pressures calculated from the gas- formula might be subject to a small correction for the volume changes accompanying dissolution. Whether such corrections are necessary can only be determined by experiment, but the evidence now available points to a very close agreement between the values observed and calculated for dilute solutions. If this identity should be confirmed, it will be possible to deduce from observations of osmotic pressure some information in reference to the surface structure of liquids, since if the agreement is exact there must be an equally exact replacement of solvent by solute in the surface of the liquid.Thus in view of the different iiiasses of the molecules that may be dis- solved in the same solute and yield identical osmotic pressures, there must be a considerable spacing between the actual molecules in the surface. Again, it may be noted that this exact replacement does not take place in the interior of the liquid, where the molecular volumes of different solvents differ widely, and when calculated in the conventional way may even have a negative value. It is, however, by no means improbable that the marshalling of the molecules 011 the frontiers of the liquid may be governed by a stricter discipline than that which prevails in the interior, and that the surface molecules may even be forced to conform to the exact regulations which govern the replacement of molecules in solid solutions or isomorphous mixtures. FORhfATION OF COMPLEXES.The formation in the solution of loose complexes of solvent with solute or of solvent molecules with one another has not yet been referred to, but presents no difficulty in the development of the theory. Such complexes are usually formed without any large change of volume, and under the stricter condi- tions prevailing at the surface no alteration in the area of surface occupied need result from the linking up of the ‘‘ residual affinities ” of the molecules.Neither need it be supposed that the rate of evaporation would be affected otherwise than by a general reduction of mobility::: due to the chemical attraction of the molecules and producing equal effects in solvent and in dilute solution. Thus, as there is 110 change of energy involved in the replace- ment of one solvent molecule in a complex by another from outside, a rapidly moving particle impinging on a complex might drive a combined solvent moleculc into the vapour space and itself occupy the vacant position in the complex, the effects produced being much the same as if no complex existed. A similar statement would apply to molecules of solvent attached to the solute in the form of labile hydrates or compounds; a free solvent molecule impinging on a combined molecule in the surface of the liquid might drive it out and take its place, but if it should impinge on the nuclear solute molecule it would be repulsed and driven back into the interior, just as it would be by an uncombined molecule of solute. cohesion in a gas as represented by the quantity * Compare the reduction of mobility caused by liquid in van der Waals’ equation, 2jaOF T H E KINETIC THEORY 21 CONCENTRATED SOLUTIONS. No attempt has been made i n the above to account quantitatively for the osmotic pressure of concentrated solutions. Even in the case of gases, the equation PV = RT only applies strictly to a material gas within narrow limits of pressure, but Morse and Fraser’s experiments indicate a much wider applicability when the formula is applied to osmotic pressures, provided only that for concentrated solutions V is interpreted as the volume of solvent used to dissolve the solute and not the total volume of the solution. It need scarcely be pointed out that this modification is very similar in type to the co-volume correction in van der WBals’ equation. OSMOTIC PRESSURE AND SURFACE TENSION. In view of the close relationship that has been indicated between osmotic pressure and surface structure it would not be surprising that a relation- ship should exist between surface tension and osmotic pressure. Such a relationship has been postulated theoretically by Traube, who supposes that osmotic pressure depends on a tendency to equalise the surface tension of the two liquids, and has been confirmed experimentally by Batelli and Stephanini (Atli. R. Acad. Lincei, 1905, (v), 14, ii. pp. 3-14), who find that solutions of equal surface tension have equal osmotic pressures, even in cases in which the solutions are not nominally equimolecular. In conclusion it may be pointed out that whilst the vicws advocated above were comparatively novel ten years ago, the idea that osmotic pressure depends on the activity of the solvent rather than on that of the solute has now become widely accepted, and has been advocated, not only by Poynting, but also by Armstrong ( E m . Brit., 26. 739), Beilby, (B. A. Report, South Africa, 1905, 361), and others. At the present time, therefore, the only points for which any degree of novelty can be claimed are in reference to the mechanism by which the activity of the solvent at the surface of the liquid is reduced by the ‘‘ blocking action ” of the solute operating in one direction only, and to the possibility of deducing from the osmotic phenomena information as to the surface structure of liquids. 130, HORSEFERRY ROAD, WESTMINSTER, S.W.

ISSN:0014-7672    

DOI:10.1039/TF9070300014

出版商:RSC    

年代:1907

数据来源: RSC

摘要:

THE BEARING OF ACTUAL OSMOTIC EXPERIMENTS UPON THE CONCEPTION OF THE NATURE OF SOLUTIONS. Dr. Louis Wenberg, Professor of Physical Chemistry in the University of Wisconsin, communicated a Paper on “ The Bearing of Actual Osmotic Experiments upon the Conception of the Nature of Solutions.” Osmosis was discovered in 1748 by Nollet, and since that date a large number of physiologists, physicists, and chemists have carried on experi- mental investigations with a view to discover the nature and laws of the osmotic process. In all actual osmotic experiments with which we are here concerned there are present three phases, namely, two liquids and a septum which separates these two liquids from each other. Frequently these liquids have consisted of a solution on the one hand and pure solvent on the other, Experimental studies have been directed to ascertain what substances pass through given septa and at what rate.The influence of temperature upon the process has received consideration, and attempts have been made to determine the maximum pressure which is produced by the tendency of one of the liquids to increase in bulk during the experiments because of the influx of material from the other side of the septum. This pressure so developed has commonly been known as the osmotic pressure, As a result of all this experimental inquiry it has been conclusively demonstrated that whether osmosis will actually take place or not depends upon the nature of the septum employed and upon the nature of the liquids that bathe’ it. Furthermore, if osmosis does occur, these factors also deter- mine the direction and extent of the process. Temperature is also a factor in osmosis, but it is of very minor consequence as compared with that of the nature of the septum and the liquids employed. Again, it has developed from these experiments that osmosis always goes on in both directions, and that the rate of flow OT material through the septum in the two directions is practically never equal ; in other words, there is always a major and a minor current present in each experiment.In some special cases it has been found that this minor current is almost insignificant as compared with the major current, so that it appears as though the osmotic process were one-sided in these instances. In such cases the septum has been termed semi-permeable, and of late years interest has centred to a considerable extent upon the experimental study of osmosis in cases where such so-called semi-permeable membranes were employed.As a natural consequence “ semi-permeable ” membranes were much sought for, and in the course of this search the fact clearly developed that there really is no such thing as a semi-permeable membrane in the strict sense of the word. As already indicated, in certain specific cases-and these have been found to be relatively few in number- the major osmotic current so far outweighs the minor that an approximation to a one-sided process is reached. Until recently precipitated membranes, and among these in particular copper f errocyanide membranes, have been practically the only ones which have been found to be semi-permeable, and 22THE CONCEPTION OF T H E NATURE OF SOLUTIONS 23 that only for few substances, amongst which cane sugar is by far the most prominent.Precipitated membranes were first used by Moritz Traube in 1865, and were again prominent in the researches of Pfeffer in 1877. These botanists clearly recognised the importance of the experimental study of the peculiar osmotic properties of these precipitated membranes as bearing upon physiological processes in which frequently membranes with similar strong selective action come into play. Recently I have shown (Traizs. Wis. Acad., vol. 15 ; also Joum. Phys. Chem., March, I@) that vulcanised caoutchouc, though not a precipitated membrane, is semi-permeable in certain specific cases, among which is that of cane sugar when dissolvedin pyridine.In that Paper I have demonstrated experimentally that it is peculiarly strong selective action on the part of the septum which causes it to be approximately semi- permeable in certain instances. All the results of previous investigators also go to support this view, which is consequently incontrovertible. Indeed, from a knowledge of the peculiar selective property of, for instance, vul- canised caoutchouc, I have been able to predict whether a certain substance would pass through that septum or not ; in other words, I have been able to foretell whether in given cases the membrane would be practically semi- permeable or not. As a result of this I have actually been able to separate certain crystalloids from each other by dialysis, certain colloids from crystal- loids by having the colloid pass through the septum and the crystalloid remain behind ; and finally, I have just recently even succeeded in separating two colloidal substances from each other by dialysis.A description of the latter case has not yet been published, and I purposely refrain from giving the experimental details here, for it would seem best not to divert attention from the main issue at stake. Now this peculiar selective action exhibited by the membrane is simply due to solubility or insolubility of the substance or substances in question in that membrane. This I have demonstrated experimentally (EL), and thc work of earlier experimenters all goes to support this claim, Thus, in order that a substance may pass through a given septum it is necessary (I) that the substance be dissolved in, or, as some would choose to put it, that it be taken up or imbibed by the iiiembrane ; (2) that the liquid on the other side of the septum have the power to dissolve the substance with such a degree of avidity as to enable it to rob the septum of some of the substance.These same forces which cause a substance to pass through a septum-that is to say, which cause the liquid on one side of the septum to increase-clearly produce the pressure, the so-called osmotic pressure, which is manifested when such increase in bulk is hindered by mechanical contrivances. It is clear, therefore, that the production of osmotic pressure is due to the same forces that cause solution to take place, and that when we have an explana- tion of the process of solution we also shall have an explanation of the osmotic process and the production of osmotic pressure.Obviously osmosis is a more complicated process than simple solution, for in osmosis three phases are striving to get into equilibrium with one another, and that too against an external pressure when mechanical contrivances are used to prevent the process from going on, as when it is sought to measure osmotic pressures. From this consideration it would follow that we must not look to osmotic experiments to obtain a better conception of the nature of solutions and the process by which they are formed ; but we should rather expect a clearer understanding of osmosis to come from a study of the simpler process of solution through other channels.By regarding an osmotic membrane simply as a sieve with meshes of definite size, it has at times been hoped by some that the size of molecules24 BEARING OF ACTUAL OSMOTIC EXPERIMENTS UPON might be ascertained. However, experiments have conclusively shown that molecules which must be considered to be larger frequently go through a membrane very much more readily than molecules which we are bound to consider as smaller. The sieve theory is consequently untenable. To my mind an experimental study of the simple process of solution leads to the conclusion that it is essentially chemical in character. It is unnecessary to rehearse here the specific reasons for this view, for they have already been expressed in print (compare Phil.Mag., Feb., 1905 ; also Chemiker Zeitung, 29, No. 81, 1905). My view, therefore, is that osmosis is due to essentially the same specific attractions that are commonly termed chemical affinity, and that the osmotic pressure is also produced by the same force. Osmotic pressures may consequently be quite variable as different septa and different liquids are employed ; furthermore, such pressures may frequently be very great indeed, and this accords with what has been found experimentally. By thus cognising that the act of solution is chemical in character, and that osmosis and osmotic pressure are caused by the same agency as that which causes changes called chemical by common consent, we have, of course, not explained what chemical affinity itself is, though we have clearly made a definite gain by recognising an important relation which experiment indicates exists.Personally I incline to the opinion that once solutions are again commonly looked upon as chemical combinations between solvent and solute, as at one time they practically always were by those who busied themselves experimentally with solutions in various solvents, we shall by application of our improved ,experimental means be in a better position to acquire a thorough knowledge of the laws regulating chemical change, even though we may not hope thus to unravel the mystery of what chemical affinity in itself is. If now we inquire as to the reason why so-called semi-permeable mem- branes have of recent years been chiefly employed in experimental osmotic work, it appears that this is because of the bearing which such work has upon the theory of solution of Van’t Hoff.The latter, using Pfeffer’s data of the osmotic pressures developed when aqueous solutions of sugar are separated from water by means of septa of copper ferrocyanide, pointed out that the results are approximately such as may be computed by use of the simple gas equation. Here, then, seemed to be experimental evidence for putting into quantitative form the long-recognised fact that between a substance in the gaseous state and one in solution there is a certain degree of analogy. It was soon promulgated that just as gaseous pressure is caused by the bom- bardment of the molecules against the sides of the containing vessel, so osmotic pressure is caused by the bombardment of the semi-permeable membrane by the dissolved molecules, a view which, though quite irrecon- cilable with experimental facts, is yet even at present taught in prominent texts.I t has boldly been maintained that as long as the membrane is ‘( semi-permeable ” the osmotic pressure is necessarily the same in the case of any septum for a given solution against the pure solvent. This is contrary to experience. Even an inspection of Pfeffer’s data secured by separating aqueous sugar solutions from water by means of septa of Prussian blue and calcium phosphate instead of copper ferrocyanide indicate the specific influence of the septum which my own experimental researches further demonstrate conclusivcly. By conceiving solvent abstracted from a dilute solution contained in a cylinder fitted with a semi-permeable piston and afterwards returning this solvent to the solution in a reversible Carnot’s cycle, relations have been deduced between the latent heat of fusion of the solvent and the freezingTHE CONCEPTION OF THE NATURE OF SOLUTIONS 25 point of the solution, also between the boiling-point of the solution and the latent heat of vaporisation of the solvent.In deducing these relations the usual thermodynamic formulae have been applied, and the assumption has been made that the gas equation holds for dilute solutions. In thus com- puting the so-called molecular lowering of the freezing-point and molecular elevation of the boiling-point for various solvents from a knowledge of the freezing-point and latent heat of fusion, and of the boiling-point and latent heat of vaporisation of the solvent, results are obtained which are frequently far from the experimental values found for different solutes in case of one and the same solvent.But by assuming that the solutes are either dissociated or polymerised or united with a certain amount of solvent, or that any two or all three of these processes take place simultaneously, the experimental data are readily explained, and it is fondly imagined that thus we get a better insight into the nature of solutions. To justify such assumptions of dissocia- tion and polymerisation it has been pointed out that this is exactly parallel with what takes place in the case of gases, and reference is made to the classical experimental researches of Saint Claire Deville.As frequently aqueous solutions that conduct eIectricity yield greater molecular lowerings of the freezing-point and greater molecular elevations of the boiling-point than is required by the formulae deduced as above stated, it is claimed that the solutes are in such solutions (( electrolytically ” dissociated. Finally, the attempt is made to explain all the various physical, chemical, and physio- logical properties on the basis of these various assumptions. In a previous communication to the Faraday Society I have already discussed the weak- nesses of such explanations, so that it is unnecessary to dwell upon them at present. I should like to add, however, that it has recently been demon- strated in my laboratory (compare Sammis, gourn.Plzys. Chem., Nov., 1906) that lead will replace copper from copper oleate in hydrocarbon solutions, which are the best of insulators. With this discovery it may now be asserted that, as to type, all chemical reactions may be produced in the best of insula- tors just as they take place in excellent electrolytes. Furthermore, we have shown conclusively by experiment that in such typical cases as sugar inver- sion, ester catalysis, and solution of metals in acids, the conductivity of the solution may be materially altered by the introduction of such substances as benzene and acetone without causing anything like a proportional change in the speed of the reaction. In fact, it has been shown that the electrical conductivity and the rate of reaction may be varied practically independently of each other.The different molecular lowerings of the freezing-point and elevations of the boiling-point due to the introduction of various substances into one and the same solvent are very simply explained by the assumption that they result from different degrees of affinity or attraction between solvent and solute (compare Chemiker Zeituizg, Z.C.). From the freezing-points, boiling-points, and vapour-tensions of solutions their so-called osmotic pressures have conversely been computed by using formulae established with the 3id of thermodynamic reasoning. In many cases, too, the so-called osmotic pressure has been directly calculated from the concentration of the solution by simple application of the gas laws, the (( degree of electrolytic dissociation ” being strenuously considered in the case of an electrolyte.This practice has been termed the (‘indirect” method of determining osmotic pressures, Now, all thermodyitaiitic deductions OJ, relalions between osmotic pressure opt ihe one hand, and vapour tensions, freezing-points, or boiling-points on the other hand, involve the assumption of a membrane which is semi-permeable, and which at the same time is quite passive; that is to say, which has ?to selective action. VOL. III-T226 OSMOTIC EXPERIMENTS However, as stated above, all experimental evidence we have goes to demonstrate (I) that there is no such thing as a semi-permeable membrane in the strict sense of the word, and (2) the more nearly a membrane is semi-permeable in character in practice, the grcater is its selective action ; in other words, it is the Pronounced selective action of the membrane wlzich makes it approximately semi-permeable.I t is clear, therefore, lhut such a membrane as thermodynamic reasoning postu- lates can never be realised in practice, nor can we hope by experiment to produce even approximately such a membrane ; for as we experimentally approximate toward fuljilling the first requirement, semi-permeability, we do this at the sacrifice of the second requirement, passivity. The semi-permeable membrane of the thermodynamicist is therefore a thing which has no counterpart in actual osmotic experiments, not even approximately. Consequently the semi-per- meable membrane and the osmotic pressure of the thermodynamicist are quite different things from the actual osmotic septa and osmotic pressures of experimental practice. A clear recognition of this will do much to clear up existing confusion and misunderstanding.Mr. Whetham’s recent admission (Nature, July 26, 1906) that actual experimental osmotic pressures and SO- called thermodynamic osmotic pressures are quite different things goes far toward arriving at an understanding. We need also to grasp clearly the fact that actual membranes as used in osmotic experiments are quite different from the septa postulated in the thermodynamic deductions. The reasons for this have already been given above. When summed up, then, the situation is as follows: Actual osmotic experiments have suggested that the concentration of a solution might be imagined to be changed reversibly by the application of a hypothetical membrane, which is simultaneously semi-permeable and devoid of specific selective action.By imagining a reversible cyclic process to take place in which the concentration of the solution is altered by the hypothetical means mentioned, relations between the freezing-point and vapour tensions of solutions on the one hand, and the theoretical equilibrium pressure required to be put on the hypothetical semi-permeable piston on the other hand, have been deduced. For this hypothetical equilibrium pressure Mr. Whetham (Z.C.) proposes the name “ thermodynamic osmotic pressure ” in contradis- tinction to the osmotic pressure developed in actual osmotic experiments, which he suggests should be termed ‘‘ experimental osmotic pressure.” I fear, however, that as long as the term “osmotic” is in any way attached to this hypothetic thermodynamic quantity, there will be more or less confusion. It is to be hoped, therefore, that another term will be invented for the so- called thermodynamic osmotic pressure, which does not involve the use of the term ‘‘ osmotic.” Further, the semi-permeable membrane assumed in thermodynamic considerations ought to be called a thermodynamic semi- permeable membrane, so as to distinguish it from actual experimental osmotic membranes which are approximately semi-permeable because of their selcctive properties.In conclusion, may I be permitted once more to emphasise the fact that to speak of the experimental osmotic pressure of any solution-the only osmotic pressure which has any counterpart in reality- without mentioning specifically just what septum is used to separate that solution from what other liquid is quite devoid of all meaning.LABORATORY OF PHYSICAL CHEMISTRY, MADISON, January 14, 1907. UNIVERSITY OF WISCONSIN, THE BEARING OF ACTUAL OSMOTIC EXPERIMENTS UPON THE CONCEPTION OF THE NATURE OF SOLUTIONS. Dr. Louis Wenberg, Professor of Physical Chemistry in the University of Wisconsin, communicated a Paper on “ The Bearing of Actual Osmotic Experiments upon the Conception of the Nature of Solutions.” Osmosis was discovered in 1748 by Nollet, and since that date a large number of physiologists, physicists, and chemists have carried on experi- mental investigations with a view to discover the nature and laws of the osmotic process.In all actual osmotic experiments with which we are here concerned there are present three phases, namely, two liquids and a septum which separates these two liquids from each other. Frequently these liquids have consisted of a solution on the one hand and pure solvent on the other, Experimental studies have been directed to ascertain what substances pass through given septa and at what rate. The influence of temperature upon the process has received consideration, and attempts have been made to determine the maximum pressure which is produced by the tendency of one of the liquids to increase in bulk during the experiments because of the influx of material from the other side of the septum. This pressure so developed has commonly been known as the osmotic pressure, As a result of all this experimental inquiry it has been conclusively demonstrated that whether osmosis will actually take place or not depends upon the nature of the septum employed and upon the nature of the liquids that bathe’ it.Furthermore, if osmosis does occur, these factors also deter- mine the direction and extent of the process. Temperature is also a factor in osmosis, but it is of very minor consequence as compared with that of the nature of the septum and the liquids employed. Again, it has developed from these experiments that osmosis always goes on in both directions, and that the rate of flow OT material through the septum in the two directions is practically never equal ; in other words, there is always a major and a minor current present in each experiment. In some special cases it has been found that this minor current is almost insignificant as compared with the major current, so that it appears as though the osmotic process were one-sided in these instances.In such cases the septum has been termed semi-permeable, and of late years interest has centred to a considerable extent upon the experimental study of osmosis in cases where such so-called semi-permeable membranes were employed. As a natural consequence “ semi-permeable ” membranes were much sought for, and in the course of this search the fact clearly developed that there really is no such thing as a semi-permeable membrane in the strict sense of the word.As already indicated, in certain specific cases-and these have been found to be relatively few in number- the major osmotic current so far outweighs the minor that an approximation to a one-sided process is reached. Until recently precipitated membranes, and among these in particular copper f errocyanide membranes, have been practically the only ones which have been found to be semi-permeable, and 22THE CONCEPTION OF T H E NATURE OF SOLUTIONS 23 that only for few substances, amongst which cane sugar is by far the most prominent. Precipitated membranes were first used by Moritz Traube in 1865, and were again prominent in the researches of Pfeffer in 1877. These botanists clearly recognised the importance of the experimental study of the peculiar osmotic properties of these precipitated membranes as bearing upon physiological processes in which frequently membranes with similar strong selective action come into play.Recently I have shown (Traizs. Wis. Acad., vol. 15 ; also Joum. Phys. Chem., March, I@) that vulcanised caoutchouc, though not a precipitated membrane, is semi-permeable in certain specific cases, among which is that of cane sugar when dissolvedin pyridine. In that Paper I have demonstrated experimentally that it is peculiarly strong selective action on the part of the septum which causes it to be approximately semi- permeable in certain instances. All the results of previous investigators also go to support this view, which is consequently incontrovertible. Indeed, from a knowledge of the peculiar selective property of, for instance, vul- canised caoutchouc, I have been able to predict whether a certain substance would pass through that septum or not ; in other words, I have been able to foretell whether in given cases the membrane would be practically semi- permeable or not.As a result of this I have actually been able to separate certain crystalloids from each other by dialysis, certain colloids from crystal- loids by having the colloid pass through the septum and the crystalloid remain behind ; and finally, I have just recently even succeeded in separating two colloidal substances from each other by dialysis. A description of the latter case has not yet been published, and I purposely refrain from giving the experimental details here, for it would seem best not to divert attention from the main issue at stake.Now this peculiar selective action exhibited by the membrane is simply due to solubility or insolubility of the substance or substances in question in that membrane. This I have demonstrated experimentally (EL), and thc work of earlier experimenters all goes to support this claim, Thus, in order that a substance may pass through a given septum it is necessary (I) that the substance be dissolved in, or, as some would choose to put it, that it be taken up or imbibed by the iiiembrane ; (2) that the liquid on the other side of the septum have the power to dissolve the substance with such a degree of avidity as to enable it to rob the septum of some of the substance. These same forces which cause a substance to pass through a septum-that is to say, which cause the liquid on one side of the septum to increase-clearly produce the pressure, the so-called osmotic pressure, which is manifested when such increase in bulk is hindered by mechanical contrivances.It is clear, therefore, that the production of osmotic pressure is due to the same forces that cause solution to take place, and that when we have an explana- tion of the process of solution we also shall have an explanation of the osmotic process and the production of osmotic pressure. Obviously osmosis is a more complicated process than simple solution, for in osmosis three phases are striving to get into equilibrium with one another, and that too against an external pressure when mechanical contrivances are used to prevent the process from going on, as when it is sought to measure osmotic pressures.From this consideration it would follow that we must not look to osmotic experiments to obtain a better conception of the nature of solutions and the process by which they are formed ; but we should rather expect a clearer understanding of osmosis to come from a study of the simpler process of solution through other channels. By regarding an osmotic membrane simply as a sieve with meshes of definite size, it has at times been hoped by some that the size of molecules24 BEARING OF ACTUAL OSMOTIC EXPERIMENTS UPON might be ascertained. However, experiments have conclusively shown that molecules which must be considered to be larger frequently go through a membrane very much more readily than molecules which we are bound to consider as smaller. The sieve theory is consequently untenable.To my mind an experimental study of the simple process of solution leads to the conclusion that it is essentially chemical in character. It is unnecessary to rehearse here the specific reasons for this view, for they have already been expressed in print (compare Phil. Mag., Feb., 1905 ; also Chemiker Zeitung, 29, No. 81, 1905). My view, therefore, is that osmosis is due to essentially the same specific attractions that are commonly termed chemical affinity, and that the osmotic pressure is also produced by the same force. Osmotic pressures may consequently be quite variable as different septa and different liquids are employed ; furthermore, such pressures may frequently be very great indeed, and this accords with what has been found experimentally.By thus cognising that the act of solution is chemical in character, and that osmosis and osmotic pressure are caused by the same agency as that which causes changes called chemical by common consent, we have, of course, not explained what chemical affinity itself is, though we have clearly made a definite gain by recognising an important relation which experiment indicates exists. Personally I incline to the opinion that once solutions are again commonly looked upon as chemical combinations between solvent and solute, as at one time they practically always were by those who busied themselves experimentally with solutions in various solvents, we shall by application of our improved ,experimental means be in a better position to acquire a thorough knowledge of the laws regulating chemical change, even though we may not hope thus to unravel the mystery of what chemical affinity in itself is.If now we inquire as to the reason why so-called semi-permeable mem- branes have of recent years been chiefly employed in experimental osmotic work, it appears that this is because of the bearing which such work has upon the theory of solution of Van’t Hoff. The latter, using Pfeffer’s data of the osmotic pressures developed when aqueous solutions of sugar are separated from water by means of septa of copper ferrocyanide, pointed out that the results are approximately such as may be computed by use of the simple gas equation. Here, then, seemed to be experimental evidence for putting into quantitative form the long-recognised fact that between a substance in the gaseous state and one in solution there is a certain degree of analogy.It was soon promulgated that just as gaseous pressure is caused by the bom- bardment of the molecules against the sides of the containing vessel, so osmotic pressure is caused by the bombardment of the semi-permeable membrane by the dissolved molecules, a view which, though quite irrecon- cilable with experimental facts, is yet even at present taught in prominent texts. I t has boldly been maintained that as long as the membrane is ‘( semi-permeable ” the osmotic pressure is necessarily the same in the case of any septum for a given solution against the pure solvent.This is contrary to experience. Even an inspection of Pfeffer’s data secured by separating aqueous sugar solutions from water by means of septa of Prussian blue and calcium phosphate instead of copper ferrocyanide indicate the specific influence of the septum which my own experimental researches further demonstrate conclusivcly. By conceiving solvent abstracted from a dilute solution contained in a cylinder fitted with a semi-permeable piston and afterwards returning this solvent to the solution in a reversible Carnot’s cycle, relations have been deduced between the latent heat of fusion of the solvent and the freezingTHE CONCEPTION OF THE NATURE OF SOLUTIONS 25 point of the solution, also between the boiling-point of the solution and the latent heat of vaporisation of the solvent. In deducing these relations the usual thermodynamic formulae have been applied, and the assumption has been made that the gas equation holds for dilute solutions.In thus com- puting the so-called molecular lowering of the freezing-point and molecular elevation of the boiling-point for various solvents from a knowledge of the freezing-point and latent heat of fusion, and of the boiling-point and latent heat of vaporisation of the solvent, results are obtained which are frequently far from the experimental values found for different solutes in case of one and the same solvent. But by assuming that the solutes are either dissociated or polymerised or united with a certain amount of solvent, or that any two or all three of these processes take place simultaneously, the experimental data are readily explained, and it is fondly imagined that thus we get a better insight into the nature of solutions.To justify such assumptions of dissocia- tion and polymerisation it has been pointed out that this is exactly parallel with what takes place in the case of gases, and reference is made to the classical experimental researches of Saint Claire Deville. As frequently aqueous solutions that conduct eIectricity yield greater molecular lowerings of the freezing-point and greater molecular elevations of the boiling-point than is required by the formulae deduced as above stated, it is claimed that the solutes are in such solutions (( electrolytically ” dissociated. Finally, the attempt is made to explain all the various physical, chemical, and physio- logical properties on the basis of these various assumptions.In a previous communication to the Faraday Society I have already discussed the weak- nesses of such explanations, so that it is unnecessary to dwell upon them at present. I should like to add, however, that it has recently been demon- strated in my laboratory (compare Sammis, gourn. Plzys. Chem., Nov., 1906) that lead will replace copper from copper oleate in hydrocarbon solutions, which are the best of insulators. With this discovery it may now be asserted that, as to type, all chemical reactions may be produced in the best of insula- tors just as they take place in excellent electrolytes. Furthermore, we have shown conclusively by experiment that in such typical cases as sugar inver- sion, ester catalysis, and solution of metals in acids, the conductivity of the solution may be materially altered by the introduction of such substances as benzene and acetone without causing anything like a proportional change in the speed of the reaction. In fact, it has been shown that the electrical conductivity and the rate of reaction may be varied practically independently of each other.The different molecular lowerings of the freezing-point and elevations of the boiling-point due to the introduction of various substances into one and the same solvent are very simply explained by the assumption that they result from different degrees of affinity or attraction between solvent and solute (compare Chemiker Zeituizg, Z.C.). From the freezing-points, boiling-points, and vapour-tensions of solutions their so-called osmotic pressures have conversely been computed by using formulae established with the 3id of thermodynamic reasoning.In many cases, too, the so-called osmotic pressure has been directly calculated from the concentration of the solution by simple application of the gas laws, the (( degree of electrolytic dissociation ” being strenuously considered in the case of an electrolyte. This practice has been termed the (‘indirect” method of determining osmotic pressures, Now, all thermodyitaiitic deductions OJ, relalions between osmotic pressure opt ihe one hand, and vapour tensions, freezing-points, or boiling-points on the other hand, involve the assumption of a membrane which is semi-permeable, and which at the same time is quite passive; that is to say, which has ?to selective action.VOL. III-T226 OSMOTIC EXPERIMENTS However, as stated above, all experimental evidence we have goes to demonstrate (I) that there is no such thing as a semi-permeable membrane in the strict sense of the word, and (2) the more nearly a membrane is semi-permeable in character in practice, the grcater is its selective action ; in other words, it is the Pronounced selective action of the membrane wlzich makes it approximately semi-permeable. I t is clear, therefore, lhut such a membrane as thermodynamic reasoning postu- lates can never be realised in practice, nor can we hope by experiment to produce even approximately such a membrane ; for as we experimentally approximate toward fuljilling the first requirement, semi-permeability, we do this at the sacrifice of the second requirement, passivity.The semi-permeable membrane of the thermodynamicist is therefore a thing which has no counterpart in actual osmotic experiments, not even approximately. Consequently the semi-per- meable membrane and the osmotic pressure of the thermodynamicist are quite different things from the actual osmotic septa and osmotic pressures of experimental practice. A clear recognition of this will do much to clear up existing confusion and misunderstanding. Mr. Whetham’s recent admission (Nature, July 26, 1906) that actual experimental osmotic pressures and SO- called thermodynamic osmotic pressures are quite different things goes far toward arriving at an understanding.We need also to grasp clearly the fact that actual membranes as used in osmotic experiments are quite different from the septa postulated in the thermodynamic deductions. The reasons for this have already been given above. When summed up, then, the situation is as follows: Actual osmotic experiments have suggested that the concentration of a solution might be imagined to be changed reversibly by the application of a hypothetical membrane, which is simultaneously semi-permeable and devoid of specific selective action. By imagining a reversible cyclic process to take place in which the concentration of the solution is altered by the hypothetical means mentioned, relations between the freezing-point and vapour tensions of solutions on the one hand, and the theoretical equilibrium pressure required to be put on the hypothetical semi-permeable piston on the other hand, have been deduced. For this hypothetical equilibrium pressure Mr. Whetham (Z.C.) proposes the name “ thermodynamic osmotic pressure ” in contradis- tinction to the osmotic pressure developed in actual osmotic experiments, which he suggests should be termed ‘‘ experimental osmotic pressure.” I fear, however, that as long as the term “osmotic” is in any way attached to this hypothetic thermodynamic quantity, there will be more or less confusion. It is to be hoped, therefore, that another term will be invented for the so- called thermodynamic osmotic pressure, which does not involve the use of the term ‘‘ osmotic.” Further, the semi-permeable membrane assumed in thermodynamic considerations ought to be called a thermodynamic semi- permeable membrane, so as to distinguish it from actual experimental osmotic membranes which are approximately semi-permeable because of their selcctive properties. In conclusion, may I be permitted once more to emphasise the fact that to speak of the experimental osmotic pressure of any solution-the only osmotic pressure which has any counterpart in reality- without mentioning specifically just what septum is used to separate that solution from what other liquid is quite devoid of all meaning. LABORATORY OF PHYSICAL CHEMISTRY, MADISON, January 14, 1907. UNIVERSITY OF WISCONSIN,

ISSN:0014-7672    

DOI:10.1039/TF9070300022

出版商:RSC    

年代:1907

数据来源: RSC

摘要:

DISCUSSION DISCUSS I0 N. Mr. H. N. Morse (communicated) : I thank the Faraday Society for the kind invitation to contribute towards the symposium on '' Osmotic Pressure " which has been arranged for January 29th. I send herewith four tables which present in the briefest form results which have been or are about to be published. Table I. contains a summary of the results which were obtained with glucose. The full account of this TABLE I.-SUMMARY OF RESULTS. Determination of the Osmotic Pressure of Glucose. Series I., 1906. 0'1 0'1 0'2 0'2 0.3 0 3 0.4 0.4 0.5 0'5 0.6 0.6 0-6 0.7 0.7 0.7 0.8 0.8 0.8 0.9 0'9 0'9 1'0 1'0 1'0 H = I I 2 I 2 I 2 I 2 I 2 I 2 I 2 I 2 I 2 I 2 3 3 3 3 3 - Degrees C. 24-10 25-10 24.10 2493 23'48 26-90 26.60 2 1-86 24.17 22-57 22.40 22-30 22-26 25'43 22-70 23-00 23.28 23.80 22.58 23.10 22-60 22'20 23'64 22'20 22'10 2-39 2-42 4'76 4'77 7-12 7.17 9-70 9-65 I 2.07 14'56 14-32 14.29 16.82 16-96 16.75 19-27 19.16 19-25 21-64 21' 9 2 1 -23 24'12 24.00 24-03 12'00 - 0.03 - 0'09 - 0'09 - 0.08 - 0.08 + 0.04 + 0'10 - 0.08 - 0'16 - 0'01 - 0'10 - 0'12 - 0'12 - 0'0 - 0.08 - 0-13 - 0.04 - 0.17 - 0.16 + 0.04 - 0.04 - 0'10 - 0'21 - 0'11 - 0'11 c ' z .zg g ??E 3;;; u 03% 9 2.E g ."c; 181*r 179' 3 181.77 1d270 181.38 180.73 180.21 181.02 178.12 180.57 177.62 180.42 1 9'55 179.16 I 80.39 179'73 '79'87 I 80.49 179;60 178046 179'60 179.07 180S88 1791 Ig0.16 Mol.Wt. Glucose = 178.74 Mean Mol. Wt. 180.08 work will appear in the April number of the American Chemical Journal. Tables 11. and 111. contain a similar statement of the results which have been obtained with cane sugar.Table IV. presents the results with both cane sugar and glucose in the form of ratios of observcd osmotic to calculated gas pressure at the same temperature, The conclusions which we deduce from the results presented in the tables are : (I) That, in thc vicinity of 20°, the osmotic pressure exerted by both cane sugar and glucose is equal to that which a molecular-equivalent28 OSMOTIC EXPERIMENTS quantity of a gas would exert if its volume were reduced, at the same temperature, to the volume of the solvent in the pure state ; (2) that, at the temperature at which the measurements were made ( i e , , between 18" and 26O, in round numbersj, both cane sugar and glucose in solution are in the anhydrous condition.It is to be remembered in this connection that our solutions are all made TABLE II.-SUMMARY OF RESULTS. Determination of the Osmotic Pressure of Cane Sugar. Series I., 1905. Am. Chem, Tourn., xxxiv. I, z . gs iY 2s & $ s o WE: 0'1 0'1 0'2 0'2 0.25 0.25 0.3 0'3 0.4 0'4 0'5 0.5 0.6 0.6 0.7 0'7 0.8 0.8 0.89101 0.9 1'0 1'0 1'0 H = I I 2 I 2 I 2 I 2 I 2 I 2 I 2 I 2 I 2 I I I 2 3 - Degrees C. I 8.85 18-71 20'59 20.9 I 23'45 21-46 19'28 20'94 20.84 20'32 21-38 24'45 20.14 24-62 17-89 19-56 19.84 ;g:$j 19'34 23-32 22'44 21'60 2'43 2.42 4'73 4'79 608 5'94 7.1 8 7.18 9'45 9'65 I 1-93 12.06 I 4.2 2 14'37 16.66 I 6.89 19-20 19-35 21-03 2 I -68 24-56 24.28 24-17 2.38 2.3 8 4'79 4-80 6-05 6-02 7'17 7'13 9'55 9.6 I 11-99 14-45 14'59 26-77 17-03 19.03 19-13 2 1-30 21-55 24.26 24-15 24'09 12'01 E! ;* g QJmg 2 3 z ;$$ $06 $ W E : -- + 0'05 + 0'05 - 0.06 + 0.03 - 0.08 + 0'01 + 0.05 + 0.04 - 0.08 + 0.07 - 0.23 - 0.14 + 0.17 + 0'22 - 0.27 + 0'13 + 0.30 + 0.13 + 0.08 - 0'01 - 0'10 - 0'22 - 0'11 3 3 4'47 334'08 344'50 340.03 338'05 344.60 339'44 337'65 343'21 338'54 341.92 337'85 345'09 344'89 342.18 342'41 336'46 335'86 343'90 337'55 335'50 337'95 338'54 3 34'28 342'27 341'33 338'55 339'89 340.88 344'99 342.30 336.16 343'90 337'55 337'33 ~- Final Mean of Mol. Wts, 339.95 Theoretical Mol.Wt. 339'60 up with a fixed mass of the solvent, and that, therefore, if the pressures are found to be strictly proportional to the supposed concentration, the dissolved material cannot have appropriated any portion of the solvent. It may be of interest to you to learn that we have also measured the osmotic pressure of cane sugar and glucose solutions at a temperature just above oo, and that, in the case of both substances, we find pressures somewhat in excess of the calculated gas pressures.An account of this later work will probably appear in the May and June numbers of the American Chemical Journal. We are just about ready to begin work with the electrolytes, havingDISCUSSION 29 perfected a cell which is expected to enable us to determine simultaneously osmotic pressure and electrolytic dissociation. JOHNS HOPKINS UNIVERSITY, 3'niauary 19, 1907. Dr. J. C. Philip, in connection with Dr. Lowry's statement that the validity of the equation PV = RT rested purely on an experimental basis, spoke of the results of applying thermodynamics to dilute solutions.If he was not mistaken, Dr. Larmor, in a paper published in Nafure in 1897, had reached the conclusion that, so long as a solution might be regarded as consisting of a certain number of solute nuclei, each surrounded by, and TABLE III.-SUBIMARY OF RESULTS. Deicriniitaiion of the Osritolic Pressure of Canc Sugar. Scries II., 1906. Atn. Claent. Journ., xxxvi. 39. 0'1 0'1 0'2 0'2 0'2 0'3 0.3 0.4 0'4 0'5 0.6 0.6 0.6 0'7 0'7 0.8 0.8 0'9 0.9 0'5 1'0 1'0 I 2 I 2 I 2 I 2 I 2 I 2 3 3 I 2 I 2 I 2 I 2 H = I & 0 Q ) . k r : 2 ti2 n c y H Dcgrees c. 24.06 2423 20'90 2 1.38 21.77 21-67 19-88 21-63 22':s 22'02 23'70 24-38 24'24 24-10 23-64 23'00 23-63 23-69 24'79 24'76 23'56 24-58 2-51 2'55 4'72 4.8 I 7'24 7-20 9'64 9'69 12.06 I 2.23 14'74 14-70 14'77 16-95 I 6-96 19-30 19'39 2 1.82 21-91 24'42 4'78 23'98 2.42 2'43 4'79 4.80 4.8 I 7.2 I 7.16 9.6 I 9-63 12.06 14'55 14.55 14-54 16-94 16.93 19'35 19-36 2 1-86 2 1.86 24' I c) 24-28 12'10 + 0.09 + 0'12 - 0.07 + 0.03 + 0.04 + 0.03 + 0.06 + 0.19 + 0'15 + 0'23 + 0'01 + 0.03 - 0.05 + 0.03 - 0.04 + 0'05 -k 0'23 - 0.30 - 0'02 0'00 0'00 + 0.13 327'85 322*j6 3 4 4 9 340'90 339'74 338'37 338.20 338'73 337'50 339'59 336.19 335'32 336.07 334'32 339'39 339'50 340'48 339'08 340.19 338'82 336'39 342'72 E: i .s "1 gs .!:z aa% 5 3 n j 0s aJ u 325.21 341.87 338'29 338.12 337'89 335'24 339'45 339'78 339'51 339'56 Final Mean of Mol.Wts. 337'49 Theoretical Mol. Wt. 339'60 possibly attached to, a large number of solvent molecules, the application of thermodynamics leads to the result that osmotic pressure must vary inversely as the volume and directly as the absolute temperature.This result is independent of any particular views which may be entertained as to the nature and origin of osmotic effects. Mr. John Rhodin remarked that he happened to be a studeiit of Upsala30 OSMOTIC EXPERIMENTS University when the great controversy was taking place between Arrhenius and Mendeleeff with regard to the theory of solutions, and ever since that time he had taken a great interest in the question. With regard to osmotic pressure, he thought it was one of the few things connected with the physical chemistry of the last ten or fifteen years which had a distinct meaning, and which one of only moderate understanding could grasp.The remarkable thing about osmotic pressure which struck him was that it moved in the wrong direction. One would expect the higher pressure to cause a motion TABLE IV. Ratio of Observed Osnaofic to Calculated Gas Pressure at the same Temperature, the Volume of the Gas being that of the Solveiat in the Pure Stale. Weight. Normal Concen- tration. - -__ 0'1 0'1 0'2 0'2 0'2 0'25 0'25 0'3 0.3 0.4 0'4 0.5 0'5 0.6 0.6 0.6 0.7 0'7 0'7 0.8 0.8 0-8 0.89101 0'9 0.9 0.9 1'0 1'0 1'0 Number 01 Experi- ment. I 2 I 2 3 I 2 I 2 I 2 I 2 I 2 3 I 2 I 2 3 3 I I 2 3 I 2 3 CAXE SUGAR. Series I. Tempera. ture. Degrees C 18.85 I 8.7 I 20'59 20'9 I 23'45 21-46 19.28 17-65 I 8.8 I 20'94 20.84 20-32 21-38 24'45 20.14 24-62 17-89 19-56 19'34 19-84 23-32 22-44 2 1-60 Ratio. I '02 I 1.017 0.987 0.998 I '005 0'987 1.007 0'990 I *004 0'993 I *006 1'001 0'984 0.984 0'993 0.992 I '009 1'012 0'987 1.006 1'012 1.005 I so03 Series 11. Tempera- ture.Degrees C 24.06 24'23 20.90 2 1-38 21'77 2 I -67 19.88 21-63 22.15 22-62 23'70 24-38 24.70 24-10 23'64 23-90 23.60 23-69 24'79 24-76 23'59 24-58 Ratio. 1'037 1'049 0.985 0.996 I '000 I '004 I ,006 I '003 1.006 I '000 1'010 1'010 1'000 I '002 1.013 1.016 0.997 I '002 0.998 1'002 1'010 0,988 GLUCOSE. Series I. Tempera- ture. Degrees C . 2 5'10 24-10 24-10 24'93 22'20 23-48 26-90 26.60 2 1-86 24.17 22'57 22.46 22-30 22-26 25'43 22.70 23-00 23-28 23-80 22.58 23-10 22-60 23 '64 22'20 22-10 Ratio. 0.988 0.98 I 0.98 I 0.996 0.986 0.992 0.988 1'003 0.990 I '007 0'994 0.988 0.998 0'995 0.992 0.998 0.991 0'995 0'903 0.990 0w5 0'995 0'998 0'989 1'002 in the direction of the lower pressure. There was one fact, however, which was not taken into consideration, namely, the incompressibility of liquids.To his mind the direction taken by the osmotic pressure, i.e., of the kinetic motion which took place, depended upon the volume of the system under consideration, which must be a constant. One might just as well say that, from the top of the side where the solution was, there was a kind ofDISCUSSION 31 weightless and frictionless piston which was being moved upwards, and that the resistance of the semi-permeable membrane against the influx of the solvent was nil. But even then the gripping point of the solution seemed to be rather remarkable, and hence he had found that nearly all writers on osmotic pressure had taken to the indirect proof of the direction of osmotic pressure mentioned by Dr.Lowry. I t was not easy to consider the direction of motion of osmotic pressure. Everybody could see it, but at the same time the only explanation which had some kind of scientific basis was that of evaporation from one surface, and that one could not get a constant distillation in one direction to exert a pressure in the other, because perpetual motion would then be obtained. Another thing had struck him as being remarkable in connection with osmotic pressure, and semi-permeable mem- branes in particular, although Dr. Lowry’s remarks had partly explained the difficulty. Why was it considered that only the ions conducted in a solution ? If a semi-permeable membrane were taken, one would not expect to be able to pass electric current through it into another conductor standing outside, but from thousands of measurements he had made himself it took place with the greatest of ease.The semi-permeable membrane did not seem to offer any greater resistance, except ohmic resistance, than an ordinary membrane. Supposing a solution of sulphate of soda were placed in a porous pot, and outside it some caustic soda ; if a copper anode was put in sulphate of soda, and a cathode of another kind outside, and the current were passed through, it would be found that the sulphate of soda would decompose, and the sodium would go through the semi-permeable membrane without any hindrance whatever, forming caustic soda outside, and forming inside sulphate of copper, which could not permeate the membrane because it deposited oxide of copper and some composition, the properties of which were not known, on the semi-permeable membrane.From experiments which he carried out extending over six weeks he discovered that the process went on as nearly as possible according to Faraday’s law. There did not seem to be any impediment whatever. In modern text-books on physical chemistry very audacious views with regard to the use of the method of least squares were to be found, results being given which were supposed to be constants, but which differed by 25 or 30 per cent. For instance, in Arrhenius’s Electrochemistry, he could mention figures where 6 or 7 per cent. was the ordinary deviation from the mean.The method of least squares was an extremely useful one, showing how to find out what was a theoretical error, but it had now become the fashion to utilise that method in physical chemistry to such an extent that almost anything could be proved by it. With regard to the question of solution pressure, Professor Palmaer, of Stockholm, wrote a paper some years ago on the solution velocity of zinc in hydrochloric acid, and gave some tabulated figures which he had obtained with regard to the evolution of hydrogen as compared with certain formulz. It was a kind of integral which could be got from anything. He developed from those figures a theory with regard to the solution velocity of zinc in hydrochloric acid as depending upon electromotive force by means of what he called dissolution pressures.When the problem was scrutinised, it was found that the dissolution pressures had to be determined experimentally, and when that had been done, the experimenter simply calculated what he had found before. Dr. A. Findlay, after thanking the Society for allowing him to be present, stated that he had come to the meeting more for the purpose of learning than of imparting information, and he hoped his remarks would be considered32 OSMOTIC EXPERIMENTS in that spirit. He supposed there was scarcely any subject about which there had been more beating of the air than that of osmotic pressure. A large part of the discussion in recent years might have been useful, had one not misunderstood what was the problem actually to be investigated.He thought it was an unfortunate thing that a number of the workers on the subject in recent years, while exercising a vigorous criticism, had failed to make themselves thoroughly conversant with the van’t Hoff point of view and theory of solutions. As the members were well aware, the well- known van’t Hoff relationship, PV=RT for solutions, was put forward by van’t Hoff only for dilute solutions, the true osmotic formula involving other factors which were not contained in that equation. With regard to the assertions made about equilibrium, he would like to refer to the objections raised to the use of semi-permeable membranes in the thermodynamic deduction of osmotic pressure. He thought the whole of Kahlenberg’s objection to the postulating of semi-permeable membranes rested on a total misunderstanding as to the meaning of a thermodynamically passive membrane.In thermodynamics, they did not consider at all how the membrane acted, provided that the changes in energy were equal in opposite directions, If this condition were satisfied the membrane would be thermodynamically passive, no matter whether it acted as a selective absorbent for different substances or however otherwise it might act. He could not, therefore, agree to the assertion made by Kahlenberg that the semi-permeable membrane of the thermodynamicist had no counterpart in actual osmotic experiments, not even approximately, It does have a counterpart, sometimes, indeed, a practically exact counterpart, as, for example, in the case of a membrane of copper ferrocyanide employed with aqueous solutions of cane-sugar.He was, however, quite ready to agree with Professor Kahlenberg that the thermodynamic osmotic pressure and the osmotic pressure of experimental practice were, or at least might be, quite different things; and he heartily endorsed the statement that a full recognition of this would do much to clear up the existing coiifusion and misunderstanding. At the same time it was only fair to previous workers in the domain of osmotic pressure to say that this confusion and misunderstanding were to a large extent due to Kahlenberg himself. Van’t Hoff and the other physical chemists who gave a general acceptance to his theory had always, so far as he was aware, been quite cognisant of the difference referred to, and had always used the term osmotic pressure as the thermodynamic osmotic pressure.Indeed, it was only when so employed that the osmotic pressure had in itself any definite meaning and represented a definite property of the solution. He was sure, however, that all would hail with delight the signs that the confusion and misunderstandings werc being dissipated. So far as the problem of osmotic pressure was concerned it could, he thought, be divided into three parts, namely, the experimental part, the quantitative theoretical part, and the speculative part concerning the mechanism of osmotic pressure. The quantitative theoretical part had been already practically completely worked out on the basis of the second law of thermodynamics. He was surprised that Mr. Whetham seemed to doubt whether, after all, the thermodynamic formula which Mr.Spens obtained under his direction for the relationship between vapour pressure and osmotic pressure was the correct thing, and when he stated that it must be left to experiment to decide. He thought that statement was probably due to Mr. Whetham’s modesty. He accepted the thermodynamic relationship as being the correct relationship (always granting, of course, that the assumptions made in its deduction were essentially correct), and also that experimentOSMOTIC PRESSURE 33 could only inform them within a certain error how nearly those ideal conditions of the thermodynamic deductions were reached. He considered the quantitative theoretical relationship between concentration and osmotic pressure had been essentially solved.As to the experimental verification of those relationships, he thought they shouId all be pleased that Morse and Fraser in Amerka and the Earl of Berkeley and Mr. Hartley in this country had so completely obtained concordance between experiment and theory. One could not fail to be amazed, not only at the ingenuity of the apparatus employed, but also at the accuracy of the work which the Earl of Berkeley had explained that evening. He thought, therefore, it might be taken for granted that there was nearly absolute agreement between the experimental work which had been done and the van’t Hoff thermodynamic equation. Coming to the speculative point of view, inquiry as to what the nature of osmotic pressure was, and how it was caused, Dr.Lowry had given a very interesting modification of the bombardment kinetic theory, to which, however, he (Dr. Findlay) did not take kindly at first sight. It wasvery ingenious in many ways, but he did not think it was quite applicable to osmotic pressure. So far as he could see, the two things-the production of a vapour pressure and osmotic pressure- were not necessarily the same, although it was quite trde that if one obtained an expression for the lowering of the vapour pressure, it would also apply to osmotic pressure, because in both cases they were measuring the work done in separating the solvent from the solution. He did not think that Dr. Lowry’s explanation could be applied directly to the osmotic pressure. In any case Dr. Lowry was dealing with a two-phase system, whereas in the case of osmotic pressure experiments they were dealing in principle with a one-phase system.Moreover, the theory put forward by Dr. Lowry neglected the influence of surface tension in producing an alteration of concentration in the surface layers of solution. From this it followed that in equimolecular solutions there would not be an equimolecular distribution of solute molecules in the surface layer, and consequently such solutions should not, according to Dr. Lowry’s theory, exhibit the same lowering of vapour pressure as compared with the pure solvent. However, there was a danger of giving too large a space in their dis- cussions to the consideration of the various mechanical pictures made of the mechanism of osmotic pressure processes.He did not in the slightest object to the formation of such pictures, provided they were fruitful in sug- gesting further investigation and showed themselves capable of being developed so as to give a quantitative-not merely qualitative-expression of the phenomena. He was not convinced, however, that any of the pictures so far suggested had this merit. For this reason he welcomed all the more the accurate experimental investigation of osmotic pressures now being carried out. He thought that so far as our present knowledge goes the production of osmotic pressure could be most simply and satisfactorily regarded as due to the difference of energy-experimentally verified-between the solution and the pure solvent, or if they preiferred it, to the affinity or attraction between the solvent and the solute ; and the difference of energy might be, no doubt was, partly, but not necessarily entirely, due to chemical combination of solvent and solute.They might leave out the semi-permeable membrane altogether and imagine that they had a solution with a layer of pure solvent on the top of it. They all knew thqt diffusion took place, both of solvent molecules into the solution and of solute molecules into the solvent, and the force causing the diffusion was numerically the same as the osmotic pressure. The only34 DISCUSSION effect of the introduction of a semi-permeable membrane-a true semi- permeable membrane-was to make the diffusion one sided, so that while the solvent molecules could still pass into the solution, the solute molecules could no longer pass out into the solvent, By the entrance of solvent molecules into the solution, however, a hydrostatic pressure was created, and, given a perfect semi-permeable membrane, this hydrostatic *pressure measured the osmotic pressure.In conclusion, he also desired to say he thought it was unfortunate that in so much of their work in the verification of van’t Hoff’s formula, they applied the formula for dilute solutions to concentrated solutions. He understood that, in the last paper which the Earl of Berkeley published along with Mr. Hartley in the Traizsactiorzs of the Royal Society, the calculations were carried out on the basis of grammes of substance in litres of solution. Morse and Fraser calculated their results on the basis of grammes of substance in grammes of solvent, or volume of solvent, which came more nearly to the quantities used in the thermodynamic equation as developed by van’t Hoff. The Earl of Berkeley and Mr.Hartley found a disagreement between their experiments and what were called the gas laws, Le., the gas laws as deduced from dilute solutions ; and it was not surprising at all that disagreement was found. He believed the remark he had made was correct, but he should like to ask the Earl of Berkeley whether his calculations were carried out on the basis of grammes of substance in volumes of solution. Mr. W. A. Davis stated that his only excuse for entering into the discus- sion was the fact that he had had the opportunity of talking over Dr. Lowry’s views with him.I t appeared to him that a great deal had been made, especially by Dr. Findlay, of the question of the agreemcnt between experi- ment and theory. The question was, what was to be meant by ‘‘ theory” ? So many theories had been advanced to explain osmotic pressure. Dr. Lowry that evening had shown that, starting with an entirely different hypothesis, it was possible to come mathematically and thermodynamically to the same result as was reached by van’t Hoff originally from his simple conception of the bombgrdment of the walls of the osmotic cell. He thought van’t Hoffs original hypothesis was open to the grave objection that it entirely neglected the influence exerted on the solute by the solvelit, and it appeared to him that Dr. Lowry’s modification of van’t Hoffs conception was necessary from that standpoint-that it was necessary to take into account the influence of the solvent on the solute.There were many theories to account for osmotic pressure. For instance, there was the theory of Batelli and Stephanini, which, as far as one could judge from the brief statement given of it in the Atti dei Lilzcei, was supported by a very considerable amount of evidence. Batelli and Stephanini’s conception of osmotic pressure was that it was due entirely to a tendency to equalise the surface tension on two sides of the osmotic cell. Dr. Lowry’s conception, it seemed to him, would help to reconcile the ordinary views of osmotic pressure with the results established experimentally by Batelli and Stephanini, which they themselves considered to be in direct conflict with the ordinary conception of osmotic pressure.Dr. T. M. Lowry (in reply) : At the present time it was not possible to claim any substantial element of novelty for the explanation that had been given of osmotic pressure from the standpoint of the kinetic theory. The theory had originally formed the subject of a paper read before the Chemical Society of the Central Technical College, but it was only recently that the relative unfamiliarity of the ideas referred to had appeared to warrant further publicity. He entirely agreed with Mr. Rhodin in reference to the direction of theOSMOTIC PRESSURE 35 forces called into play, and considered that the phenomena might be described quite as accurately by the term (( osmotic suction ” as by the more familiar term ‘( osmotic pressure.” The views he had advocated accounted for the direction of the osmotic flow as well as for the magnitude of the so-called “ osmotic pressure ” required to prevent that flow.It was interesting to note that in his equation Lord Berkeley had introduced the osmotic pressure as a negative quantity, In reply to Dr. Philip it was not at all remarkable that Boyle’s law should hold good for osmotic pressure, i.e., that osmotic pressure should be proportional to the concentration, nor even that the temperature coefficient should be identical with that for the pressure of a gas at constant volume, since this implied no more than that the osmotic pressure was a linear function of the temperature and would vanish at the absolute zero.What was really remarkable was that the constant R in the equation PV = RT should be identical in the two cases. This fact enormously increased the value of the osmotic methods of determining molecular weights, since it was impossible to argue, for instance, that the double effect produced by potassium chloride on the freezing point of water could be explained away by assuming the sugar molecule to be bimolecular. The exact equality of the two constants could not be predicted from the kinetic theory of osmotic pressure, since this in its simplest form would indicate rather that the agreement between osmotic pressure and gas pressure might be subject to a correction depending on the molecular volume of the solute and the volume changes occurring during dissolution.He felt justified, however, in working backwards from the known laws of osmotic pressure, and considered that these might throw considerable light on the surface structure of liquids, about which exceedingly little was yet known. The Earl of Berkeley : The reasoning originally applied by J. Thomson to Andrews’ experiment on gas pressures admits of extension to osmotics; and the curves which connect the osmotic pressures with the volumes of the solution (or, it may be, the volumes of solvent in the solution) may be expected to show features similar to the well-known diagram of isothermals for vapour-liquid substances such as carbonic acid. If we draw the system of p.v. curves (where p=osmotic pressure and v =volume of solution), then, when the temperature is high, p will diminish throughout the curve as v increases; on passing in succession to curves corresponding to lower temperatures, one will be reached for which the gradient or slope just vanishes at a certain pressure ; and the curves beyond this will have contrary flexure in different parts. Now, increase of osmotic pressure with increase of dilution must involve intrinsic physical instability, resulting in change of state ; and the charac- teristic equation, together with the derived equations dp/dv = 0 and dp2/d2v = 0, will determine the critical temperature (and the pressure and volume) below which supersaturation and crystallisation can set in.As crystallisation, in the case of cane-sugar solutions, can take place at ordinary temperatures, the P.v.curve, of say T=z73O, must give a point dP/dv= 0 where complete instability sets in at that temperature ; this point is the limit of supersaturation and may be determinable by experiment. If one could find a membrane which is only permeable to the solute, then when the solution is placed in contact with the pure liquid solute through this membrane, an osmotic pressure will be set up. The P.v. curves repre- senting these new osmotic pressures and volumes will show phenomena, in connection with the freezing point of the solution, entirely analogous to those described above as appertaining to the crystallising point, In other36 DISCUSSION words, the freezing and crystallising points of the solution are two extreme points, with continuous physical transition between them representable by the equations of osmotic pressure, and they coalesce in the cryohydric point.Experiments are in progress to determine both the point of super- saturation and the connection between the two osmotic pressures just mentioned. Mr. W. C. Dampier Whetham (communicafed reply): Dr. Findlay has treated the theoretical aspect of the subject so well that the words which remain to be said can but emphasise some of his remarks. I am glad that Prof. Kahlenberg sees the basis for an understanding in the controversy about osmotic pressure in what he calls my ‘< recent admission ” that his pressures as measured experimentally do not necessarily give us the values of the thermodynamic osmotic pressure as defined theoretically. Prof. Kahlenberg used his experimental results to attack the soundness of the thermodynamic theory, and I (‘ admitted ” that his results had nothing to do with his case : they merely went to show how far his experimental conditions approached those assumed in the theoretical definitions.That the theoretical osmotic pressure of a dilute solution must have the ‘( gas value ” may be deduced by thermodynamic reasoning either from the observed solubility relations of a volatile solute, or, as Prof. Larmor has proved, from the fundamental hypothesis of the molecular theory, The validity of the reasoning, and the assumption made in extending the result to involatile solutes, is amply justified, not only by direct experiments on the osmotic pressures of solutions of sugar in water with membranes of copper ferrocyanide, but also by the far tnore accurate and theoretically satisfactory determinations of the freezing points of dilute solutions of sugar and potassium chloride made by Mr.Griffiths. These results also show that in the surface of a crystal of ice growing in n solution we have an ideal semi- permeable wall. As Dr. Findlay points out, membranes of copper ferro- cyanide can be obtained which are very nearly thermodynamically semi-permeable for sugar and water, and here, in the surface of crystals and the surface of a volatile solvent containing an involatile solute, we have semi-permeable partitions which approach still more nearly the ideal state. Such considerations justify the extension of thermodynamic reasoning to the phenomena of solutions, and have enabled us to co-ordinate many of the properties of solutions which must otherwise have remained isolated facts.From the thermodynamic standpoint the mode of action of the membrane does not matter; it may well be due to selective absorption or chemical affinity. As long as the process is reversible, the thermodynamic reasoning applies, however the membrane works. I t is the strength of thermodynamic reasoning that it avoids the uncertainties of definite hypotheses about the mechanism of the connection between the two series of phenomena it correlates; it is the weakness of thermodynamic reasoning that it fails to throw light on the intimate structure of the system which such hypotheses seek to examine. Prof. Kahlenberg’s experiments are of great interest as elucidating the mechanism of osmosis, and the relations of liquids and permeable mem- branes.But, as he himself sees, they have no bearing on the general theory of solution ; they investigate the special system involved in each case, but give no ground for co-ordinating different species of phenomena, as the thermo- dynamic theory enables us to co-ordinate osmotic pressures, freezing points, and electrolytic conductivity. Having defined osmotic pressure theoretically and correlated by aid of the conception the different phenomena of solution, it remains to consider howOSMOTIC PRESSURE 37 far the different experimental investigations conform to the theoretical conditions, and how far the quantities treated in the experiments are identical with those of pure theory.And this is why I was guilty of what Dr. Findlay evidently considers to be want of faith in thermodynamic reasoning. If Lord Berkeley and Mr. Hartley really measure the true theoretical osmotic pressure, I have no doubt that their results will conform to the theoretical values indicated by the thermodynamic cycle. If their osmotic pressure as defined in the deduction of their equation had been the same as that of Mr. Spens, I have no doubt they would have arrived at the same result as he did. How far both these assumptions are justified time, and further experiments, will show. DISCUSSION DISCUSS I0 N. Mr. H. N. Morse (communicated) : I thank the Faraday Society for the kind invitation to contribute towards the symposium on '' Osmotic Pressure " which has been arranged for January 29th.I send herewith four tables which present in the briefest form results which have been or are about to be published. Table I. contains a summary of the results which were obtained with glucose. The full account of this TABLE I.-SUMMARY OF RESULTS. Determination of the Osmotic Pressure of Glucose. Series I., 1906. 0'1 0'1 0'2 0'2 0.3 0 3 0.4 0.4 0.5 0'5 0.6 0.6 0-6 0.7 0.7 0.7 0.8 0.8 0.8 0.9 0'9 0'9 1'0 1'0 1'0 H = I I 2 I 2 I 2 I 2 I 2 I 2 I 2 I 2 I 2 I 2 3 3 3 3 3 - Degrees C. 24-10 25-10 24.10 2493 23'48 26-90 26.60 2 1-86 24.17 22-57 22.40 22-30 22-26 25'43 22-70 23-00 23.28 23.80 22.58 23.10 22-60 22'20 23'64 22'20 22'10 2-39 2-42 4'76 4'77 7-12 7.17 9-70 9-65 I 2.07 14'56 14-32 14.29 16.82 16-96 16.75 19-27 19.16 19-25 21-64 21' 9 2 1 -23 24'12 24.00 24-03 12'00 - 0.03 - 0'09 - 0'09 - 0.08 - 0.08 + 0.04 + 0'10 - 0.08 - 0'16 - 0'01 - 0'10 - 0'12 - 0'12 - 0'0 - 0.08 - 0-13 - 0.04 - 0.17 - 0.16 + 0.04 - 0.04 - 0'10 - 0'21 - 0'11 - 0'11 c ' z .zg g ??E 3;;; u 03% 9 2.E g ."c; 181*r 179' 3 181.77 1d270 181.38 180.73 180.21 181.02 178.12 180.57 177.62 180.42 1 9'55 179.16 I 80.39 179'73 '79'87 I 80.49 179;60 178046 179'60 179.07 180S88 1791 Ig0.16 Mol.Wt. Glucose = 178.74 Mean Mol. Wt. 180.08 work will appear in the April number of the American Chemical Journal. Tables 11. and 111. contain a similar statement of the results which have been obtained with cane sugar. Table IV. presents the results with both cane sugar and glucose in the form of ratios of observcd osmotic to calculated gas pressure at the same temperature, The conclusions which we deduce from the results presented in the tables are : (I) That, in thc vicinity of 20°, the osmotic pressure exerted by both cane sugar and glucose is equal to that which a molecular-equivalent28 OSMOTIC EXPERIMENTS quantity of a gas would exert if its volume were reduced, at the same temperature, to the volume of the solvent in the pure state ; (2) that, at the temperature at which the measurements were made ( i e , , between 18" and 26O, in round numbersj, both cane sugar and glucose in solution are in the anhydrous condition.It is to be remembered in this connection that our solutions are all made TABLE II.-SUMMARY OF RESULTS. Determination of the Osmotic Pressure of Cane Sugar.Series I., 1905. Am. Chem, Tourn., xxxiv. I, z . gs iY 2s & $ s o WE: 0'1 0'1 0'2 0'2 0.25 0.25 0.3 0'3 0.4 0'4 0'5 0.5 0.6 0.6 0.7 0'7 0.8 0.8 0.89101 0.9 1'0 1'0 1'0 H = I I 2 I 2 I 2 I 2 I 2 I 2 I 2 I 2 I 2 I I I 2 3 - Degrees C. I 8.85 18-71 20'59 20.9 I 23'45 21-46 19'28 20'94 20.84 20'32 21-38 24'45 20.14 24-62 17-89 19-56 19.84 ;g:$j 19'34 23-32 22'44 21'60 2'43 2.42 4'73 4'79 608 5'94 7.1 8 7.18 9'45 9'65 I 1-93 12.06 I 4.2 2 14'37 16.66 I 6.89 19-20 19-35 21-03 2 I -68 24-56 24.28 24-17 2.38 2.3 8 4'79 4-80 6-05 6-02 7'17 7'13 9'55 9.6 I 11-99 14-45 14'59 26-77 17-03 19.03 19-13 2 1-30 21-55 24.26 24-15 24'09 12'01 E! ;* g QJmg 2 3 z ;$$ $06 $ W E : -- + 0'05 + 0'05 - 0.06 + 0.03 - 0.08 + 0'01 + 0.05 + 0.04 - 0.08 + 0.07 - 0.23 - 0.14 + 0.17 + 0'22 - 0.27 + 0'13 + 0.30 + 0.13 + 0.08 - 0'01 - 0'10 - 0'22 - 0'11 3 3 4'47 334'08 344'50 340.03 338'05 344.60 339'44 337'65 343'21 338'54 341.92 337'85 345'09 344'89 342.18 342'41 336'46 335'86 343'90 337'55 335'50 337'95 338'54 3 34'28 342'27 341'33 338'55 339'89 340.88 344'99 342.30 336.16 343'90 337'55 337'33 ~- Final Mean of Mol.Wts, 339.95 Theoretical Mol. Wt. 339'60 up with a fixed mass of the solvent, and that, therefore, if the pressures are found to be strictly proportional to the supposed concentration, the dissolved material cannot have appropriated any portion of the solvent. It may be of interest to you to learn that we have also measured the osmotic pressure of cane sugar and glucose solutions at a temperature just above oo, and that, in the case of both substances, we find pressures somewhat in excess of the calculated gas pressures.An account of this later work will probably appear in the May and June numbers of the American Chemical Journal. We are just about ready to begin work with the electrolytes, havingDISCUSSION 29 perfected a cell which is expected to enable us to determine simultaneously osmotic pressure and electrolytic dissociation. JOHNS HOPKINS UNIVERSITY, 3'niauary 19, 1907. Dr. J. C. Philip, in connection with Dr. Lowry's statement that the validity of the equation PV = RT rested purely on an experimental basis, spoke of the results of applying thermodynamics to dilute solutions. If he was not mistaken, Dr. Larmor, in a paper published in Nafure in 1897, had reached the conclusion that, so long as a solution might be regarded as consisting of a certain number of solute nuclei, each surrounded by, and TABLE III.-SUBIMARY OF RESULTS.Deicriniitaiion of the Osritolic Pressure of Canc Sugar. Scries II., 1906. Atn. Claent. Journ., xxxvi. 39. 0'1 0'1 0'2 0'2 0'2 0'3 0.3 0.4 0'4 0'5 0.6 0.6 0.6 0'7 0'7 0.8 0.8 0'9 0.9 0'5 1'0 1'0 I 2 I 2 I 2 I 2 I 2 I 2 3 3 I 2 I 2 I 2 I 2 H = I & 0 Q ) . k r : 2 ti2 n c y H Dcgrees c. 24.06 2423 20'90 2 1.38 21.77 21-67 19-88 21-63 22':s 22'02 23'70 24-38 24'24 24-10 23-64 23'00 23-63 23-69 24'79 24'76 23'56 24-58 2-51 2'55 4'72 4.8 I 7'24 7-20 9'64 9'69 12.06 I 2.23 14'74 14-70 14'77 16-95 I 6-96 19-30 19'39 2 1.82 21-91 24'42 4'78 23'98 2.42 2'43 4'79 4.80 4.8 I 7.2 I 7.16 9.6 I 9-63 12.06 14'55 14.55 14-54 16-94 16.93 19'35 19-36 2 1-86 2 1.86 24' I c) 24-28 12'10 + 0.09 + 0'12 - 0.07 + 0.03 + 0.04 + 0.03 + 0.06 + 0.19 + 0'15 + 0'23 + 0'01 + 0.03 - 0.05 + 0.03 - 0.04 + 0'05 -k 0'23 - 0.30 - 0'02 0'00 0'00 + 0.13 327'85 322*j6 3 4 4 9 340'90 339'74 338'37 338.20 338'73 337'50 339'59 336.19 335'32 336.07 334'32 339'39 339'50 340'48 339'08 340.19 338'82 336'39 342'72 E: i .s "1 gs .!:z aa% 5 3 n j 0s aJ u 325.21 341.87 338'29 338.12 337'89 335'24 339'45 339'78 339'51 339'56 Final Mean of Mol.Wts. 337'49 Theoretical Mol. Wt. 339'60 possibly attached to, a large number of solvent molecules, the application of thermodynamics leads to the result that osmotic pressure must vary inversely as the volume and directly as the absolute temperature. This result is independent of any particular views which may be entertained as to the nature and origin of osmotic effects.Mr. John Rhodin remarked that he happened to be a studeiit of Upsala30 OSMOTIC EXPERIMENTS University when the great controversy was taking place between Arrhenius and Mendeleeff with regard to the theory of solutions, and ever since that time he had taken a great interest in the question. With regard to osmotic pressure, he thought it was one of the few things connected with the physical chemistry of the last ten or fifteen years which had a distinct meaning, and which one of only moderate understanding could grasp. The remarkable thing about osmotic pressure which struck him was that it moved in the wrong direction. One would expect the higher pressure to cause a motion TABLE IV.Ratio of Observed Osnaofic to Calculated Gas Pressure at the same Temperature, the Volume of the Gas being that of the Solveiat in the Pure Stale. Weight. Normal Concen- tration. - -__ 0'1 0'1 0'2 0'2 0'2 0'25 0'25 0'3 0.3 0.4 0'4 0.5 0'5 0.6 0.6 0.6 0.7 0'7 0'7 0.8 0.8 0-8 0.89101 0'9 0.9 0.9 1'0 1'0 1'0 Number 01 Experi- ment. I 2 I 2 3 I 2 I 2 I 2 I 2 I 2 3 I 2 I 2 3 3 I I 2 3 I 2 3 CAXE SUGAR. Series I. Tempera. ture. Degrees C 18.85 I 8.7 I 20'59 20'9 I 23'45 21-46 19.28 17-65 I 8.8 I 20'94 20.84 20-32 21-38 24'45 20.14 24-62 17-89 19-56 19'34 19-84 23-32 22-44 2 1-60 Ratio. I '02 I 1.017 0.987 0.998 I '005 0'987 1.007 0'990 I *004 0'993 I *006 1'001 0'984 0.984 0'993 0.992 I '009 1'012 0'987 1.006 1'012 1.005 I so03 Series 11. Tempera- ture.Degrees C 24.06 24'23 20.90 2 1-38 21'77 2 I -67 19.88 21-63 22.15 22-62 23'70 24-38 24.70 24-10 23'64 23-90 23.60 23-69 24'79 24-76 23'59 24-58 Ratio. 1'037 1'049 0.985 0.996 I '000 I '004 I ,006 I '003 1.006 I '000 1'010 1'010 1'000 I '002 1.013 1.016 0.997 I '002 0.998 1'002 1'010 0,988 GLUCOSE. Series I. Tempera- ture. Degrees C . 2 5'10 24-10 24-10 24'93 22'20 23-48 26-90 26.60 2 1-86 24.17 22'57 22.46 22-30 22-26 25'43 22.70 23-00 23-28 23-80 22.58 23-10 22-60 23 '64 22'20 22-10 Ratio. 0.988 0.98 I 0.98 I 0.996 0.986 0.992 0.988 1'003 0.990 I '007 0'994 0.988 0.998 0'995 0.992 0.998 0.991 0'995 0'903 0.990 0w5 0'995 0'998 0'989 1'002 in the direction of the lower pressure. There was one fact, however, which was not taken into consideration, namely, the incompressibility of liquids.To his mind the direction taken by the osmotic pressure, i.e., of the kinetic motion which took place, depended upon the volume of the system under consideration, which must be a constant. One might just as well say that, from the top of the side where the solution was, there was a kind ofDISCUSSION 31 weightless and frictionless piston which was being moved upwards, and that the resistance of the semi-permeable membrane against the influx of the solvent was nil. But even then the gripping point of the solution seemed to be rather remarkable, and hence he had found that nearly all writers on osmotic pressure had taken to the indirect proof of the direction of osmotic pressure mentioned by Dr. Lowry. I t was not easy to consider the direction of motion of osmotic pressure.Everybody could see it, but at the same time the only explanation which had some kind of scientific basis was that of evaporation from one surface, and that one could not get a constant distillation in one direction to exert a pressure in the other, because perpetual motion would then be obtained. Another thing had struck him as being remarkable in connection with osmotic pressure, and semi-permeable mem- branes in particular, although Dr. Lowry’s remarks had partly explained the difficulty. Why was it considered that only the ions conducted in a solution ? If a semi-permeable membrane were taken, one would not expect to be able to pass electric current through it into another conductor standing outside, but from thousands of measurements he had made himself it took place with the greatest of ease.The semi-permeable membrane did not seem to offer any greater resistance, except ohmic resistance, than an ordinary membrane. Supposing a solution of sulphate of soda were placed in a porous pot, and outside it some caustic soda ; if a copper anode was put in sulphate of soda, and a cathode of another kind outside, and the current were passed through, it would be found that the sulphate of soda would decompose, and the sodium would go through the semi-permeable membrane without any hindrance whatever, forming caustic soda outside, and forming inside sulphate of copper, which could not permeate the membrane because it deposited oxide of copper and some composition, the properties of which were not known, on the semi-permeable membrane.From experiments which he carried out extending over six weeks he discovered that the process went on as nearly as possible according to Faraday’s law. There did not seem to be any impediment whatever. In modern text-books on physical chemistry very audacious views with regard to the use of the method of least squares were to be found, results being given which were supposed to be constants, but which differed by 25 or 30 per cent. For instance, in Arrhenius’s Electrochemistry, he could mention figures where 6 or 7 per cent. was the ordinary deviation from the mean. The method of least squares was an extremely useful one, showing how to find out what was a theoretical error, but it had now become the fashion to utilise that method in physical chemistry to such an extent that almost anything could be proved by it.With regard to the question of solution pressure, Professor Palmaer, of Stockholm, wrote a paper some years ago on the solution velocity of zinc in hydrochloric acid, and gave some tabulated figures which he had obtained with regard to the evolution of hydrogen as compared with certain formulz. It was a kind of integral which could be got from anything. He developed from those figures a theory with regard to the solution velocity of zinc in hydrochloric acid as depending upon electromotive force by means of what he called dissolution pressures. When the problem was scrutinised, it was found that the dissolution pressures had to be determined experimentally, and when that had been done, the experimenter simply calculated what he had found before.Dr. A. Findlay, after thanking the Society for allowing him to be present, stated that he had come to the meeting more for the purpose of learning than of imparting information, and he hoped his remarks would be considered32 OSMOTIC EXPERIMENTS in that spirit. He supposed there was scarcely any subject about which there had been more beating of the air than that of osmotic pressure. A large part of the discussion in recent years might have been useful, had one not misunderstood what was the problem actually to be investigated. He thought it was an unfortunate thing that a number of the workers on the subject in recent years, while exercising a vigorous criticism, had failed to make themselves thoroughly conversant with the van’t Hoff point of view and theory of solutions.As the members were well aware, the well- known van’t Hoff relationship, PV=RT for solutions, was put forward by van’t Hoff only for dilute solutions, the true osmotic formula involving other factors which were not contained in that equation. With regard to the assertions made about equilibrium, he would like to refer to the objections raised to the use of semi-permeable membranes in the thermodynamic deduction of osmotic pressure. He thought the whole of Kahlenberg’s objection to the postulating of semi-permeable membranes rested on a total misunderstanding as to the meaning of a thermodynamically passive membrane. In thermodynamics, they did not consider at all how the membrane acted, provided that the changes in energy were equal in opposite directions, If this condition were satisfied the membrane would be thermodynamically passive, no matter whether it acted as a selective absorbent for different substances or however otherwise it might act.He could not, therefore, agree to the assertion made by Kahlenberg that the semi-permeable membrane of the thermodynamicist had no counterpart in actual osmotic experiments, not even approximately, It does have a counterpart, sometimes, indeed, a practically exact counterpart, as, for example, in the case of a membrane of copper ferrocyanide employed with aqueous solutions of cane-sugar. He was, however, quite ready to agree with Professor Kahlenberg that the thermodynamic osmotic pressure and the osmotic pressure of experimental practice were, or at least might be, quite different things; and he heartily endorsed the statement that a full recognition of this would do much to clear up the existing coiifusion and misunderstanding. At the same time it was only fair to previous workers in the domain of osmotic pressure to say that this confusion and misunderstanding were to a large extent due to Kahlenberg himself.Van’t Hoff and the other physical chemists who gave a general acceptance to his theory had always, so far as he was aware, been quite cognisant of the difference referred to, and had always used the term osmotic pressure as the thermodynamic osmotic pressure. Indeed, it was only when so employed that the osmotic pressure had in itself any definite meaning and represented a definite property of the solution.He was sure, however, that all would hail with delight the signs that the confusion and misunderstandings werc being dissipated. So far as the problem of osmotic pressure was concerned it could, he thought, be divided into three parts, namely, the experimental part, the quantitative theoretical part, and the speculative part concerning the mechanism of osmotic pressure. The quantitative theoretical part had been already practically completely worked out on the basis of the second law of thermodynamics. He was surprised that Mr. Whetham seemed to doubt whether, after all, the thermodynamic formula which Mr. Spens obtained under his direction for the relationship between vapour pressure and osmotic pressure was the correct thing, and when he stated that it must be left to experiment to decide. He thought that statement was probably due to Mr.Whetham’s modesty. He accepted the thermodynamic relationship as being the correct relationship (always granting, of course, that the assumptions made in its deduction were essentially correct), and also that experimentOSMOTIC PRESSURE 33 could only inform them within a certain error how nearly those ideal conditions of the thermodynamic deductions were reached. He considered the quantitative theoretical relationship between concentration and osmotic pressure had been essentially solved. As to the experimental verification of those relationships, he thought they shouId all be pleased that Morse and Fraser in Amerka and the Earl of Berkeley and Mr.Hartley in this country had so completely obtained concordance between experiment and theory. One could not fail to be amazed, not only at the ingenuity of the apparatus employed, but also at the accuracy of the work which the Earl of Berkeley had explained that evening. He thought, therefore, it might be taken for granted that there was nearly absolute agreement between the experimental work which had been done and the van’t Hoff thermodynamic equation. Coming to the speculative point of view, inquiry as to what the nature of osmotic pressure was, and how it was caused, Dr. Lowry had given a very interesting modification of the bombardment kinetic theory, to which, however, he (Dr. Findlay) did not take kindly at first sight.It wasvery ingenious in many ways, but he did not think it was quite applicable to osmotic pressure. So far as he could see, the two things-the production of a vapour pressure and osmotic pressure- were not necessarily the same, although it was quite trde that if one obtained an expression for the lowering of the vapour pressure, it would also apply to osmotic pressure, because in both cases they were measuring the work done in separating the solvent from the solution. He did not think that Dr. Lowry’s explanation could be applied directly to the osmotic pressure. In any case Dr. Lowry was dealing with a two-phase system, whereas in the case of osmotic pressure experiments they were dealing in principle with a one-phase system. Moreover, the theory put forward by Dr.Lowry neglected the influence of surface tension in producing an alteration of concentration in the surface layers of solution. From this it followed that in equimolecular solutions there would not be an equimolecular distribution of solute molecules in the surface layer, and consequently such solutions should not, according to Dr. Lowry’s theory, exhibit the same lowering of vapour pressure as compared with the pure solvent. However, there was a danger of giving too large a space in their dis- cussions to the consideration of the various mechanical pictures made of the mechanism of osmotic pressure processes. He did not in the slightest object to the formation of such pictures, provided they were fruitful in sug- gesting further investigation and showed themselves capable of being developed so as to give a quantitative-not merely qualitative-expression of the phenomena.He was not convinced, however, that any of the pictures so far suggested had this merit. For this reason he welcomed all the more the accurate experimental investigation of osmotic pressures now being carried out. He thought that so far as our present knowledge goes the production of osmotic pressure could be most simply and satisfactorily regarded as due to the difference of energy-experimentally verified-between the solution and the pure solvent, or if they preiferred it, to the affinity or attraction between the solvent and the solute ; and the difference of energy might be, no doubt was, partly, but not necessarily entirely, due to chemical combination of solvent and solute.They might leave out the semi-permeable membrane altogether and imagine that they had a solution with a layer of pure solvent on the top of it. They all knew thqt diffusion took place, both of solvent molecules into the solution and of solute molecules into the solvent, and the force causing the diffusion was numerically the same as the osmotic pressure. The only34 DISCUSSION effect of the introduction of a semi-permeable membrane-a true semi- permeable membrane-was to make the diffusion one sided, so that while the solvent molecules could still pass into the solution, the solute molecules could no longer pass out into the solvent, By the entrance of solvent molecules into the solution, however, a hydrostatic pressure was created, and, given a perfect semi-permeable membrane, this hydrostatic *pressure measured the osmotic pressure.In conclusion, he also desired to say he thought it was unfortunate that in so much of their work in the verification of van’t Hoff’s formula, they applied the formula for dilute solutions to concentrated solutions. He understood that, in the last paper which the Earl of Berkeley published along with Mr. Hartley in the Traizsactiorzs of the Royal Society, the calculations were carried out on the basis of grammes of substance in litres of solution. Morse and Fraser calculated their results on the basis of grammes of substance in grammes of solvent, or volume of solvent, which came more nearly to the quantities used in the thermodynamic equation as developed by van’t Hoff.The Earl of Berkeley and Mr. Hartley found a disagreement between their experiments and what were called the gas laws, Le., the gas laws as deduced from dilute solutions ; and it was not surprising at all that disagreement was found. He believed the remark he had made was correct, but he should like to ask the Earl of Berkeley whether his calculations were carried out on the basis of grammes of substance in volumes of solution. Mr. W. A. Davis stated that his only excuse for entering into the discus- sion was the fact that he had had the opportunity of talking over Dr. Lowry’s views with him. I t appeared to him that a great deal had been made, especially by Dr. Findlay, of the question of the agreemcnt between experi- ment and theory.The question was, what was to be meant by ‘‘ theory” ? So many theories had been advanced to explain osmotic pressure. Dr. Lowry that evening had shown that, starting with an entirely different hypothesis, it was possible to come mathematically and thermodynamically to the same result as was reached by van’t Hoff originally from his simple conception of the bombgrdment of the walls of the osmotic cell. He thought van’t Hoffs original hypothesis was open to the grave objection that it entirely neglected the influence exerted on the solute by the solvelit, and it appeared to him that Dr. Lowry’s modification of van’t Hoffs conception was necessary from that standpoint-that it was necessary to take into account the influence of the solvent on the solute.There were many theories to account for osmotic pressure. For instance, there was the theory of Batelli and Stephanini, which, as far as one could judge from the brief statement given of it in the Atti dei Lilzcei, was supported by a very considerable amount of evidence. Batelli and Stephanini’s conception of osmotic pressure was that it was due entirely to a tendency to equalise the surface tension on two sides of the osmotic cell. Dr. Lowry’s conception, it seemed to him, would help to reconcile the ordinary views of osmotic pressure with the results established experimentally by Batelli and Stephanini, which they themselves considered to be in direct conflict with the ordinary conception of osmotic pressure. Dr. T. M. Lowry (in reply) : At the present time it was not possible to claim any substantial element of novelty for the explanation that had been given of osmotic pressure from the standpoint of the kinetic theory.The theory had originally formed the subject of a paper read before the Chemical Society of the Central Technical College, but it was only recently that the relative unfamiliarity of the ideas referred to had appeared to warrant further publicity. He entirely agreed with Mr. Rhodin in reference to the direction of theOSMOTIC PRESSURE 35 forces called into play, and considered that the phenomena might be described quite as accurately by the term (( osmotic suction ” as by the more familiar term ‘( osmotic pressure.” The views he had advocated accounted for the direction of the osmotic flow as well as for the magnitude of the so-called “ osmotic pressure ” required to prevent that flow.It was interesting to note that in his equation Lord Berkeley had introduced the osmotic pressure as a negative quantity, In reply to Dr. Philip it was not at all remarkable that Boyle’s law should hold good for osmotic pressure, i.e., that osmotic pressure should be proportional to the concentration, nor even that the temperature coefficient should be identical with that for the pressure of a gas at constant volume, since this implied no more than that the osmotic pressure was a linear function of the temperature and would vanish at the absolute zero. What was really remarkable was that the constant R in the equation PV = RT should be identical in the two cases. This fact enormously increased the value of the osmotic methods of determining molecular weights, since it was impossible to argue, for instance, that the double effect produced by potassium chloride on the freezing point of water could be explained away by assuming the sugar molecule to be bimolecular.The exact equality of the two constants could not be predicted from the kinetic theory of osmotic pressure, since this in its simplest form would indicate rather that the agreement between osmotic pressure and gas pressure might be subject to a correction depending on the molecular volume of the solute and the volume changes occurring during dissolution. He felt justified, however, in working backwards from the known laws of osmotic pressure, and considered that these might throw considerable light on the surface structure of liquids, about which exceedingly little was yet known.The Earl of Berkeley : The reasoning originally applied by J. Thomson to Andrews’ experiment on gas pressures admits of extension to osmotics; and the curves which connect the osmotic pressures with the volumes of the solution (or, it may be, the volumes of solvent in the solution) may be expected to show features similar to the well-known diagram of isothermals for vapour-liquid substances such as carbonic acid. If we draw the system of p.v. curves (where p=osmotic pressure and v =volume of solution), then, when the temperature is high, p will diminish throughout the curve as v increases; on passing in succession to curves corresponding to lower temperatures, one will be reached for which the gradient or slope just vanishes at a certain pressure ; and the curves beyond this will have contrary flexure in different parts.Now, increase of osmotic pressure with increase of dilution must involve intrinsic physical instability, resulting in change of state ; and the charac- teristic equation, together with the derived equations dp/dv = 0 and dp2/d2v = 0, will determine the critical temperature (and the pressure and volume) below which supersaturation and crystallisation can set in. As crystallisation, in the case of cane-sugar solutions, can take place at ordinary temperatures, the P.v. curve, of say T=z73O, must give a point dP/dv= 0 where complete instability sets in at that temperature ; this point is the limit of supersaturation and may be determinable by experiment.If one could find a membrane which is only permeable to the solute, then when the solution is placed in contact with the pure liquid solute through this membrane, an osmotic pressure will be set up. The P.v. curves repre- senting these new osmotic pressures and volumes will show phenomena, in connection with the freezing point of the solution, entirely analogous to those described above as appertaining to the crystallising point, In other36 DISCUSSION words, the freezing and crystallising points of the solution are two extreme points, with continuous physical transition between them representable by the equations of osmotic pressure, and they coalesce in the cryohydric point. Experiments are in progress to determine both the point of super- saturation and the connection between the two osmotic pressures just mentioned.Mr. W. C. Dampier Whetham (communicafed reply): Dr. Findlay has treated the theoretical aspect of the subject so well that the words which remain to be said can but emphasise some of his remarks. I am glad that Prof. Kahlenberg sees the basis for an understanding in the controversy about osmotic pressure in what he calls my ‘< recent admission ” that his pressures as measured experimentally do not necessarily give us the values of the thermodynamic osmotic pressure as defined theoretically. Prof. Kahlenberg used his experimental results to attack the soundness of the thermodynamic theory, and I (‘ admitted ” that his results had nothing to do with his case : they merely went to show how far his experimental conditions approached those assumed in the theoretical definitions.That the theoretical osmotic pressure of a dilute solution must have the ‘( gas value ” may be deduced by thermodynamic reasoning either from the observed solubility relations of a volatile solute, or, as Prof. Larmor has proved, from the fundamental hypothesis of the molecular theory, The validity of the reasoning, and the assumption made in extending the result to involatile solutes, is amply justified, not only by direct experiments on the osmotic pressures of solutions of sugar in water with membranes of copper ferrocyanide, but also by the far tnore accurate and theoretically satisfactory determinations of the freezing points of dilute solutions of sugar and potassium chloride made by Mr. Griffiths. These results also show that in the surface of a crystal of ice growing in n solution we have an ideal semi- permeable wall. As Dr. Findlay points out, membranes of copper ferro- cyanide can be obtained which are very nearly thermodynamically semi-permeable for sugar and water, and here, in the surface of crystals and the surface of a volatile solvent containing an involatile solute, we have semi-permeable partitions which approach still more nearly the ideal state. Such considerations justify the extension of thermodynamic reasoning to the phenomena of solutions, and have enabled us to co-ordinate many of the properties of solutions which must otherwise have remained isolated facts. From the thermodynamic standpoint the mode of action of the membrane does not matter; it may well be due to selective absorption or chemical affinity. As long as the process is reversible, the thermodynamic reasoning applies, however the membrane works. I t is the strength of thermodynamic reasoning that it avoids the uncertainties of definite hypotheses about the mechanism of the connection between the two series of phenomena it correlates; it is the weakness of thermodynamic reasoning that it fails to throw light on the intimate structure of the system which such hypotheses seek to examine. Prof. Kahlenberg’s experiments are of great interest as elucidating the mechanism of osmosis, and the relations of liquids and permeable mem- branes. But, as he himself sees, they have no bearing on the general theory of solution ; they investigate the special system involved in each case, but give no ground for co-ordinating different species of phenomena, as the thermo- dynamic theory enables us to co-ordinate osmotic pressures, freezing points, and electrolytic conductivity. Having defined osmotic pressure theoretically and correlated by aid of the conception the different phenomena of solution, it remains to consider howOSMOTIC PRESSURE 37 far the different experimental investigations conform to the theoretical conditions, and how far the quantities treated in the experiments are identical with those of pure theory. And this is why I was guilty of what Dr. Findlay evidently considers to be want of faith in thermodynamic reasoning. If Lord Berkeley and Mr. Hartley really measure the true theoretical osmotic pressure, I have no doubt that their results will conform to the theoretical values indicated by the thermodynamic cycle. If their osmotic pressure as defined in the deduction of their equation had been the same as that of Mr. Spens, I have no doubt they would have arrived at the same result as he did. How far both these assumptions are justified time, and further experiments, will show.

ISSN:0014-7672    

DOI:10.1039/TF9070300027

出版商:RSC    

年代:1907

数据来源: RSC

摘要:

THE PRESENT POSITION AND FUTURE PROSPECTS OF THE ELECTROLYTIC ALKALI AND BLEACH INDUSTRY. BY JOHN B. C. KERSHAW, F.I.C. ( A Paper read before thc Faraday Society on Tuesday, February 19, 1907, DR. T. M. LOWRY in the Chair.) I. INTRODUCTION. Over fifty-five years have passed away since Charles Watt described in some detail how sodium or potassium hydrate and chlorine compounds could be produced by electrolysis of the corresponding chloride in a diaphragm cell, and nearly twenty years have passed since a process worked upon this principle was developed industrially at Griesheim in Germany. The claim made that Watt’s patent, No. 13,755 of 1851, should be recognised as the master-patent of the electrochemical and electrometallurgical industries is substantiated, the more one studies it.The author has lately been instru- mental in having this remarkable patent reprinted by H.M. Patent Office, and all interested in the historical side of the development of these new industries can now obtain and study it for themselves. It is unnecessary to explain to this audience why Watt’s ideas lay dormant for thirty-five years, or why it was only towards the end of the eighties that electricians and chemists commenced to experiment upon an industrial scale with cells designed for the electrolytic production of alkali and chlorine from sodium or potassium chloride. The earliest attempts appear to have been made in Germany. According to Lunge,* in 1884 three German firms con- tributed to a common fund for carrying out experiments upon the electro- lysis of the alkali chlorides.It may be remarked here that this method of obtaining funds for industrial research work is widely employed in Germany, and judging by the results attained it has much to recommend it. As fruit of these experiments, which were continued for four years, the ‘( Elektroit ” diaphragm type of cell was evolved, and a small experimental plant was erected at Griesheim, near Frankfort, in 1889, to test it upon a larger scale. This plant commenced work with 200 H.P. in the following year. In 1892 this plant was doubled in size and was visited by Lunge, who convinced himself that it was working economically. A larger plant using the same cell and process was erected at Bitterfeld, in Saxony, in 1893 ; this commenced work in 1894, and wasdouhled in size in 1895.It is not the author’s purpose in this paper to trace the later developments of the ‘‘ Elektron ” cell and pro- cess. It will suffice to say that it is now adopted in the majority of the European works, and that as far as one can judge from the financial returns, it works satisfactorily and economically. Potassium chloride is the salt electrolysed in several of these works on account of the higher yield and more valuable endproducts. The diaphragms used in the cells are stated by Haber to be made of cement. With great care it is said they can be made to last two years, but the correctness of this statement may be doubted. Turning now to the history of the English processes, we find that the earliest industrial trials were made with Greenwood’s asbestos diaphragm Zcits.f. angew. Chemie, 1896, p. 517. 38ELECTROLYTIC ALKALI AND BLEACH INDUSTRY 39 cell at Snodland, in Kent, in 1891-92. These were unsuccessful, and were followed by the trials a t the same place of the Richardson and Holland bell type of cell patented in 1890. The trials of this cell at Snodland led to the promotion of the EEectrochemicaZ Company with a capital of ;GI~O,OOO, and to the erection in 1894-95 of a large works at St. Helens, in Lancashire. These works were started in December, 1895, with plant utilising 1,100 H.P. After a chequered career these works were finally closed down in 1900. The Castner cell was patented in 1892 and 1893. After trials at Oldbury the cell was exploited upon a large scale at the Castner Kellner Works, near Runcorn, erected in 1895-96, and started in 1897.The Middlewich Works of the EZectroZytic AEkaEi Company followed in 18gg-1g00, a very long period of trial having been allowed to elapse between the patenting of the Hargreaves-Bird type of cell in 1892-93 and its industrial development upon a large scale. The early experiments with this cell were carried out at Farnworth, near Widnes. As regards America, the earliest electrolytic alkali works was erected at Rumford Falls in 1892. The Le Sueur diaphragm cell was employed here, and three tons of bleach were produced per day. The Mathieson Works at Niagara Falls, using Castner’s cell, was not erected until 1897-98, and started in the latter year with 2,000 H.P. This works has since been taken over by the Castner Electrolytic Company, and has been greatly enlarged.These historical details show clearly that, although Charles Watt was the originator of the idea underlying the electrolytic alkali processes, German engineers and chemists by steady and long-continued industrial research work, were the first to develop it. German capitalists and engineers have also reaped some advantage from this early work, for the larger number of the electrolytic alkali works on the Continent of Europe are now using the ‘‘ EZektrorz” cell, and are under the technical control of the Griesheim firm. Though the financial results of the developments have in many cases been disappointing, the orders for plant and machinery placed in Germany, and the demand for chemists and managers to take charge of these subsidiary works, likewise drawn from the Fatherland, will, no doubt, have been regarded as satisfactory compensation for the small dividend-earning capacity of certain of these undertakings, especially when floated, as many were, with the aid of foreign capital. In the following pages it is not the author’s intention to give a detailed description of each cell and process now in actual use for the production of alkalies and chlorine compounds by the electrolysis of chlorides.The majority of the members of this Society are already well acquainted with these details, or know exactly where to find them when required. What the author proposes to give is a list of works now operating in Europe and America, with a brief summary of the facts relating to the power used, type of cell and process employed, and products made.This information is based as far as possible upon the replies to a circular letter sent out to all the companies operating these processes in the spring of I@. The replies to this letter were, however, disappointing, since in a very large number of instances a letter stating that no information could be given was received. It has therefore been necessary to supplement the official information by facts and figures drawn from reliable technical journals and from other sources, and from private reports sent to the author by correspon- dents in France and other countries. Only those who have attempted to obtain information as to the progress of the electrochemical industries will realise the excessive modesty of electrochemical firms about their achieve- ments, or the objection there is on the part of manufacturers to allow the40 POSITION AND FUTURE PROSPECTS OF THE world to know what is being done and what progress is being made.Whether this reticence is a sign of prosperity or the reverse, the writer will not attempt to decide. 11. DETAILS OF PROCESSES AND W O R K S . UNITED KINGDOM. I. The Castner Kellner Alkali Company. Established 1898. New works opened I@. Works: Weston Point, Process.-Castner mercury process with Kellner’s modification. Products.-Caustic soda, bleaching powder, sodium, sodium peroxide, zinc chloride. Power.-Weston Point, 5,000 to 7,000 H.P., Mond gas-producer plant and gas engines. Wallsend, 2,000 H.P., to be purchased from Supply Company.Cheshire ; Wallsend, Newcastle. 2. The Electrolytic Alkali Company. Established 1901. Process.-Hargreaves-Bird diaphragm. Products.-Carbonate of soda, bleaching powder, salt, caustic lime. Power.-3,000 H.P., steam. The Electrochemical Company, which worked the Richardson and Holland bell cell at St. Helens from 1895 to I ~ O , failed to attain financial success, and liquidated its assets in I ~ I . Messrs. Brunner, M o d 6 Co. manufacture bleach by the Hoepfner process at Winnington, in Cheshire, but this works can hardly be classed as an electrolytic alkali and bleach works. FRANCE. Works ; Middlewich, Cheshire. 3. Sociktd Industrielle des Produits Chimiques. Established 1900. Works at La Motte-Breuil. Process.--“ Elektron,” Products.-Caustic soda and bleaching powder.P0wer.-2,000 H,P., steam. The following works have ceased the manufacture of alkali and bleach by the electrolytic method in France during the last few years :- Societk des Soudieres Electrolytiques . . . . . . Les Clavaux. Sociktb la Volta Lyonnaise . . . . . . . . . . . . Montiers. Sociktk des Fives-Lille . . . . . . . . . . . . Bozel. Societ6 des Produits Chimiques . . . . . . . . . Monthey. In some of these works subsidiary electrolytic manufactures, such as the production of sodium and sodium peroxide, are now being carried on, and, no doubt, if the financial position in the alkali industry improved, some portion of the power available would again be devoted to the production of alkali and bleach. SWITZERLAND. 4. Socidtd la Volta Suisse de l’lndustrie Electrochimique.Works .- Chevrks, near Geneva. Process.-Outhenin Chalandre diaphragm. Produck-Caustic soda and bleaching powder, Power.-r,ooo H.P., water,ELECTROLYTIC ALKALI AND BLEACH INDUSTRY 41 GERMANY. 5 and 6. Chemische Fabrik Griesheim (( Elektron.” Established 189. Works at Griesheim, near Frankfort, and at Bitterfeld. Process.--“ Elektron.” Produck-Caustic potash, caustic soda, potassium, sodium, magnesium, Power.-Griesheim, 400 H.P., steam. Established 1894. Process.--“ Elektron.” Produck-Alkalies and bleach, sodium, magnesium. Power.-Bitterfeld, 3,500 H.Y., steam. These last two works have recently been taken over by the Chemische Fabrik Griesheim “Elektron,” and the four works are now under the one management and control. and bleaching powder.Bitterfeld, 3,600 H.P., steam. 7 and 8. Elektrochemische Werke. G.m.b.H. Works at Bitterfeld and at Rheinfelden. Rheinfelden, 2,800 H.P., water. 9. Badische Anilin und Soda Fabrik. Works : Ludwigshafen. Process.-(‘ E 1 e k t ron . ” Products.-Caustic soda and potash and chlorine compounds. Power.-3,3oo H.P., steam. 10. Consolidierte Alkali Works. Process.--“ Elektron.” Products.-Caustic potash and bleaching powder. Power.-1,300 H.P., steam. Established 1896. Works : Osternienberg. Process.-Solvay mercury process. Products.-Caustic potash and bleaching powder. Power.-1,5oo H.P., steam. Works : Westeregeln. 11, Deutsche Solvay Company. AUSTRIA. 12. Oest. Verein f. Chemische Productioiz. Works : Aussig. Established I 8%. Process.-Bell gravity process.Products.-Caustic soda and bleaching powder. Power.--2,ooo H.P., steam. Established 1900. Process.-Kellner mercury process. Products.-Alkali and chlorine compounds. Power.-Golling, 200 H.P. ; Jaice, 1,000 H.P. ; water. The author is informed by his correspondent in Austria that the projected 13 and 14. Consortium f. Elektrochem. Industrien. Works : Golling and Jaice in Bosnia. enlargement of the experimental works at Golling has not been carried out. RUSSIA. 15. Lubinoff Solvay G Cie. Established I ~ I . Works : Donetz. Process.-Solvay mercury. Products.-Caustic soda and bleaching powder, Powe~.-1,500 H.P., steam,42 POSITION AND FUTURE PROSPECTS OF THE 16. Zabkowickie Zaklady Elektrochemiczne. T. A. Elektrycznon. Established 18%. Works : Zabkowickie. Process.- Produck-Caustic soda and bleaching powder.Power.-I,zoo H.P., steam. 17. Gesellschaft Russki Elektron.” Established 1900. Works ; Slaviansk. Process.--“ Elektron.” Products.-Caustic soda and bleaching powder. Puwer.--I,ooo H.P., steam. In view of the disturbed state of Russian finance and industry, it may be doubted whether these three works are now operating, and in the tabular statement at the elid of this section they have been classed as ‘‘ suspended.” ITALY. 18. Societa Elettrica ed Elettrochimica del Cajaro. Established I@. Process.- Produck-Caustic soda and bleaching powder. Works ; Caffaro Falls. 19 and 20. Societa Elettrochimica Ztaliana. Works : Piano dorte and Bussi. Process.-Outhenin-Chalandre. Produck-Alkalies and bleaching powder, chlorates, carbon tetrachloride, Power.4,ooo H.P., water.Elter, in a recent report, states that the financial results obtained in these two works have been disappointing, owing to their unfavourable position and to the high price of salt. pure hydrochloric acid. SPAIN. 21. Sociedad Electro-Quimica de Flix. Established 190. Process.--“ Elektron.” Products.-Caustic soda, bleaching powder, and hypochlorite. Power.-3,ooo H.P., water. 22. Sociedad Anon. Electra del Besaya. Established 1 9 1 . Works : Santander. Process.-Outhenin-Chalandre. Products.-Caustic soda and bleaching powder. Power.-7oo H. P. 23. L’ Usine d’Aboizos. Works: Flix on the Ebro. Works : Gijon. Gas power. At present stopped. BELGIUM. 24. Solvay & Cie. Established 1898. Works : Jemeppe-sur-Sambre. Process.-Solvay mercury process.Produck-Caustic soda and bleaching powder Power.-1,5oo H.P., steam:ELECTROLYTIC ALKALI AND BLEACH INDUSTRY 43 UNITED STATES. Works : Niagara Falls. 25. Castrier Electrolytic Alkali Company. Established IS@. Process.-Castner Kellner mercury. Produck-Caustic soda and bleaching powder. Power.4,ooo H.P., water. 26. The Pennsylvania Salt Company. Established . Works : Wyandotte, Michigan. Process.-Bell Brothers’ mercury. Products.-Caustic soda and bleaching powder. Power. - H.P., gas. 27. The Roberts Chemical Company. Established . Works : Niagara Falls. Process.-Ro bert s’ diaphragm. Products.-Caustic potash, pure hydrochloric acid. Power.-soo H.P., water. This works is reported to have been burnt down recently. 28. The Acker Process Company.Established 18%. Works : Niagara Falls. Process.-Acker fusion with lead cathode. Produck-Caustic soda, bleaching powder, carbon tretrachloride and tin Power.--z,ooo H.P., water. Established 1900. Process.-Le Sueur diaphragm. Products.-Bleaching liquors only. Power.-7oo H.P., water. 30. The Dow Process Company. Established . Works : Midland, Michigan. Process.- Do w diaphragm. Products.-Bleaching liquors, bromine. Power,-- H.P., steam. tetrachloride, sulphur bichloride. 29. The Burgess Sulfihite WDod Pulp Company. Works ; Berlin Falls. CANADA. Established ~gor. Works : Sault Sainte Marie. Process.-Rhodin. Products.-Caustic soda and bleaching powder. Power.-2,ooo H.P., water. This works only operated from January, 1901, to September, 1902, and is 31. The American Alkali Company.now shut down. A new works, using the Townsend diaphragm cell, is about to be started at Niagara Falls. There are, in addition, in America a considerable number of small electro- lytic alkali installations worked in connection with wood-pulp mills. These employ, as a rule, a diaphragm cell process, and use the caustic soda liquors and chlorine directly in the pulp mill. The installation of the MacDonald cell at the Clarion Paper Mills, Johnsonburg, Pa., is of this type. The same cell and process are being employed at Colorado City, for providing a solution to open up gold-bearing ores, These plants, however, cannot be correctly classed as electrolytic alkali works.44 POSITION AND FUTURE PROSPECTS OF T H E Summarising the details given in this section of the paper, we obtain the following totals :- 2 I I 7 3 3 0 2 I 7 27 -- Country.I 0 0 0 0 0 0 0 0 I 2 United Kingdom ... France . . . . . . Switzerland ... Germany ...... Austria ...... Russia . . . . . . Italy . . . . . . Spain . . . . . . Belgium ... United States aI;'ci Canada ...... Totals. . . . . . Works. Operating. I Closed. juspended 0 0 0 0 0 I 0 0 4 3 8 Building. I 0 0 0 0 0 0 0 0 0 I Horse Power.* The totals show that about 55,000 H.P. are now being devoted to the production of alkalies and bleach by the electrolytic method, and that plant representing about 13,000 H.P. is lying in reserve, ready to be put into operation when the local conditions improve. Assuming that all the plants are being worked to the best advantage, the production of 70 per cent.caustic soda at present would be about I 10,000 tons per annum, with an equivalent of 231,000 tons of 35 per cent. bleaching powder (2 tons of caustic and 4'2 tons of bleach per E.H.P. year). There are no figures available for showing how these totals compare with the production of alkalies and bleach by the old Le Blanc Works, but it is of interest to note that the exports of bleaching powder by this country since 1900 have averaged about 50,000 tons per annum. In the year 1896, which is the last year for which the figures are available, 360,000 tons of salt were decomposed by the Le Blanc Works in the United Kingdom. At the rate of 36 cwts. of salt per ton of bleach this would be equivalent to a production of 200,000 tons of bleaching powder.111. THE FUTURE OF THE ELECTROLYTIC INDUSTRY. Prophecy in these modern days, when so many influences are at work to disturb the general lines of progress, is more uncertain and hazardous than in the days of our ancestors. It would be a mistake, however, to close this paper without some attempt to forecast the future development of the electrolytic alkali industry, and to indicate the lines upon which its further progress is likely to occur. The figures given at the close of the second section of this paper prove that the development of the industry since 1g0a has undergone a check, for Professor Borchers, in a paper published in the Zeits. f. Elektrochemie, of July 20, 1899, gave a list of works engaged in this industry, and estimated the aggregate production of bleaching powder in that year at 225,000 tons.Further evidence of this check is found in the reports received by the writer of the present position of the industry in * These figures must be regarded as estimates only, and as not of official value.ELECTROLYTIC ALKALI AND BLEACH INDUSTRY 45 Bleaching Powder. Price per Ton (f.0.r.). France and Spain, from correspondents who are well acquainted with the facts. In France the industry is stated to have had a set-back during the last five years, an experience partly due to the defects of the processes themselves, and partly due to the competition from the old Le Blanc Works. These have sold bleaching powder at so low a price that its sale has not yielded any profit to the electrolytic works, and the competition has thus discouraged the financial development of newer and more efficient electro- lytic processes, In Spain it is stated that the prospects of the industryat present are not bright.The Le Blanc Works in Europe are, in fact, making a better fight than was expected ten years ago, and it would seem probable that they will continue to exist side by side with the electrolytic works for many years to come. It is interesting to note that when the Ammonia Soda process was introduced into this country, the early decease of the older Le Blanc process was likewise confidently predicted. Threatened industries, however, like ailing people, have sometimes a long life. In the writer’s opinion all three types of works, Le Blaitc, Electrolytic, and Ainmoiria Soda, will find a place in the chemical industry of the future.The character of the chief manufactured products in each type of works may change, but the works themselves will survive. Some indications of this change already may be observed, for bleaching powder and caustic or carbonate of soda are no longer providing large profits for their producers by the Le Blanc or electrolytic processes, and it is the by-products and special manufactures of each type of works that are now providing the dividends, The following tabular statement, giving the average price of bleaching powder and 58 per cent. soda ash in December of each year for the period 1895-1906, is of interest in this connection, since, if studied in coniiection with the balance-sheets of the various companies, it serves to indicate the source of the profits of the three processes used in the manufacture of alkali and bleach :- 58 per Cent.Soda Ash. Price per Ton (f.0.r.). Year. & s. d. 7 0 0 6 1 2 6 6 2 6 6 0 0 6 17 6 6 1 7 6 4 0 0 4 I 0 0 4 I 0 0 4 I 3 6 6 5 0 4 1 6 ;;7 2 4 7 ; 3 I0 0 4 0 0 4 12 4 I 0 0 4 I 0 0 4 I 0 0 4 I 0 0 4 I 0 0 3 1 7 6 4 I7 6 (Prices compiled by The Chemical Trade gournal.) The writer’s conclusions regarding the future of the alkali and bleach industry generally may be summarised as follows :- The Le Blanc Works will continue to produce caustic and carbonated alkali and bleaching powder, but in diminishing quantities, and the dividends earned by manufacture of these staple products (which once yielded large profits) will declinc. But compensation for this loss will be found in the46 POSITION AND FUTURE PROSPECTS OF THE manufacture of sulphuric acid, sodium sulphate, hyposulphite, sulphide, and other derivatives from these sulphur salts which cannot be manufactured by the processes of either of their rivals, and even the impurities of the pyrites burnt in the kilns for the lead chamber process of sulphuric acid manufacture may be looked upon as an important source of revenue in years to come.Copper sulphate is another by-product, yielding its makers handsome profits, which can be most successfully made by the Le Blanc manufacturers. The Le Blanc Alkali Works will then depend more and more upon by-products, and less and less upon the staple products, soda and bleach, for their future profits. With this change will come the need for increased foresight and scientific supervision in their management and control.The Electrolytic Works, when operating a good process, and when under wise financial management, will gradually take the place of the Le Blanc Works as producers of bleaching powder, chlorates, and other chlorine pro- ducts. The manufactiire of metallic sodium and its derivatives, such as cyanides and peroxides, will also fall into the hands of the electrolytic manu- facturers. The cost of operating the electrolytic process of alkali manufacture is, however, greater than was at first estimated, and only the best processes and best equipped works will survive in the struggle for existence. The Ammonia Soda Works, both in this country and abroad, will continue to make enormous profits out of the manufacture of carbonate and bi- carbonate of soda by the Solway process. But what is regarded by many as the best feature of these works, namely, the absence of by-products, is, judged from another standpoint, a source of weakness. The Ammonia Soda Works can never entirely take the place of the Le Blanc or Electrolytic Works, since neither sulphur nor chlorine products can be made a subsidiary and successful part of the manufacture of soda by the Solvay process.All attempts hitherto made to extract chlorine success- fully from the waste calcium chloride liquors of these works have failed, and the Hoepfner process of zinc extraction, which has been in operation at Winnington for some years, cannot be considered to meet the demand for a satisfactory and profitable use for these waste liquors of the Solvay process.If such a process be ever found and successfully developed, the forecasts given in this article will require revision, and their author may have to accept the fate of many another prophet and to confess that in this world l1 nothing is certain but the unforeseen.” APPENDIX. MERCURY TYPE OF CELLS. I. Castner Kellner. The Electrical Review (London), March 31, 1899. Zeits. f. Elektrochemie, September 8, 1905. Electrochemical Industry, November, 1905. 2. Solvay. DIAPHRAGM TYPE OF CELL. I. Hargreaves-Bird. The Electrician, February 12, 1897, and February 18, 1898. The Electrician, March 14, 1902. Electricity (New York), April 17, IYI. Electrochemical Industry, May, 1905.Electrochemical Industry, January, I+. 2. Outlzeitiia-Chalaiadre. 3. Townsend. 4 Zerr and Whitham.ELECTROLYTIC ALKALI AND BLEACH INDUSTRY 47 FUSION TYPE OF CELL. I. Hulin. 2 . Acker. Electrician, March 4, 1898. Electrician, October 25, 1901. GRAVITY TYPE OF CELL. I. Aussag bell" cell. Electrician, February 7, q o z . Zeits. f. Elektrochemie, October 10, 1 9 1 . Electrochemical Industry, June, 1904. GENERAL ARTICLES. Siebert. Frarake. Elektrochemische Zeitschrift, November, 1g01 American Electrician, March, 1901. THE PRESENT POSITION AND FUTURE PROSPECTS OF THE ELECTROLYTIC ALKALI AND BLEACH INDUSTRY. BY JOHN B. C. KERSHAW, F.I.C. ( A Paper read before thc Faraday Society on Tuesday, February 19, 1907, DR. T. M. LOWRY in the Chair.) I. INTRODUCTION.Over fifty-five years have passed away since Charles Watt described in some detail how sodium or potassium hydrate and chlorine compounds could be produced by electrolysis of the corresponding chloride in a diaphragm cell, and nearly twenty years have passed since a process worked upon this principle was developed industrially at Griesheim in Germany. The claim made that Watt’s patent, No. 13,755 of 1851, should be recognised as the master-patent of the electrochemical and electrometallurgical industries is substantiated, the more one studies it. The author has lately been instru- mental in having this remarkable patent reprinted by H.M. Patent Office, and all interested in the historical side of the development of these new industries can now obtain and study it for themselves.It is unnecessary to explain to this audience why Watt’s ideas lay dormant for thirty-five years, or why it was only towards the end of the eighties that electricians and chemists commenced to experiment upon an industrial scale with cells designed for the electrolytic production of alkali and chlorine from sodium or potassium chloride. The earliest attempts appear to have been made in Germany. According to Lunge,* in 1884 three German firms con- tributed to a common fund for carrying out experiments upon the electro- lysis of the alkali chlorides. It may be remarked here that this method of obtaining funds for industrial research work is widely employed in Germany, and judging by the results attained it has much to recommend it. As fruit of these experiments, which were continued for four years, the ‘( Elektroit ” diaphragm type of cell was evolved, and a small experimental plant was erected at Griesheim, near Frankfort, in 1889, to test it upon a larger scale.This plant commenced work with 200 H.P. in the following year. In 1892 this plant was doubled in size and was visited by Lunge, who convinced himself that it was working economically. A larger plant using the same cell and process was erected at Bitterfeld, in Saxony, in 1893 ; this commenced work in 1894, and wasdouhled in size in 1895. It is not the author’s purpose in this paper to trace the later developments of the ‘‘ Elektron ” cell and pro- cess. It will suffice to say that it is now adopted in the majority of the European works, and that as far as one can judge from the financial returns, it works satisfactorily and economically.Potassium chloride is the salt electrolysed in several of these works on account of the higher yield and more valuable endproducts. The diaphragms used in the cells are stated by Haber to be made of cement. With great care it is said they can be made to last two years, but the correctness of this statement may be doubted. Turning now to the history of the English processes, we find that the earliest industrial trials were made with Greenwood’s asbestos diaphragm Zcits. f. angew. Chemie, 1896, p. 517. 38ELECTROLYTIC ALKALI AND BLEACH INDUSTRY 39 cell at Snodland, in Kent, in 1891-92. These were unsuccessful, and were followed by the trials a t the same place of the Richardson and Holland bell type of cell patented in 1890.The trials of this cell at Snodland led to the promotion of the EEectrochemicaZ Company with a capital of ;GI~O,OOO, and to the erection in 1894-95 of a large works at St. Helens, in Lancashire. These works were started in December, 1895, with plant utilising 1,100 H.P. After a chequered career these works were finally closed down in 1900. The Castner cell was patented in 1892 and 1893. After trials at Oldbury the cell was exploited upon a large scale at the Castner Kellner Works, near Runcorn, erected in 1895-96, and started in 1897. The Middlewich Works of the EZectroZytic AEkaEi Company followed in 18gg-1g00, a very long period of trial having been allowed to elapse between the patenting of the Hargreaves-Bird type of cell in 1892-93 and its industrial development upon a large scale.The early experiments with this cell were carried out at Farnworth, near Widnes. As regards America, the earliest electrolytic alkali works was erected at Rumford Falls in 1892. The Le Sueur diaphragm cell was employed here, and three tons of bleach were produced per day. The Mathieson Works at Niagara Falls, using Castner’s cell, was not erected until 1897-98, and started in the latter year with 2,000 H.P. This works has since been taken over by the Castner Electrolytic Company, and has been greatly enlarged. These historical details show clearly that, although Charles Watt was the originator of the idea underlying the electrolytic alkali processes, German engineers and chemists by steady and long-continued industrial research work, were the first to develop it.German capitalists and engineers have also reaped some advantage from this early work, for the larger number of the electrolytic alkali works on the Continent of Europe are now using the ‘‘ EZektrorz” cell, and are under the technical control of the Griesheim firm. Though the financial results of the developments have in many cases been disappointing, the orders for plant and machinery placed in Germany, and the demand for chemists and managers to take charge of these subsidiary works, likewise drawn from the Fatherland, will, no doubt, have been regarded as satisfactory compensation for the small dividend-earning capacity of certain of these undertakings, especially when floated, as many were, with the aid of foreign capital.In the following pages it is not the author’s intention to give a detailed description of each cell and process now in actual use for the production of alkalies and chlorine compounds by the electrolysis of chlorides. The majority of the members of this Society are already well acquainted with these details, or know exactly where to find them when required. What the author proposes to give is a list of works now operating in Europe and America, with a brief summary of the facts relating to the power used, type of cell and process employed, and products made. This information is based as far as possible upon the replies to a circular letter sent out to all the companies operating these processes in the spring of I@. The replies to this letter were, however, disappointing, since in a very large number of instances a letter stating that no information could be given was received.It has therefore been necessary to supplement the official information by facts and figures drawn from reliable technical journals and from other sources, and from private reports sent to the author by correspon- dents in France and other countries. Only those who have attempted to obtain information as to the progress of the electrochemical industries will realise the excessive modesty of electrochemical firms about their achieve- ments, or the objection there is on the part of manufacturers to allow the40 POSITION AND FUTURE PROSPECTS OF THE world to know what is being done and what progress is being made.Whether this reticence is a sign of prosperity or the reverse, the writer will not attempt to decide. 11. DETAILS OF PROCESSES AND W O R K S . UNITED KINGDOM. I. The Castner Kellner Alkali Company. Established 1898. New works opened I@. Works: Weston Point, Process.-Castner mercury process with Kellner’s modification. Products.-Caustic soda, bleaching powder, sodium, sodium peroxide, zinc chloride. Power.-Weston Point, 5,000 to 7,000 H.P., Mond gas-producer plant and gas engines. Wallsend, 2,000 H.P., to be purchased from Supply Company. Cheshire ; Wallsend, Newcastle. 2. The Electrolytic Alkali Company. Established 1901. Process.-Hargreaves-Bird diaphragm. Products.-Carbonate of soda, bleaching powder, salt, caustic lime. Power.-3,000 H.P., steam. The Electrochemical Company, which worked the Richardson and Holland bell cell at St.Helens from 1895 to I ~ O , failed to attain financial success, and liquidated its assets in I ~ I . Messrs. Brunner, M o d 6 Co. manufacture bleach by the Hoepfner process at Winnington, in Cheshire, but this works can hardly be classed as an electrolytic alkali and bleach works. FRANCE. Works ; Middlewich, Cheshire. 3. Sociktd Industrielle des Produits Chimiques. Established 1900. Works at La Motte-Breuil. Process.--“ Elektron,” Products.-Caustic soda and bleaching powder. P0wer.-2,000 H,P., steam. The following works have ceased the manufacture of alkali and bleach by the electrolytic method in France during the last few years :- Societk des Soudieres Electrolytiques . . . . .. Les Clavaux. Sociktb la Volta Lyonnaise . . . . . . . . . . . . Montiers. Sociktk des Fives-Lille . . . . . . . . . . . . Bozel. Societ6 des Produits Chimiques . . . . . . . . . Monthey. In some of these works subsidiary electrolytic manufactures, such as the production of sodium and sodium peroxide, are now being carried on, and, no doubt, if the financial position in the alkali industry improved, some portion of the power available would again be devoted to the production of alkali and bleach. SWITZERLAND. 4. Socidtd la Volta Suisse de l’lndustrie Electrochimique. Works .- Chevrks, near Geneva. Process.-Outhenin Chalandre diaphragm. Produck-Caustic soda and bleaching powder, Power.-r,ooo H.P., water,ELECTROLYTIC ALKALI AND BLEACH INDUSTRY 41 GERMANY. 5 and 6.Chemische Fabrik Griesheim (( Elektron.” Established 189. Works at Griesheim, near Frankfort, and at Bitterfeld. Process.--“ Elektron.” Produck-Caustic potash, caustic soda, potassium, sodium, magnesium, Power.-Griesheim, 400 H.P., steam. Established 1894. Process.--“ Elektron.” Produck-Alkalies and bleach, sodium, magnesium. Power.-Bitterfeld, 3,500 H.Y., steam. These last two works have recently been taken over by the Chemische Fabrik Griesheim “Elektron,” and the four works are now under the one management and control. and bleaching powder. Bitterfeld, 3,600 H.P., steam. 7 and 8. Elektrochemische Werke. G.m.b.H. Works at Bitterfeld and at Rheinfelden. Rheinfelden, 2,800 H.P., water. 9. Badische Anilin und Soda Fabrik. Works : Ludwigshafen. Process.-(‘ E 1 e k t ron .” Products.-Caustic soda and potash and chlorine compounds. Power.-3,3oo H.P., steam. 10. Consolidierte Alkali Works. Process.--“ Elektron.” Products.-Caustic potash and bleaching powder. Power.-1,300 H.P., steam. Established 1896. Works : Osternienberg. Process.-Solvay mercury process. Products.-Caustic potash and bleaching powder. Power.-1,5oo H.P., steam. Works : Westeregeln. 11, Deutsche Solvay Company. AUSTRIA. 12. Oest. Verein f. Chemische Productioiz. Works : Aussig. Established I 8%. Process.-Bell gravity process. Products.-Caustic soda and bleaching powder. Power.--2,ooo H.P., steam. Established 1900. Process.-Kellner mercury process. Products.-Alkali and chlorine compounds. Power.-Golling, 200 H.P. ; Jaice, 1,000 H.P. ; water. The author is informed by his correspondent in Austria that the projected 13 and 14. Consortium f.Elektrochem. Industrien. Works : Golling and Jaice in Bosnia. enlargement of the experimental works at Golling has not been carried out. RUSSIA. 15. Lubinoff Solvay G Cie. Established I ~ I . Works : Donetz. Process.-Solvay mercury. Products.-Caustic soda and bleaching powder, Powe~.-1,500 H.P., steam,42 POSITION AND FUTURE PROSPECTS OF THE 16. Zabkowickie Zaklady Elektrochemiczne. T. A. Elektrycznon. Established 18%. Works : Zabkowickie. Process.- Produck-Caustic soda and bleaching powder. Power.-I,zoo H.P., steam. 17. Gesellschaft Russki Elektron.” Established 1900. Works ; Slaviansk. Process.--“ Elektron.” Products.-Caustic soda and bleaching powder. Puwer.--I,ooo H.P., steam.In view of the disturbed state of Russian finance and industry, it may be doubted whether these three works are now operating, and in the tabular statement at the elid of this section they have been classed as ‘‘ suspended.” ITALY. 18. Societa Elettrica ed Elettrochimica del Cajaro. Established I@. Process.- Produck-Caustic soda and bleaching powder. Works ; Caffaro Falls. 19 and 20. Societa Elettrochimica Ztaliana. Works : Piano dorte and Bussi. Process.-Outhenin-Chalandre. Produck-Alkalies and bleaching powder, chlorates, carbon tetrachloride, Power.4,ooo H.P., water. Elter, in a recent report, states that the financial results obtained in these two works have been disappointing, owing to their unfavourable position and to the high price of salt. pure hydrochloric acid.SPAIN. 21. Sociedad Electro-Quimica de Flix. Established 190. Process.--“ Elektron.” Products.-Caustic soda, bleaching powder, and hypochlorite. Power.-3,ooo H.P., water. 22. Sociedad Anon. Electra del Besaya. Established 1 9 1 . Works : Santander. Process.-Outhenin-Chalandre. Products.-Caustic soda and bleaching powder. Power.-7oo H. P. 23. L’ Usine d’Aboizos. Works: Flix on the Ebro. Works : Gijon. Gas power. At present stopped. BELGIUM. 24. Solvay & Cie. Established 1898. Works : Jemeppe-sur-Sambre. Process.-Solvay mercury process. Produck-Caustic soda and bleaching powder Power.-1,5oo H.P., steam:ELECTROLYTIC ALKALI AND BLEACH INDUSTRY 43 UNITED STATES. Works : Niagara Falls. 25. Castrier Electrolytic Alkali Company. Established IS@.Process.-Castner Kellner mercury. Produck-Caustic soda and bleaching powder. Power.4,ooo H.P., water. 26. The Pennsylvania Salt Company. Established . Works : Wyandotte, Michigan. Process.-Bell Brothers’ mercury. Products.-Caustic soda and bleaching powder. Power. - H.P., gas. 27. The Roberts Chemical Company. Established . Works : Niagara Falls. Process.-Ro bert s’ diaphragm. Products.-Caustic potash, pure hydrochloric acid. Power.-soo H.P., water. This works is reported to have been burnt down recently. 28. The Acker Process Company. Established 18%. Works : Niagara Falls. Process.-Acker fusion with lead cathode. Produck-Caustic soda, bleaching powder, carbon tretrachloride and tin Power.--z,ooo H.P., water. Established 1900. Process.-Le Sueur diaphragm. Products.-Bleaching liquors only.Power.-7oo H.P., water. 30. The Dow Process Company. Established . Works : Midland, Michigan. Process.- Do w diaphragm. Products.-Bleaching liquors, bromine. Power,-- H.P., steam. tetrachloride, sulphur bichloride. 29. The Burgess Sulfihite WDod Pulp Company. Works ; Berlin Falls. CANADA. Established ~gor. Works : Sault Sainte Marie. Process.-Rhodin. Products.-Caustic soda and bleaching powder. Power.-2,ooo H.P., water. This works only operated from January, 1901, to September, 1902, and is 31. The American Alkali Company. now shut down. A new works, using the Townsend diaphragm cell, is about to be started at Niagara Falls. There are, in addition, in America a considerable number of small electro- lytic alkali installations worked in connection with wood-pulp mills.These employ, as a rule, a diaphragm cell process, and use the caustic soda liquors and chlorine directly in the pulp mill. The installation of the MacDonald cell at the Clarion Paper Mills, Johnsonburg, Pa., is of this type. The same cell and process are being employed at Colorado City, for providing a solution to open up gold-bearing ores, These plants, however, cannot be correctly classed as electrolytic alkali works.44 POSITION AND FUTURE PROSPECTS OF T H E Summarising the details given in this section of the paper, we obtain the following totals :- 2 I I 7 3 3 0 2 I 7 27 -- Country. I 0 0 0 0 0 0 0 0 I 2 United Kingdom ... France . . . . . . Switzerland ... Germany ...... Austria ...... Russia . . . . . . Italy . .. . . . Spain . . . . . . Belgium ... United States aI;'ci Canada ...... Totals. . . . . . Works. Operating. I Closed. juspended 0 0 0 0 0 I 0 0 4 3 8 Building. I 0 0 0 0 0 0 0 0 0 I Horse Power.* The totals show that about 55,000 H.P. are now being devoted to the production of alkalies and bleach by the electrolytic method, and that plant representing about 13,000 H.P. is lying in reserve, ready to be put into operation when the local conditions improve. Assuming that all the plants are being worked to the best advantage, the production of 70 per cent. caustic soda at present would be about I 10,000 tons per annum, with an equivalent of 231,000 tons of 35 per cent. bleaching powder (2 tons of caustic and 4'2 tons of bleach per E.H.P. year). There are no figures available for showing how these totals compare with the production of alkalies and bleach by the old Le Blanc Works, but it is of interest to note that the exports of bleaching powder by this country since 1900 have averaged about 50,000 tons per annum.In the year 1896, which is the last year for which the figures are available, 360,000 tons of salt were decomposed by the Le Blanc Works in the United Kingdom. At the rate of 36 cwts. of salt per ton of bleach this would be equivalent to a production of 200,000 tons of bleaching powder. 111. THE FUTURE OF THE ELECTROLYTIC INDUSTRY. Prophecy in these modern days, when so many influences are at work to disturb the general lines of progress, is more uncertain and hazardous than in the days of our ancestors. It would be a mistake, however, to close this paper without some attempt to forecast the future development of the electrolytic alkali industry, and to indicate the lines upon which its further progress is likely to occur.The figures given at the close of the second section of this paper prove that the development of the industry since 1g0a has undergone a check, for Professor Borchers, in a paper published in the Zeits. f. Elektrochemie, of July 20, 1899, gave a list of works engaged in this industry, and estimated the aggregate production of bleaching powder in that year at 225,000 tons. Further evidence of this check is found in the reports received by the writer of the present position of the industry in * These figures must be regarded as estimates only, and as not of official value.ELECTROLYTIC ALKALI AND BLEACH INDUSTRY 45 Bleaching Powder.Price per Ton (f.0.r.). France and Spain, from correspondents who are well acquainted with the facts. In France the industry is stated to have had a set-back during the last five years, an experience partly due to the defects of the processes themselves, and partly due to the competition from the old Le Blanc Works. These have sold bleaching powder at so low a price that its sale has not yielded any profit to the electrolytic works, and the competition has thus discouraged the financial development of newer and more efficient electro- lytic processes, In Spain it is stated that the prospects of the industryat present are not bright. The Le Blanc Works in Europe are, in fact, making a better fight than was expected ten years ago, and it would seem probable that they will continue to exist side by side with the electrolytic works for many years to come.It is interesting to note that when the Ammonia Soda process was introduced into this country, the early decease of the older Le Blanc process was likewise confidently predicted. Threatened industries, however, like ailing people, have sometimes a long life. In the writer’s opinion all three types of works, Le Blaitc, Electrolytic, and Ainmoiria Soda, will find a place in the chemical industry of the future. The character of the chief manufactured products in each type of works may change, but the works themselves will survive. Some indications of this change already may be observed, for bleaching powder and caustic or carbonate of soda are no longer providing large profits for their producers by the Le Blanc or electrolytic processes, and it is the by-products and special manufactures of each type of works that are now providing the dividends, The following tabular statement, giving the average price of bleaching powder and 58 per cent.soda ash in December of each year for the period 1895-1906, is of interest in this connection, since, if studied in coniiection with the balance-sheets of the various companies, it serves to indicate the source of the profits of the three processes used in the manufacture of alkali and bleach :- 58 per Cent. Soda Ash. Price per Ton (f.0.r.). Year. & s. d. 7 0 0 6 1 2 6 6 2 6 6 0 0 6 17 6 6 1 7 6 4 0 0 4 I 0 0 4 I 0 0 4 I 3 6 6 5 0 4 1 6 ;;7 2 4 7 ; 3 I0 0 4 0 0 4 12 4 I 0 0 4 I 0 0 4 I 0 0 4 I 0 0 4 I 0 0 3 1 7 6 4 I7 6 (Prices compiled by The Chemical Trade gournal.) The writer’s conclusions regarding the future of the alkali and bleach industry generally may be summarised as follows :- The Le Blanc Works will continue to produce caustic and carbonated alkali and bleaching powder, but in diminishing quantities, and the dividends earned by manufacture of these staple products (which once yielded large profits) will declinc.But compensation for this loss will be found in the46 POSITION AND FUTURE PROSPECTS OF THE manufacture of sulphuric acid, sodium sulphate, hyposulphite, sulphide, and other derivatives from these sulphur salts which cannot be manufactured by the processes of either of their rivals, and even the impurities of the pyrites burnt in the kilns for the lead chamber process of sulphuric acid manufacture may be looked upon as an important source of revenue in years to come.Copper sulphate is another by-product, yielding its makers handsome profits, which can be most successfully made by the Le Blanc manufacturers. The Le Blanc Alkali Works will then depend more and more upon by-products, and less and less upon the staple products, soda and bleach, for their future profits. With this change will come the need for increased foresight and scientific supervision in their management and control. The Electrolytic Works, when operating a good process, and when under wise financial management, will gradually take the place of the Le Blanc Works as producers of bleaching powder, chlorates, and other chlorine pro- ducts.The manufactiire of metallic sodium and its derivatives, such as cyanides and peroxides, will also fall into the hands of the electrolytic manu- facturers. The cost of operating the electrolytic process of alkali manufacture is, however, greater than was at first estimated, and only the best processes and best equipped works will survive in the struggle for existence. The Ammonia Soda Works, both in this country and abroad, will continue to make enormous profits out of the manufacture of carbonate and bi- carbonate of soda by the Solway process. But what is regarded by many as the best feature of these works, namely, the absence of by-products, is, judged from another standpoint, a source of weakness. The Ammonia Soda Works can never entirely take the place of the Le Blanc or Electrolytic Works, since neither sulphur nor chlorine products can be made a subsidiary and successful part of the manufacture of soda by the Solvay process. All attempts hitherto made to extract chlorine success- fully from the waste calcium chloride liquors of these works have failed, and the Hoepfner process of zinc extraction, which has been in operation at Winnington for some years, cannot be considered to meet the demand for a satisfactory and profitable use for these waste liquors of the Solvay process. If such a process be ever found and successfully developed, the forecasts given in this article will require revision, and their author may have to accept the fate of many another prophet and to confess that in this world l1 nothing is certain but the unforeseen.” APPENDIX. MERCURY TYPE OF CELLS. I. Castner Kellner. The Electrical Review (London), March 31, 1899. Zeits. f. Elektrochemie, September 8, 1905. Electrochemical Industry, November, 1905. 2. Solvay. DIAPHRAGM TYPE OF CELL. I. Hargreaves-Bird. The Electrician, February 12, 1897, and February 18, 1898. The Electrician, March 14, 1902. Electricity (New York), April 17, IYI. Electrochemical Industry, May, 1905. Electrochemical Industry, January, I+. 2. Outlzeitiia-Chalaiadre. 3. Townsend. 4 Zerr and Whitham.ELECTROLYTIC ALKALI AND BLEACH INDUSTRY 47 FUSION TYPE OF CELL. I. Hulin. 2 . Acker. Electrician, March 4, 1898. Electrician, October 25, 1901. GRAVITY TYPE OF CELL. I. Aussag bell" cell. Electrician, February 7, q o z . Zeits. f. Elektrochemie, October 10, 1 9 1 . Electrochemical Industry, June, 1904. GENERAL ARTICLES. Siebert. Frarake. Elektrochemische Zeitschrift, November, 1g01 American Electrician, March, 1901.

ISSN:0014-7672    

DOI:10.1039/TF9070300038

出版商:RSC    

年代:1907

数据来源: RSC

摘要:

ELECTROLYTIC ALKALI AND BLEACH INDUSTRY 47 DISCUSSION. Mr. W. Pollard Digby said his own personal interest in the subject was confined to the comparatively small industry of hypochlorite manu- facture, but many points that arose in the larger alkali industry were applicable to the smaller one, and he shared the author’s regret, therefore, at the reticence of the manufacturers of caustic alkali and bleach. Such being the case, he feared that the chief interest in the paper was on the archzeological side. But on the practical side he would have liked to have had information on such questions as the life of graphite anodes, the com- parative value of, say, Acheson graphite and retort carbon anodes, and on the best current densities to use having regard to the life of the anodes; at what rate those working on a large scale were prepared to sacrifice carbon and gain efficiency.He wished the author could have told them more about diaphragms ; the life of asbestos and cement diaphragms ; the difficulties cabsed by clogging, and the means adopted for overcoming these. The only information available on such points was that to be found in patent specifications, and it was unnecessary to point out that just the essential devices which spelt success were those most jealously guarded by inventors. But it must remain a matter for regret that so little was known about so important an industry ; he felt sure this could not, on the whole, be beneficial, even to the industry itself. Mr. John G. A. Rhodin expected that more interest would have been taken in the discussion, seeing that the alkali industry was almost the only one in which electrochemical methods had had a good chance to show their value.He himself had been intimately connected with the industry in its early days, and early in the nineties he had reported on the Acker and Kellner processes and stated his belief in the future of mercury processes. Later on he had reported on the improvements introduced by Mr. Castner, which resulted, as everybody knew, in a lawsuit, which he would not refer to further except to state as his opinion that the reckless expenditure of money to uphold worthless patents was the main reason why we in England had fallen behind in the electrochemical-alkali industry. He protested against the suggestion that the subject had been neglected in England on the48 POSITION AND FUTURE PROSPECTS OF THE technical side ; on the contrary, possibly more pioneering work had been done here than in any other country.It was true that there had been some mis- calculations in the early days; for example, the cost of the apparatus had been very greatly underestimated, and the life of the carbon-anodes very greatly overestimated, but the procesbes were workable, and the real reasons why they did not pay were financial reasons-inflated capital and reckless speculation. The future, of course, would depend entirely on the cost of power. Dr. R. Seligman drew attention to an omission in the table on page 40, where no mention was made of the works at Winnington, where a large amount of bleach was being made electrolytically. There was another small omission on page 43 ; he believed the Acker Process Company made sulphur chlorides.He would ask the author whether the Dow Process Company did not make bromine electrolytically. He welcomed Mr. Kershaw’s prophecies as old and familiar friends. Dr. H. Borns, referring to Watt’s original patent, asked where Watt had been working, and whether he had brought his cell from the experimental to the practical stage. He regretted that secrecy still appeared to be enforced in electrochemical manufacture, and that etiquette kept the manufacturers away from discussions of this kind, and induced the few competent men present on such occasions to restrict themselves to references to historical questions and patent litiga- tions.For the same reason it would probably be useless to press Mr. Kershaw as to technical details. The troubles caused by the clogging of diaphragms that had been referred to in connection with the Middlewich plant and elsewhere appeared to be caused by the presence of calcium and magnesium salts, which were redeposited in the diaphragms. Mr. J. C. Richardson, who had been associated with Alexander Watt, a brother of Charles Watt, in the early eighties, said that there was no practical value in Watt’s work ; and as to his ideas, most of those came direct from Faraday. As a matter of fact, all kinds of electrolytic work on‘ a commercial scale had to wait on the evolution and perfection of the dynamo, although, as in the case of electroplating, machines of poor efficiency had been long previously employed.He, the speaker, many years ago concluded that the “ bell-jar” type of cell would turn out to be a practicable form for commercial work, and he had experimented successfully with tanks embodying the gravity principle. He found that if salt was fed into the anode compartment of such gravity cells, an ampere-efficiency of 84 per cent. was attainable, but the alkaline solution run off was discoloured and carried an excess of salt, which had to be separated. But in any type of electrolytic (wet) plant for alkali manufacture the apparatus was much too bulky and expensive in proportion to yield. Hence inventors had turned their attention to fusion methods. We were still waiting for an indestructible anode upon which to liberate chlorine.The trouble caused by the evolution of carbon dioxide at the anode, which prevented the making of strong bleach, was now easily obviated by passing the chlorine gas through a sohtion of hypochlorite to absorb the dioxide. Cement diaphragms had been referred to, and to a certain extent these were a success. Clogging, however, was very great with these, and their resistance went up enormously on that account. With regard to the utilisation of the hydrogen produced, he had proposedELECTROLYTIC ALKALI AND BLEACH INDUSTRY 49 to absorb it by means of oxide of copper, although probably that was too dear a method. Mr. Rhodin, interposing, said the hydrogen loss was equivalent to only about one-eighth of the total power (i.e.? on the basis of a P.D.on each tank Mr. J. C. Richardson, in conclusion, referred to one or two omissions the author had made. Chlorate of potash was being manufactured with success at Niagara. Mr. Greenwood, he might add, had never been to Snodland. Mr. J. B. C. Kershaw (commuizicafed reply) : With reference to the general complaint that runs through the remarks of the gentlemen who have joined in this discussion, I may state that this paper was never intended to be descriptive in character. All information that can at present be made public relative to types of cell employed and methods of working is already available for the study of those specially interested in the electrolytic alkali and bleach processes, and Mr. Digby is mistaken in thinking that the pub- lished information is merely a recapitulation of that contained in patent specifications, with what one most desires to know left out.For the benefit of those members who do not appear to have made themselves acquainted with the literature of the subject, I have appended to my paper a list of articles in English and foreign technical journals worthy of careful reading and study. In these articles Mr. Digby will find the information he desires respecting current densities, the life of carbon anodes, and the methods of making diaphragms ; while Mr. Richardson will find information about a “bell” type of cell which contains many points of resemblance to that worked at St. Helens. This cell is operated at Aussig, in Austria, and at two works in Germany, with entirely successful results.Mr. Rhodin’s and Mr. Richardson’s remarks concerning the causes of failure of the processes associated with their names are interesting. I must differ with Mr. Richardson, however, in his estimate of the value of Charles Watt’s patent of 1851. It is a truism to remark that inventors, as a class, are inclined to over-rate the value of their own work and patents. I am, there- fore, afraid Mr. Richardson would not be accepted in a court of law as an absolutely unprejudiced witness upon the scientific and practical value of Charles Watt’s ideas. Mr. Seligman draws my attention to one or two omissions in the table on page 40, and I have corrected the table in accordance with his suggestions. In reply to Dr. Borns, Charles Watt carried out his work in Kennington, and I believe tried his cell upon a laboratory scale of working.The reasons that no larger installation was attempted are, of course, so obvious that I thought it unnecessary to explain them in detail, but Mr. Richardson has kindly supplied this deficiency in the paper as read. With regard to the troubles arising from calcium and magnesium salts, these appear to be met with wherever rock-salt or brine without previous purification is employed in the cells, and some method of removing these impurities is now adopted in the better managed works. The trouble from this cause affects not only the diaphragm process, but also the mercury process. With reference to Mr. Richardson’s correction that chlorate of potash is being manufactured at Niagara, I may state that chlorate of potash is being manufactured electrolytically at seven works in Europe and America, but that these are omitted from my list since chlorate of potash is not usually classed as an alkali or as bleaching powder.of 4 volts). ELECTROLYTIC ALKALI AND BLEACH INDUSTRY 47 DISCUSSION. Mr. W. Pollard Digby said his own personal interest in the subject was confined to the comparatively small industry of hypochlorite manu- facture, but many points that arose in the larger alkali industry were applicable to the smaller one, and he shared the author’s regret, therefore, at the reticence of the manufacturers of caustic alkali and bleach. Such being the case, he feared that the chief interest in the paper was on the archzeological side. But on the practical side he would have liked to have had information on such questions as the life of graphite anodes, the com- parative value of, say, Acheson graphite and retort carbon anodes, and on the best current densities to use having regard to the life of the anodes; at what rate those working on a large scale were prepared to sacrifice carbon and gain efficiency.He wished the author could have told them more about diaphragms ; the life of asbestos and cement diaphragms ; the difficulties cabsed by clogging, and the means adopted for overcoming these. The only information available on such points was that to be found in patent specifications, and it was unnecessary to point out that just the essential devices which spelt success were those most jealously guarded by inventors. But it must remain a matter for regret that so little was known about so important an industry ; he felt sure this could not, on the whole, be beneficial, even to the industry itself.Mr. John G. A. Rhodin expected that more interest would have been taken in the discussion, seeing that the alkali industry was almost the only one in which electrochemical methods had had a good chance to show their value. He himself had been intimately connected with the industry in its early days, and early in the nineties he had reported on the Acker and Kellner processes and stated his belief in the future of mercury processes. Later on he had reported on the improvements introduced by Mr. Castner, which resulted, as everybody knew, in a lawsuit, which he would not refer to further except to state as his opinion that the reckless expenditure of money to uphold worthless patents was the main reason why we in England had fallen behind in the electrochemical-alkali industry.He protested against the suggestion that the subject had been neglected in England on the48 POSITION AND FUTURE PROSPECTS OF THE technical side ; on the contrary, possibly more pioneering work had been done here than in any other country. It was true that there had been some mis- calculations in the early days; for example, the cost of the apparatus had been very greatly underestimated, and the life of the carbon-anodes very greatly overestimated, but the procesbes were workable, and the real reasons why they did not pay were financial reasons-inflated capital and reckless speculation.The future, of course, would depend entirely on the cost of power. Dr. R. Seligman drew attention to an omission in the table on page 40, where no mention was made of the works at Winnington, where a large amount of bleach was being made electrolytically. There was another small omission on page 43 ; he believed the Acker Process Company made sulphur chlorides. He would ask the author whether the Dow Process Company did not make bromine electrolytically. He welcomed Mr. Kershaw’s prophecies as old and familiar friends. Dr. H. Borns, referring to Watt’s original patent, asked where Watt had been working, and whether he had brought his cell from the experimental to the practical stage. He regretted that secrecy still appeared to be enforced in electrochemical manufacture, and that etiquette kept the manufacturers away from discussions of this kind, and induced the few competent men present on such occasions to restrict themselves to references to historical questions and patent litiga- tions.For the same reason it would probably be useless to press Mr. Kershaw as to technical details. The troubles caused by the clogging of diaphragms that had been referred to in connection with the Middlewich plant and elsewhere appeared to be caused by the presence of calcium and magnesium salts, which were redeposited in the diaphragms. Mr. J. C. Richardson, who had been associated with Alexander Watt, a brother of Charles Watt, in the early eighties, said that there was no practical value in Watt’s work ; and as to his ideas, most of those came direct from Faraday.As a matter of fact, all kinds of electrolytic work on‘ a commercial scale had to wait on the evolution and perfection of the dynamo, although, as in the case of electroplating, machines of poor efficiency had been long previously employed. He, the speaker, many years ago concluded that the “ bell-jar” type of cell would turn out to be a practicable form for commercial work, and he had experimented successfully with tanks embodying the gravity principle. He found that if salt was fed into the anode compartment of such gravity cells, an ampere-efficiency of 84 per cent. was attainable, but the alkaline solution run off was discoloured and carried an excess of salt, which had to be separated. But in any type of electrolytic (wet) plant for alkali manufacture the apparatus was much too bulky and expensive in proportion to yield.Hence inventors had turned their attention to fusion methods. We were still waiting for an indestructible anode upon which to liberate chlorine. The trouble caused by the evolution of carbon dioxide at the anode, which prevented the making of strong bleach, was now easily obviated by passing the chlorine gas through a sohtion of hypochlorite to absorb the dioxide. Cement diaphragms had been referred to, and to a certain extent these were a success. Clogging, however, was very great with these, and their resistance went up enormously on that account. With regard to the utilisation of the hydrogen produced, he had proposedELECTROLYTIC ALKALI AND BLEACH INDUSTRY 49 to absorb it by means of oxide of copper, although probably that was too dear a method.Mr. Rhodin, interposing, said the hydrogen loss was equivalent to only about one-eighth of the total power (i.e.? on the basis of a P.D. on each tank Mr. J. C. Richardson, in conclusion, referred to one or two omissions the author had made. Chlorate of potash was being manufactured with success at Niagara. Mr. Greenwood, he might add, had never been to Snodland. Mr. J. B. C. Kershaw (commuizicafed reply) : With reference to the general complaint that runs through the remarks of the gentlemen who have joined in this discussion, I may state that this paper was never intended to be descriptive in character. All information that can at present be made public relative to types of cell employed and methods of working is already available for the study of those specially interested in the electrolytic alkali and bleach processes, and Mr.Digby is mistaken in thinking that the pub- lished information is merely a recapitulation of that contained in patent specifications, with what one most desires to know left out. For the benefit of those members who do not appear to have made themselves acquainted with the literature of the subject, I have appended to my paper a list of articles in English and foreign technical journals worthy of careful reading and study. In these articles Mr. Digby will find the information he desires respecting current densities, the life of carbon anodes, and the methods of making diaphragms ; while Mr.Richardson will find information about a “bell” type of cell which contains many points of resemblance to that worked at St. Helens. This cell is operated at Aussig, in Austria, and at two works in Germany, with entirely successful results. Mr. Rhodin’s and Mr. Richardson’s remarks concerning the causes of failure of the processes associated with their names are interesting. I must differ with Mr. Richardson, however, in his estimate of the value of Charles Watt’s patent of 1851. It is a truism to remark that inventors, as a class, are inclined to over-rate the value of their own work and patents. I am, there- fore, afraid Mr. Richardson would not be accepted in a court of law as an absolutely unprejudiced witness upon the scientific and practical value of Charles Watt’s ideas. Mr. Seligman draws my attention to one or two omissions in the table on page 40, and I have corrected the table in accordance with his suggestions. In reply to Dr. Borns, Charles Watt carried out his work in Kennington, and I believe tried his cell upon a laboratory scale of working. The reasons that no larger installation was attempted are, of course, so obvious that I thought it unnecessary to explain them in detail, but Mr. Richardson has kindly supplied this deficiency in the paper as read. With regard to the troubles arising from calcium and magnesium salts, these appear to be met with wherever rock-salt or brine without previous purification is employed in the cells, and some method of removing these impurities is now adopted in the better managed works. The trouble from this cause affects not only the diaphragm process, but also the mercury process. With reference to Mr. Richardson’s correction that chlorate of potash is being manufactured at Niagara, I may state that chlorate of potash is being manufactured electrolytically at seven works in Europe and America, but that these are omitted from my list since chlorate of potash is not usually classed as an alkali or as bleaching powder. of 4 volts).

ISSN:0014-7672    

DOI:10.1039/TF9070300047

出版商:RSC    

年代:1907

数据来源: RSC

摘要:

THE POTENTIAL OF HYDROGEN LIBERATED FROM METALLIC SURFACES. BY HARRY NUTTON AND HERBERT DRAKE LAW. ( A Paper read before the Faraday Society on Tuesday, March 19, 1907, Dr. T. M. LOWRY, in the Chair.) During the past few years electrolytic methods of reduction have been gradually introduced, and this has drawn attention to the use of different electrodes. It was recognised long ago by Caspari that the potential of hydrogen liberated from a cathode surface has by no means a fixed value, but varies from metal to metal within fairly wide limits. These early experiments were considerably extended by Tafel, and this investigator was the first to point out the connection between chemical reaction and ‘‘ supertension.” As a result of numerous experiments, the conclusion arrived at was that the reduction of an organic substance proceeds furthest when the potential of the hydrogen bringing about this result is the highest.In this work the metals were arranged in the following order : Mercury, lead, cadmium, tin, silver, bismuth, gold, nickel, platinum (black), the first metal being most capable of bringing about the reduction of a compound not readily attacked, while platinised platinum possessed the property in the lowest degree. Working in an entirely different region of chemistry with inorganic com- pounds, the effects of metallic impurities on the reducing action of zinc in acid medium were studied by Law and Chapman (Analyst, 1906, s), and the results of Caspari and Tafel were confirmed, but a difficulty occurred in the case of iron.This latter metal was extremely irregular in its behaviour. Further, it became of interest to fix the relative positions of cadmium and zinc, as these two metals promise to take up a prominent place in analytical chem- istry. A little later than this platinum was found to behave in a very curious manner when used in a freshly platinised condition (Law, Tram. Faraday SOC., vol. ii., May, 1906, p. 72), and still later copper, aluminium, and nickel also were found to behave anomalously. Both copper and platinum (black, exhibit a remarkable activity in the reduction of aromatic aldehydes, and iron and aluminium can at times cause a complete resinification of the product. The activity of copper, for example, is seriously affected by external circumstances, and so by the substitution of an alkaline electrolyte for an acid one the abnormal reaction of the first case falls into line with a large class of others.Results similar to these have been obtained by S . Fokin (2eitschr.f. Elektrochem. 1906, 12, 749), who ascribes the activity to the intermediate formation of metallic hydrides. From considerations such as above it seemed of interest to study the question of supertension under different conditions, in the hope of being able to pick out the purely chemical from the physical effects, The conclusions previously arrived at may be briefly summarised in the following : With an organic compound resinification can take place wherever a double link exists. The effect of the supertension is to produce the largest amount of resinous matter when the highest potential is 50THE POTENTIAL OF HYDROGEN 51 used.Bearing these facts in mind, it is possible to explain many apparent anomalies. The reduction of oleic acid to stearic acid takes place better on copper and nickel cathodes (Zoc. cit.) than on lead (Petersen, Zeifschr. f. Elektro- chern. 1906, 11, 549), in spite of the greater E.M.F. in the case of the latter metal. This is due to the resin formation which clogs the cathode, while with nickel the potential never rises high enough to form these complex compounds, In the reduction of benzaldehyde in acid solution the reaction proceeds far more quickly on copper cathodes than on lead, Here it is possible to follow the reaction. At first the lead is most vigorous, but as the resin increases in quantity the velocity falls far behind that of copper (Law, Trans.Chem. Soc. 1906, 8g, 1524), and finally the reduction ceases altogether. Another phenomenon often met with in the case of metallic electrodes is that the reduction of a substance does not set in immediately after starting the current, but only after the elapse of some considerable time. This is very noticeable in the case of iron and nickel, and to a smaller extent with tin. The interpretation is simple, as is shown by the experiments with these metals. The supertension is by no means a fixed quantity, and is influenced very considerably by the physical condition of the metallic surface of the cathode within wide limits. Very small quantities of impurities exercise a large and varying influence, which is overcome only on long standing in contact with the electrolyte.A cathode then, working below its normal value as a reducing agent, also shows an E.M.F. less than usual. On stand- ing in contact with the electrolyte the condition of the electrode surface gradually becomes readjusted, and the reducing power is recovered. It has already been shown that metallic impurities and other such influences can be overcome by the chemical deposition of small quantities of such metals as cadmium (loc. cit.) on the reducing surface. To confirm the use of such a process by physical measurements the experiments with cadmium deposited electrolytically on platinum were undertaken. The result proves conclusively that a complete cover can be deposited only when the greatest care is taken to obtain a compact, non-crystalline deposition, which bears out what has previously been stated (loc.cit.) in connection with zinc in the reduction of arsenious oxide. The above refers to physical conditions, but chemical changes are equally effective in bringing about an entire modification of the course of the reduction. This is seen best in the case of copper cathodes. With this metal the reduction of benzaldehyde in alkaline solution proceeds simply and smoothly to a mixture of hydrobenzoins according to the following equation :- 2C,5H5. CHO + 2H = CsH5. CH(OH) t CH(OH) . CsH,. The E.M.F. of the cathode is lowered about 0.2 volt on the addition of 5 grams of the aldehyde. Very little resin is formed. With sulphuric acid as electrolyte the product is entirely different, and the reaction proceeds further, benzyl alcohol being the product- CsH5.CHO + 2H = C6HS. CH,OH. Small quantities of hydrobenzoin make their appearance, and only the most minute traces of resin. At the same time the fall of E.M.F. on the addition of 5 grams of the depolariscr is quite absent, and in some cases an actual rise was registered. From this it is quite evident that a change of E.M.F. due to the addition of a depolariser gives no indication of the vigour of a chemical reaction. Still more curious than this is the influence of temperature. The52 THE POTENTIAL OF HYDROGEN above experiment was conducted at IOO, but on repeating at 40" the benzyl alcohol formation was almost absent in the acid solution, and a good yield of hydrobenzoin was oblained.Thus at the higher temperature the least chemical reaction took place. Experiments conducted in this direction showed that the E.M.F. gave no indication of such a change as this. The phenomenon was entirely chemical. Throughout the whole of this work the formation of resistance films on the surface of the cathodes was noticed. In the case of iron and aluminium these might be classed under the head of passivity, but it is almost impossible to explain them completely on the assumption of an oxide film. They are present in the same marked degree with smooth platinum, mercury, and magnesium, and were absent only in the cases of roughened nickel, platinum, copper, and lead. In some cases this was due to the formation of a film of hydrogen on the surface of the cathode, but the cause of this is quite unknown.The conclusion arrived at may be briefly stated as follows : The super- tension of one and the same metal is by no means a fixed quantity. It is influenced by the physical and chemical nature of the surface within wide limits. Film formations also have a large influence. Chemical reactions, such as the formation of resins, are greatly increased by raising the super- tension, but these complex bodies may totally hide any other chemical reaction. The nature of the electrolyte can completely change the chemical reaction without any corresponding change in the supertension. The supertension gives small indicatioq of catalytic action. The fall of potential on adding a depolariser is only a rough guide of the nature of the reduction taking place.The small drop of E.M.F. in the case of copper and platinised platinum may be an indication of catalysis. Method.-In this work a porous pot of a capacity of 120 C.C. was used to contain the cathode, the volume of solution being 100 C.C. After being care- fully cleaned it was fitted with a trebly bored rubber stopper. Short pieces of glass tubing were fitted into two of the holes, one serving as a gas outlet and the other for fixing the cathode. The third hole contained a capillary tube. The cathode was fixed in position by means of sealing wax, and could be readily removed. The capillary tube was bent at right angles inside the porous pot, and was plugged with filter paper. This was pressed close to the inside of the cylindrical cathode, and was connected with the hydrogen electrode. The anode compartment was a stout glass jar containing a spiral platinum wire as anode.This apparatus was connected in series with twelve storage cells, an adjustable resistance, and a standard millivoltmeter shunted to read milliamperes. The E.M.F. between the cathode and the hydrogen electrode was measured on the Clark-Fisher '' Compensating Potentiometer," reading to & millivolt. When possible the cathodes were bent in the form of a cylinder, and had a superficial area of 20 sq. cms. on one side. The current in each case was started at the lowest reading, and measurements taken after an almost steady state had been arrived at. It has been pointed out by Tafel that in most cases the E.M.F.of the cathode gradually increases with a constant current, and reaches a maximum sometimes after several hours. The rise is, however, very rapid at first, and after half an hour increases very slowly. When this condition was reached the measurement was made and the current adjusted for the next reading. These results are shown in the form of curves, the abscissz being the E.M.F. measured-the supertension-and the ordinates represent the current in amperes and the current density in amperes per sq. cm.LIBERATED FROM METALLIC SURFACES 53 Cof$er.-These electrodes were cut from a sheet of electrolytic copper and cleaned with nitric acid. To ensure purity, a clean metallic surface was deposited electrolytically in several cases, but the results were in nowise altered. With 'such electrodes the following results were obtained :- TABLE I Copper iga 5 per cent.Sulphuric Acid. (1) Current Density in Amperes per sq. cm. 0*0005 0*0007 O'OOO8 0~0015 0.003 0.005 0.025 0.05 0'00 E 0'0 I 0'3342 0.3509 0.36 I 8 0'3663 0'4245 0'4530 0.5287 0'3827 0'4083 - E.M.F. IS VOLTS. (3) 5 % H*S04. 50 % Alcohol. 0.3081 093264 0.41 15 0.4600 0.4966 0'5277 - - - 0'43 I 0 0'3209 0.3362 0'3555 0.3723 0'4037 0.4249 0'4527 0'5260 - - (5) 5 % H2S04. 50 Alcohol. o*oooz gm. PtC14. 0'0377 0'0537 0.0786 0.0906 0-1 104 - - - 0'1517 0'2002 It has been suggested in a recent Paper that the activity of copper as a cathode is due to an intermediate hydride formation. This being so, it might be expected that for a constant current density the E.M.F.would rise gradually until the hydrogen absorption should be complete. This is exactly what does take place, but the addition of benzaldehyde as a depolariser has only a very small effect on the supertension (4), although a very vigorous chemical action takes place. In some cases even the cathode potential was increased, and this may indicate that some catalytic action is taking place on the metallic surface due to an unknown property of the copper. The absence of resinous matter serves to keep this action unimpaired throughout an experi- ment. It is quite probable that hydrides of copper are formed, but it is difficult to see how these can take any direct part in the reduction of benz- aldehyde. Very similar to this, Tafel has already quoted a case where the supertension of lead is slightly increased in the presence of a depolariser, although no hydride formation is possible here.As a rule the potential of copper is slightly decreased on the addition of alcohol for the same amount of polarisation (2and 3). No. 3 does not show this phenomenon with the higher values, as the polarisation was continued for some considerable time. As the maximum E.M.F. is not reached immediately, it is quite possible that these values are slightly low, due to incomplete polarisation. This point we are at present investigating. In the last column (5) the effect of a very small addition of platinum chloride is seen. This explains the inefficiency of electrodes which have long been in use without being cleaned, and has been shown in a previous Paper (Zoc.cit.).54 THE POTENTIAL OF HYDROGEN TABLE 11. Copper in Potassium Hydroxide. E.M.F. IN VOLTS. (1) Current Density in Amperes per sq. cm. 0.0005 0*0007 0*0009 0.001g 0.003 0.005 0.0075 0.05 0'00 I (2) 5 % KOH. 50 % Alcohol. 0.4072 0'4343 0'4575 0'4743 0'5OOo 0.5146 0.5273 0'5934 0'452 I 0.2285 0.2372 0.2422 0'2444 0'2570 0'2607 0.2682 0.2752 0.3634 In this case also the E.M.F. was lowered slightly on the addition of alcohol, but benzaldehyde made a very considerable change not observed in sulphuric acid solutions. The product of reduction was hydrobenzoir,, but in the previous case it was chiefly benzyl alcohol; that is, the drop in the E.M.F. is greatest where least chemical action takes place, while the reverse was expected. It is quite unlikely that the presence of alkali prevents the formation of an intermediate hydride of copper, and as a consequence we are quite unable to explain this seemingly anomalous be- haviour.In the case of both alkaline and acid solutions the resin formation was almost absent. This is entirely a property of reduction processes at a low potential even when unsaturated carbon atoms are present, These values by no means represent the maximum E.M.F., measurements being made when the continued rise became small. In this case also the addition of alcohol lowered the supertension, but this again might have been due to insufficient polarisation. With benzaldehyde quite a large fall of E.M.F. was noticed quite different from the case of copper. The product of the reaction contained in the case of platinum far more resin and less benzyl alcohol than in the case of the previous experiments.The gradual rise of cathodic potential for a constant current density probably indicated the slow formation of a metallic hydride. This might even become super- saturated with hydrogen, the smooth formation of the surface aiid the consequent greater surface tension preventing the ready liberation of the absorbed gas. Platinum, Polished, in Potassium Hydroxide.-These results differ only very slightly from those obtained with acid solutions. There was, however, a marked difference in the product of reduction. The resin formation was very small, but not entirely absent, and an almost theoretical yield of hydro- benzoin resulted. It is quite evident, therefore, that the constitution of the solvent has a very decided influence on reduction processes quite independent of the supertension. On the addition of alcohol the polarisation was allowed to continue for some time before making any measurements (2).This caused the E.M.F. to rise above that obtained without alcohol, but as the current density was increased the value gradually fell below that obtained in (3). This seems to prove that the polarisation takes place more slowlyLIBERATED FROM METALLIC SURFACES 55 - 0'0045 - 0.0163 - 0.0261 0'0461 0.1715 - (1) Current Density in Amperes per sq. cm. - 0-1 140 -0'Ogoo - 0.0687 - 00045 + 0.1694 - - - 0'0377 - o.Oo05 0.0007 O'oo09 0.0015 0.005 0.015 0.025 0.05 0'00 I 0'0 I 0'0249 0'0472 0.1129 0.2282 TABLE 111. Polished Platinum in Potassium Hydroxide.- 0'0524 - 0*0241 + 0'0219 + 0.- 0.203 I 0~2303 0.2508 0'2624 0.3078 0'3993 0'4567 0'5352 0.6038 - E.M.F. IN VOLTS. (3) 5 % KOH. 50 % Alcohol. 0'2505 0'281 I 0*2887 0.30 9 0'3493 0.4267 0'5203 0.5608 0'4677 - 0.1838 0'2193 0'2367 02448 0.2825 0.3225 0'3552 0.3852 0.4283 0.4931 0-1087 0'1217 0'1295 0'1325 0- I 862 - - - - - in alcoholic solutions than in those containing only dilute alkali. It is not unlikely that this phenomenon is due to the difference in viscosity of the two solutions. The measurements tabulated under (4) were made very slowly, to allow the E.M.F. to rise to its highest value. The value obtained still remained considerably below that without the aldehyde, and in nowise resembled the result obtained with copper in acid solutions.Platinum, PZatinised.-It has already been shown by one of us (luc. cit.) that freshly prepared platinum behaves quite differently from electrodes which have been in use for some time. I t became necessary, therefore, to decide whether this was due to a difference in the supertension. The electrodes were prepared and subjected to varying conditions. One, for example, was polarised for 12 hours at the anode, while a second was given a similar treatment at the cathode, but in each case the resulting E M.F. did not vary to any considerable extent. TABLE IV. Platirtised Platinum in Sulphuric Acid. E.M.F. IN VOLTS. I 0.0005 0.001g 0.003 0.005 0.025 0.05 0'00 I 0'002 0'0 I 0.0103 00148 0'0232 0-0273 0.0349 0.0890 - - - I - -56 THE POTENTIAL OF HYDROGEN % In the first set of experiments (2, 3,4) a cathode which had been in use for several experiments was used.It was not active, and no longer reduced benzaldehyde to any considerable extent. The maximum E.M.F. was reached f a r more rapidly than in the previous case, the rise on continuing the experiment with a constant current density being very small, If a hydride of platinum is formed its supertension is not much higher than that of platinum itself. Here the supertension phenomenon is not marked as in the case of polished platinum, due no doubt to the roughened surface of the metal. The addition of alcohol in this case (2, 3) caused a slight increase in the E.M.F. with the higher current densities. The fall of the voltage on adding benzaldehyde was great only where low-current densities were employed, and increased rapidly as the current density was made greater (4).In (5, 6) the same cathode was used, but was polarised for 12 hours at the anode. The E.M.F. differed only slightly in alcoholic solutions, but on the addition of benzaldehyde the voltage fell and did not rise again to the previous value, In another set of experiments, which are not recorded, the same electrode was employed, but KOH was substituted for H,SO,. The results were practically the same as in V. until benzaldehyde was added, and the E.M.F. fell as in experiment 6. Nickel.-The results obtained with this metal varied somewhat with different samples, In the first set of experiments the cathode was soft and readily soluble in nitric acid. As it dissolved the surface remained smooth and did not appear crystalline.In the second case the nickel employed was dull in appearance, and was almost insoluble in cold nitric acid. At higher temperatures it dissolved slowly, but still remained dull in appearance and crystalline. 5 % KOH. 50 % Alcohol (1) Current Density in Amperes per sq. cm. O'Ooo5 0*0007 O'ooo9 0.003 0'005 0.025 0.05 0.001 0'0 I TABLE V. Nickel in Potassium Hydroxide. E.M.F. IN VOLTS. 0'3723 0'3957 0'4131 0.4191 0'4699 0'4924 0.5203 "'5, 70 0'5377 - 0'4420 0.4637 0'4684 0'5210 0'5445 0'5754 0'6190 0.6626 I. Y 0.1641 0.2 87 0.291 I 0-3 I 30 0'3227 0'33 0 0.38986 0.2851 0*4.466 d 0.2499 0.2664 0'2795 0.2852 03368 0'3595 0.38 I 0*42& 0.4703 - 0.3232 0'3394 0'3454 0.4015 0'4202 0'4412 0.4683 0.4932 ('I) 5 % KOH.;o %.Alcohol 6 6 % CtjH5CHO. 0.1985 0'2303 0'2353 0.2649 0.2768 0.3 182 0'2202 0'2915 0'3497 \ 11. (8) 5 ob KOH. 50 % Alcohol. ooooz gm. PtC14. 0.1741 0.2 105 0.2834 0.3438 0'3741 - - 0'3091 - In set I. with the soft nickel the E.M.F. was increased by the addition of alcohol (3), but this was due to the longer polarisation, the maximumLIBERATED FROM METALLIC SURFACES 57 supertension not being reached immediately. In (6) the addition of alcohol was not so marked as in the previous case, but the supertension remained persistently below those of set I. This result was not due to metallic impurity, for the analysis showed that each was almost pure and contained only a trace of copper. Further, the addition of platinum chloride (8) to the cathode solution had only a relatively small effect on the supertension.It 1.- Nickel. -- I 0 8 Supertension. Fig. 1. was due then to the structure of the metallic surface, and corresponds to platinum, which shows an even more remarkable change. In another experiment the E.M.F. was even lower than in the one recorded, and in this case the reduction took place very slowly, and reached a normal value only after polarisation for some hours. The product of reaction was then the same as in the previous cases, and consisted of almost pure hydrobenzoin. VOL. 111-T358 THE POTENTIAL OF HYDROGEN (1) Current Density in Amperes per sq. cm. 0*0005 0-0007 0.003 0.005 0'025 0.05 0'00 I 0'0 I TABLE VI. Tin in Sulphuric Acid. 0.5287 0.5624 0.6383 0.6722 0.72 I I 0.76 I I - - (3) 5 % HzS04- 0.6249 0.6587 0.7026 0.7220 0.8367 O'gooo 0'6459 0'7784 (4) 5 % HzS04.06407 0.6627 0.6857 0.7426 0'807 I 0.8892 0.9882 0.7694 0.262 I 0.2948 0.3726 0'4107 0'4632 0'5080 - - (6) j % HzS04. 10 % Alcohol OOI gm. cuso4. 0.3856 0.4192 0'49 I 8 0.53 I 8 0'5945 0.6630 - - 0'4942 0.525 I 0'5709 0.645 I 0'6703 0.71 13 - 0'5938 0'4595 0,4708 0.4856 0.5301 0.5580 0*6041 0*7000 0'7976 The tin used in these experiments contained small traces of metallic impurities. The addition of alcohol made a small difference to the result, about the same as in alkaline solutions, and, as before, a maximum E.M.F. was not reached immediately. In experiment 2 a freshly cut electrode was used, but the resulting supertension was considerably below that obtained by Tafel, although the polarisation was continued for some considerable time.This electrode was left standing overnight in the acid solution, and during that time it had become very dull in appearance. The experiment was then repeated, and recorded in 3 and 4. The supertension had now risen very considerably, and was practically the same as that measured by Tafel. This was caused, apparently, by the formation of a pure metallic surface. It is an example of self-preparation, and is caused probably by inequalities of solution tension of different parts of the surface, i.e., by the formation of small couples and the consequent deposition of an even surface over the electrode. This action may be aided by the presence of atmospheric oxygen. The addition of impurities lowered the supertension considerably, and was greatest with platinum.This later metal is not found in commercial tin, but iron and copper are, and it became of interest, therefore, to determine the influence of these substances on a pure tin surface. In the case of copper small additions produced a very marked fall of E.M.F., but-with iron a slight increase was observed in the supertension when a decrease was expected. This extremely curious result illustrates the interesting behaviour of iron, and will be further demonstrated later. Small additions of nickel had the effect of lowering the supertension somewhat.LIBERATED FROM METALLIC SURFACES 59 TABLE VII. Tin in Potassium Hydroxide. E.M.F. IN VOLTS. I (1) Current Density in Amperes per sq. cm. O*OOo~ 0.005 0.025 0.05 0'00 I 0'0 I (a) 5 06 KOH.0.6084 0.65 I 5 0'7775 08289 0.9035 0'9575 -I I I I 0'5403 0.5963 0.7316 0.788 I 0.8589 09130 0'2920 0'3393 0'4866 0.5281 0.4069 0.4364 0'4905 0.5273 0'6047 0.6401 0'6919 0.7386 0'2560 0.2783 0'3293 0'3657 0'7764 0.8717 In alkaline solutions the fall of E.M.F. was rather greater on the addition of alcohol than in acid solutions (3). The increase of I supertension on allow- I) Tin. in KOH and ALloohoL FY !a Supmenrim In V d t r ing the metal to stand overnight in contact with the electrolyte was not as great as in the previous case, but was still observed (2). The effect of60 THE POTENTIAL OF HYDROGEN metallic impurities was very marked, but not as great as in acid solutions In this case the addition of small quantities of iron did not increase the E.M.F., but lowered it slightly.The behaviour of benzaldehyde is very curious in this case. As the current increases the E.M.F. rises continually, and continues to rise slowly for a constant current density. When this latter reaches o'ooz amperes per sq. cm., and the E.M.F. 0.5 volt, the rise in the supertension becomes very rapid. This was caused probably by the formation of a resin on the cathode, and when the latter was removed it was found to be covered with a thin coating of a sticky substance. On the other hand, it may be due to the rapid Ill.--Magnesium. Fig. 8. 0 4 0 8 10 Supartenston In Volts removal of benzaldehyde round the cathode by reduction, leaving a neigh- bouring layer free from depolariser. In acid solutions the rise of E.M.F.in the presence of benzaldehyde was a more gradual process,4no very sudden rise being observed. In these latter solutions the resin formation takes place very readily, and is formed probably at the very lowest E.M.F. in sufficient quantity to raise the supertension to its maximum value soon after the start. The rise of E.M.F. due to films in other cases will be seen later. Magnesium in Potassium Hydroxide.-The electrode was made from an ingot of pure electrolytic magnesium. In this case it was difficult to obtain concordant results. With a perfectlyLIBERATED FROM METALLIC SURFACES 61 TABLE VIII. Magnesium in Potassium Hydroxide. 5 % KOH. Current Density in Amperes per sq. cm. 0.0005 0'0015 0.003 0.005 0'0 25 0.05 0'00 I 0'0 I 0.7340 0.7930 0.8 I 70 0.847 I 0.8655 0.8868 0.8950 0.93 I 8 5 % KOH.- - - - 0'7757 0'8022 0.8287 0.8571 ~~ ~ ~ clean magnesium electrode the readings given in experiment 2 were deter- mined. On long polarisation, however, much lower results were obtained (3) ; thus after the lapse of 2 hours the readings shown in (3) resulted. Very high values may, however, be obtained, and in one case the E.M.F. rose to 3 volts at I ampere, but immediately began to fall. This fall is by no nieans rapid, and takes place over a period of several hours. At present we cannot offer any complete explanation of this phenomenon. It is evident, however, that some resistance film is set up, but at present we cannot tell the exact nature of this. TABLE IX. Iron ira Potassium Hydroxide. (1) Current Density in Amperes per sq.cm. 0.0005 o*oo I 5 0.003 0.005 0.025 0.05 0'00 I 0'01 E.M.F. IN VOLTS, (2) 5 % KOH. 0.1480 0'1741 0.2232 0.2496 0'2947 0'3795 0.4470 0'1925 - I I 0.1615 0.2234 0.2368 0.2761 0.4188 0.6869 0'9947 0'2000 0'1992 02407 0.2646 0.3030 0'3335 0-3782 0.476 I 0'4348 0'2131 0.29% 0.348 I 0'4727 0.5146 0.5287 I I The chemical nature of the products of reduction varied very consider- ably even with the same iron cathode, and depended very largely on the previous treatment of the metal. Thus a piece of soft sheet iron containing a little carbon was almost incapable of reducing benzaldehyde, but on being allowed to stand in caustic potash solution for some days the reaction pro- ceeded quite normally, hydrobenzoin being the product. This same electrode was heated in a Bunsen burner, and on then being62 THE POTENTIAL OF HYDROGEN treated with nitric acid it dissolved very slowly.On being used as a cathode large quantities of resin were produced, which was not the case in the pre- vious experiments. The first measurements were made with sheet iron which had been heated in a Bunsen burner and allowed to stand in potassium hydroxide solution to prevent rusting (2). The E.M.F. was low throughout. In the next experiment (3) the solution contained an addition of alcohol, and the E.M.F. rose to that of lead; on allowing the current to pass for some time these values began to decrease. The current was cut off and the supertension was redetermined after standing for 12 hours in potassium IV.- Iron Fig. 4 Supertension in Volts. hydroxide solution, and a low value was then obtained (4).On the addition of benzaldehyde (5) the E.M.F. rose again to the high value, but fell off as the experiment proceeded. These results show conclusively that iron exists in two forms or becomes changed on the surface. I t also demonstrates the varying properties of iron. Thus a sample of zinc containing traces of iron is incapable of completely reducing arsenious oxide, due presumably to the low supertension of the iron impurity. A little iron added to a solution con- taining a tin cathode increases the supertension slightly. The iron in this case cannot have a supertension lower than that of tin. Further, the intro- duction of iron salts in small quantities to an electrolytic reduction processLIBERATED FROM METALLIC SURFACES 63 (1) Current Density in Amperes per sq.cm. has very little effect on the reaction, but when added to the Marsh-Berzelius apparatus for estimating arsenic, a marked falling off of reducing efficiency is at once apparent. TABLE X. Aluminium in Sulphuric Acid. (3) 5 % H a 0 4 - O~Ooo5 0.0007 0'00 15 0.003 0.005 0.025 0.05 0'00 I 0'0 I 0'4942 0.5 I 25 0.5283 0.5464 0.5894 0.6537 - 0'6155 0.6917 0 O? 0 0 001 h C - m 6 e 3 Q.00 E.M.F. IN VOLTS. (7) 5 % H 8 0 4 . 50 % Alcohol. 0'4564 0-47 I I 0.4841 0'4995 0.5165 0'5404 0'5618 0'65 I 4 0.5894 V.--.Aluminium. FIE. 6 SupenenuDn.64 I THE POTENTIAL OF HYDROGEN I I Alumiizium in Sulphuric Acid.-This metal behaves like iron. A new electrode invariably causes the production of tarry matter with benzaldehyde, but at other times behaves like a metal of low supertension.As in the case of iron, the supertension rose rapidly, but began to decrease at the higher current densities (3). On repetition with the same electrode a similar phenomenon was observed, as was also the case on exposing the metal overnight (3). A new electrode was now used, and the measurements were redetermined with the addition of alcohol; the result differed only slightly from the previous ones (4). A new solution was now made contain- ing benzaldehyde, and the E.M.F. rose to the high value given in 6. The same electrode was carefully washed and allowed to stand overnight and the E.M.F. redetermined (7). In this case the supertension was low, and on the addition of benzaldehyde no repetition of the previous phenomenon was shown (8).The resin produced was also small in quantity, but was the main product in thc previous case. The question now arises as to the cause of this fall in the E.M.F. In experiment 5, therefore, platinum chloride was introduced into the cathode solution, but relatively large amounts were required to produce any considerable change in the supertension. It is probable, therefore, that this action is caused by a change of the metallic surface of the cathode and iiot to added impurities. Ziizc in Potassium Hydroxide.-It is of the greatest importance that the supertensioii of this metal should be determined. It is almost invariably used for the detection and estimation of small quantities of arsenic, but it gives results which vary very considerably when different samples are employed.TABLE XI. Zinc in Potassium Hydroxide. I E.M.F. IN VOLTS. Current Density in Amperes per sq. cm. 5 % KOH. 50 % Alcohol. 5 % KOH. 50 oh Alcohol. 0.01 gm. Fe$O.+ 0.0003 0-0007 om03 0-007 0.017 0.033 0'002 0'70 I 6 0'8649 0.9142 0.98 I 8 1.0800 1.1620 0.7548 0'6449 0.7102 0.8763 0.9472 1.0348 1.1033 - 5 % KOH. 50 % Alcohol. 0.01 gm. CuSO.+ 0'5752 0.6 136 0.69 I 7 0.7197 0.7641 0.83 I 5 - (5) 5 %, KOH. 50 %, Alcohol. ooooz gm. PtC14. 0.4827 0'5135 0.58 I I 0.6204 0.7123 0.8327 - The supertension of this metal in a pure state is very high (2), and the addition of metallic impurities lowers this value very considerably (3, 4, 5). Thus a small quantity of copper or platinum causes a fall in potential equal to about 0.3 volt (4).The addition of iron had no marked effect, but this might have been due to the precipitation of the hydroxide of iron. In any case iron can take the part of a metal of high supertension. Cadmium in Potassium Hydroxide.-As cadmium is related to zinc, it is probable that the two metals will be closely allied. It is of greatest import- ance that its relation to zinc should be determined, for it has been shownLIBERATED FROM METALLIC SURFACES 0.025 0 02 0 015 > YI C d 0 01 El a 0 0 006 Supertension. VI.-Zinc. Fig 6 TABLE XII. Cadmium in Potassium Hydroxide. I E.M.F. IN VOLTS. Current Density in Amperes per sq. cm. 5 % KOH. 0.0005 0.001g 0.003 0.005 0.025 0'05 0'001 0'0 1 0'5227 0'5569 0'5744 0'6232 0.7252 0'7702 0'8244 0.8845 0~1090 0.2137 0.2342 0.2596 02850 0.5810 0.6961 0.3854 (5) 5 % KOH. 50 % Alcohol.omoooz gm. PtCl.+ 0'3343 0,3850 - - 0.4693 0'4964 0'5425 0'5862 5 % HZS04. 0,4320 0'4323 0.5149 0'5325 0'5539 0.5895 0,6304 -66 THE POTENTIAL OF HYDROGEN quite recently that when the latter metal is behaving anomalously as a reducing agent a coat of cadmium will adjust matters. It has been noticed, however (Law and Chapman, Analyst), that zinc must be very carefully coated with cadmium before the full reducing efficiency of the former metal is recovered. In experiment 2, therefore, cadmium was deposited on 'platinum to the thickness of 0.5 mm. and the E.M.F. determined. The surface was crystalline and the whole mass fairly pliable. I t will be seen from this that the supertension is considerably lower than that of zinc.This, however, is due to the influence of the platinum. It may be assumed that this latter metal is not completely covered, and the action is the same as that of adding platinum chloride to the cathode solution. That the whole mass is behaving as a metal of low supertension is quite obvious from the experiment, in which the resin formation was small (4). These experi- ments throw a great deal of light on the method of manipulating the Marsh- Berzelius test for small traces of arsenic. It is quite evident that all particles of low supertension must be entirely covered before complete reduction can take place. The addition of alcohol caused a considerable fall of potential, but this began to rise immediately to a value which on long polarisation was equal to that obtained before.Platinum behaves the same as in other cases. Cadmium in SulPJzuric Acid.-A fresh electrode was prepared, and the E.M.F. was even lower than in the previous case, but thc coat was far more crystalline, which accounts for the discrepancy. Experiment 6 gives the result for deposited cadmium which is practically the same as with platinum alone, although apparently a complete envelope had been prepared. In the next experiment a sheet of pure cadmium, wasused, and the super- tension was practically the same as that of pure zinc. From this it may be inferred that the reducing power of both metals in a pure state is identical. It is difficult, however, to obtain commercial zinc free from metallic impuri- ties, while the salts of cadmium may be easily purified by crystallisation.For all reduction purposes, therefore, it is useless to waste time in obtaining pure zinc, for this metal may be readily coated, either chemically or electrically, with pure cadmium, and more concordant results obtained. The metal, like others showing a high supertension, is very readily affected by small traces of platinum. Lead i n Potassium Hydroxide.-These results agree closely with those given by Tafel for sulphuric acid solutions. The electrodes were very carefully prepared from pure lead by alternately oxidising and reducing the metal; they were then allowed to stand in contact with the alkali for some hours. If the readings, however, were taken immediately after preparation, the results were much lower than expected.This agrees with the results for tin. The introduction of alcohol lowers the potential somewhat, Platinum has a fairly marked effect, but its action was by no means permanent. The action of benzaldehyde was also fairly marked. With commercial lead prepared in the above manner the differences between the results obtained immediately after the introduction into the alkali and those on long standing in the cathode cell were more noticeable than above. The results agreed closely with those obtained by Tafel in acid solution. Lead in Sutphuric Acid-Commercial lead was used in this case, and the results were taken immediately after the introduction into the cathode solution. The differences were regularly between 0'2 and 0.3 volt.LIBERATED FROM METALLIC SURFACES 67 TABLE XIII.Lead in Sulphuric Acid. E.M.F. IN VOLTS. I 5 % H W 4 . 9 % Alcohol. Current Density inAmperes 1 5 % H$o4. 1 per sq. inch. I I 0.0005 O'Ooo9 0.0015 0.003 0'005 005 0'00 I 0'01 0.5701 0.6028 0~6078 0.6297 0'6633 0.6873 0'7038 0'7427 0.5260 0.5825 0'6137 0'6621 0'7361 0'5585 0'5637 0'6340 -I 0.4059 0'4234 0'4259 0'4377 0'4469 0'4545 0.4669 0'5394 It will be seen immediately that these readings are much lower than those obtained by Tafel for pure lead. On being left to stand overnight in acid solution these values increased and approached those of the pure metal. Amalgamated Metals.-Amalgamated lead (2, 3) gave values slightly below those of mercury, but this may have been due to the absence of the curious increase of potential observed with the latter metal.With zinc, however, TABLE XIV. Amalgamated It1 etals. (1) Current Densitj in Amperes per sq. cm. 0*0005 0.003 0.005 0'025 0.05 0'001 0'01 (3) Lead. 5 % KOH. 50 % Alcohol. 0'7127 0*9101 0'9544 1.0270 1-0687 0'8055 1'0012 (8) Lead. 5 % Hzs04. 50 % Alcohol. 0'7070 0.7485 0'7997 0~8210 0.8756 0.9420 1.0327 E.M.F. IN VOLTS. (4) Zinc. 5 % KOH. 50 % Alcohol. 0.9365 0'9621 0.0851 0'9973 1.1632 1'2495 I (6) Zinc. 5 % KOH. 5 % Alcohol. 1'002 gm. PtCld. 0.6145 0.6521 0.8082 0'9031 1.1085 O.gg80 - (8) Zinc. 5 % H&04. 50 % Alcohol. 0.8626 0'9536 0.9748 1'0045 1.2194 1'37% - the values were quite as high as those of mercury alone, this combination behaving in all respects like mercury (4, 6). The action of platinum is very curious in the case of lead.The introduction of 0*0002 gram of platinum chloride produced a slight increase of E.M.F. in potassium hydroxide solutions. With 0'001 gram a very small fall was obaerv d, but with 0.01 gram a very considerable change68 THE POTENTIAL OF HYDROGEN was produced, a result very little higher than that with platinised platinum being registered. In acid solutions the introduction of 0~002 platinum chloride produced an immediate fall of potential amounting to 0.1 volt. The same quantity of platinum chloride produced the same drop for zinc in both acid and alkaline solutions (5). Very small quantities of platinum chloride had, however, scarcely any effect on the supertension. Mercury.-The results obtained with this substance were much higher than was expected.This is probably due to the formation of a hydrogen film. Even on largely increasing the surface of the metal this film per- sisted. The result was practically the same as that of amalgamated zinc in potassium hydroxide (experiment 4). Violent agitation would break it down altogether. THE POTENTIAL OF HYDROGEN LIBERATED FROM METALLIC SURFACES. BY HARRY NUTTON AND HERBERT DRAKE LAW. ( A Paper read before the Faraday Society on Tuesday, March 19, 1907, Dr. T. M. LOWRY, in the Chair.) During the past few years electrolytic methods of reduction have been gradually introduced, and this has drawn attention to the use of different electrodes. It was recognised long ago by Caspari that the potential of hydrogen liberated from a cathode surface has by no means a fixed value, but varies from metal to metal within fairly wide limits.These early experiments were considerably extended by Tafel, and this investigator was the first to point out the connection between chemical reaction and ‘‘ supertension.” As a result of numerous experiments, the conclusion arrived at was that the reduction of an organic substance proceeds furthest when the potential of the hydrogen bringing about this result is the highest. In this work the metals were arranged in the following order : Mercury, lead, cadmium, tin, silver, bismuth, gold, nickel, platinum (black), the first metal being most capable of bringing about the reduction of a compound not readily attacked, while platinised platinum possessed the property in the lowest degree. Working in an entirely different region of chemistry with inorganic com- pounds, the effects of metallic impurities on the reducing action of zinc in acid medium were studied by Law and Chapman (Analyst, 1906, s), and the results of Caspari and Tafel were confirmed, but a difficulty occurred in the case of iron.This latter metal was extremely irregular in its behaviour. Further, it became of interest to fix the relative positions of cadmium and zinc, as these two metals promise to take up a prominent place in analytical chem- istry. A little later than this platinum was found to behave in a very curious manner when used in a freshly platinised condition (Law, Tram. Faraday SOC., vol. ii., May, 1906, p. 72), and still later copper, aluminium, and nickel also were found to behave anomalously.Both copper and platinum (black, exhibit a remarkable activity in the reduction of aromatic aldehydes, and iron and aluminium can at times cause a complete resinification of the product. The activity of copper, for example, is seriously affected by external circumstances, and so by the substitution of an alkaline electrolyte for an acid one the abnormal reaction of the first case falls into line with a large class of others. Results similar to these have been obtained by S . Fokin (2eitschr.f. Elektrochem. 1906, 12, 749), who ascribes the activity to the intermediate formation of metallic hydrides. From considerations such as above it seemed of interest to study the question of supertension under different conditions, in the hope of being able to pick out the purely chemical from the physical effects, The conclusions previously arrived at may be briefly summarised in the following : With an organic compound resinification can take place wherever a double link exists.The effect of the supertension is to produce the largest amount of resinous matter when the highest potential is 50THE POTENTIAL OF HYDROGEN 51 used. Bearing these facts in mind, it is possible to explain many apparent anomalies. The reduction of oleic acid to stearic acid takes place better on copper and nickel cathodes (Zoc. cit.) than on lead (Petersen, Zeifschr. f. Elektro- chern. 1906, 11, 549), in spite of the greater E.M.F. in the case of the latter metal. This is due to the resin formation which clogs the cathode, while with nickel the potential never rises high enough to form these complex compounds, In the reduction of benzaldehyde in acid solution the reaction proceeds far more quickly on copper cathodes than on lead, Here it is possible to follow the reaction.At first the lead is most vigorous, but as the resin increases in quantity the velocity falls far behind that of copper (Law, Trans. Chem. Soc. 1906, 8g, 1524), and finally the reduction ceases altogether. Another phenomenon often met with in the case of metallic electrodes is that the reduction of a substance does not set in immediately after starting the current, but only after the elapse of some considerable time. This is very noticeable in the case of iron and nickel, and to a smaller extent with tin. The interpretation is simple, as is shown by the experiments with these metals.The supertension is by no means a fixed quantity, and is influenced very considerably by the physical condition of the metallic surface of the cathode within wide limits. Very small quantities of impurities exercise a large and varying influence, which is overcome only on long standing in contact with the electrolyte. A cathode then, working below its normal value as a reducing agent, also shows an E.M.F. less than usual. On stand- ing in contact with the electrolyte the condition of the electrode surface gradually becomes readjusted, and the reducing power is recovered. It has already been shown that metallic impurities and other such influences can be overcome by the chemical deposition of small quantities of such metals as cadmium (loc.cit.) on the reducing surface. To confirm the use of such a process by physical measurements the experiments with cadmium deposited electrolytically on platinum were undertaken. The result proves conclusively that a complete cover can be deposited only when the greatest care is taken to obtain a compact, non-crystalline deposition, which bears out what has previously been stated (loc. cit.) in connection with zinc in the reduction of arsenious oxide. The above refers to physical conditions, but chemical changes are equally effective in bringing about an entire modification of the course of the reduction. This is seen best in the case of copper cathodes. With this metal the reduction of benzaldehyde in alkaline solution proceeds simply and smoothly to a mixture of hydrobenzoins according to the following equation :- 2C,5H5.CHO + 2H = CsH5. CH(OH) t CH(OH) . CsH,. The E.M.F. of the cathode is lowered about 0.2 volt on the addition of 5 grams of the aldehyde. Very little resin is formed. With sulphuric acid as electrolyte the product is entirely different, and the reaction proceeds further, benzyl alcohol being the product- CsH5. CHO + 2H = C6HS. CH,OH. Small quantities of hydrobenzoin make their appearance, and only the most minute traces of resin. At the same time the fall of E.M.F. on the addition of 5 grams of the depolariscr is quite absent, and in some cases an actual rise was registered. From this it is quite evident that a change of E.M.F. due to the addition of a depolariser gives no indication of the vigour of a chemical reaction.Still more curious than this is the influence of temperature. The52 THE POTENTIAL OF HYDROGEN above experiment was conducted at IOO, but on repeating at 40" the benzyl alcohol formation was almost absent in the acid solution, and a good yield of hydrobenzoin was oblained. Thus at the higher temperature the least chemical reaction took place. Experiments conducted in this direction showed that the E.M.F. gave no indication of such a change as this. The phenomenon was entirely chemical. Throughout the whole of this work the formation of resistance films on the surface of the cathodes was noticed. In the case of iron and aluminium these might be classed under the head of passivity, but it is almost impossible to explain them completely on the assumption of an oxide film.They are present in the same marked degree with smooth platinum, mercury, and magnesium, and were absent only in the cases of roughened nickel, platinum, copper, and lead. In some cases this was due to the formation of a film of hydrogen on the surface of the cathode, but the cause of this is quite unknown. The conclusion arrived at may be briefly stated as follows : The super- tension of one and the same metal is by no means a fixed quantity. It is influenced by the physical and chemical nature of the surface within wide limits. Film formations also have a large influence. Chemical reactions, such as the formation of resins, are greatly increased by raising the super- tension, but these complex bodies may totally hide any other chemical reaction.The nature of the electrolyte can completely change the chemical reaction without any corresponding change in the supertension. The supertension gives small indicatioq of catalytic action. The fall of potential on adding a depolariser is only a rough guide of the nature of the reduction taking place. The small drop of E.M.F. in the case of copper and platinised platinum may be an indication of catalysis. Method.-In this work a porous pot of a capacity of 120 C.C. was used to contain the cathode, the volume of solution being 100 C.C. After being care- fully cleaned it was fitted with a trebly bored rubber stopper. Short pieces of glass tubing were fitted into two of the holes, one serving as a gas outlet and the other for fixing the cathode. The third hole contained a capillary tube.The cathode was fixed in position by means of sealing wax, and could be readily removed. The capillary tube was bent at right angles inside the porous pot, and was plugged with filter paper. This was pressed close to the inside of the cylindrical cathode, and was connected with the hydrogen electrode. The anode compartment was a stout glass jar containing a spiral platinum wire as anode. This apparatus was connected in series with twelve storage cells, an adjustable resistance, and a standard millivoltmeter shunted to read milliamperes. The E.M.F. between the cathode and the hydrogen electrode was measured on the Clark-Fisher '' Compensating Potentiometer," reading to & millivolt. When possible the cathodes were bent in the form of a cylinder, and had a superficial area of 20 sq.cms. on one side. The current in each case was started at the lowest reading, and measurements taken after an almost steady state had been arrived at. It has been pointed out by Tafel that in most cases the E.M.F. of the cathode gradually increases with a constant current, and reaches a maximum sometimes after several hours. The rise is, however, very rapid at first, and after half an hour increases very slowly. When this condition was reached the measurement was made and the current adjusted for the next reading. These results are shown in the form of curves, the abscissz being the E.M.F. measured-the supertension-and the ordinates represent the current in amperes and the current density in amperes per sq.cm.LIBERATED FROM METALLIC SURFACES 53 Cof$er.-These electrodes were cut from a sheet of electrolytic copper and cleaned with nitric acid. To ensure purity, a clean metallic surface was deposited electrolytically in several cases, but the results were in nowise altered. With 'such electrodes the following results were obtained :- TABLE I Copper iga 5 per cent. Sulphuric Acid. (1) Current Density in Amperes per sq. cm. 0*0005 0*0007 O'OOO8 0~0015 0.003 0.005 0.025 0.05 0'00 E 0'0 I 0'3342 0.3509 0.36 I 8 0'3663 0'4245 0'4530 0.5287 0'3827 0'4083 - E.M.F. IS VOLTS. (3) 5 % H*S04. 50 % Alcohol. 0.3081 093264 0.41 15 0.4600 0.4966 0'5277 - - - 0'43 I 0 0'3209 0.3362 0'3555 0.3723 0'4037 0.4249 0'4527 0'5260 - - (5) 5 % H2S04.50 Alcohol. o*oooz gm. PtC14. 0'0377 0'0537 0.0786 0.0906 0-1 104 - - - 0'1517 0'2002 It has been suggested in a recent Paper that the activity of copper as a cathode is due to an intermediate hydride formation. This being so, it might be expected that for a constant current density the E.M.F. would rise gradually until the hydrogen absorption should be complete. This is exactly what does take place, but the addition of benzaldehyde as a depolariser has only a very small effect on the supertension (4), although a very vigorous chemical action takes place. In some cases even the cathode potential was increased, and this may indicate that some catalytic action is taking place on the metallic surface due to an unknown property of the copper. The absence of resinous matter serves to keep this action unimpaired throughout an experi- ment.It is quite probable that hydrides of copper are formed, but it is difficult to see how these can take any direct part in the reduction of benz- aldehyde. Very similar to this, Tafel has already quoted a case where the supertension of lead is slightly increased in the presence of a depolariser, although no hydride formation is possible here. As a rule the potential of copper is slightly decreased on the addition of alcohol for the same amount of polarisation (2and 3). No. 3 does not show this phenomenon with the higher values, as the polarisation was continued for some considerable time. As the maximum E.M.F. is not reached immediately, it is quite possible that these values are slightly low, due to incomplete polarisation.This point we are at present investigating. In the last column (5) the effect of a very small addition of platinum chloride is seen. This explains the inefficiency of electrodes which have long been in use without being cleaned, and has been shown in a previous Paper (Zoc. cit.).54 THE POTENTIAL OF HYDROGEN TABLE 11. Copper in Potassium Hydroxide. E.M.F. IN VOLTS. (1) Current Density in Amperes per sq. cm. 0.0005 0*0007 0*0009 0.001g 0.003 0.005 0.0075 0.05 0'00 I (2) 5 % KOH. 50 % Alcohol. 0.4072 0'4343 0'4575 0'4743 0'5OOo 0.5146 0.5273 0'5934 0'452 I 0.2285 0.2372 0.2422 0'2444 0'2570 0'2607 0.2682 0.2752 0.3634 In this case also the E.M.F. was lowered slightly on the addition of alcohol, but benzaldehyde made a very considerable change not observed in sulphuric acid solutions.The product of reduction was hydrobenzoir,, but in the previous case it was chiefly benzyl alcohol; that is, the drop in the E.M.F. is greatest where least chemical action takes place, while the reverse was expected. It is quite unlikely that the presence of alkali prevents the formation of an intermediate hydride of copper, and as a consequence we are quite unable to explain this seemingly anomalous be- haviour. In the case of both alkaline and acid solutions the resin formation was almost absent. This is entirely a property of reduction processes at a low potential even when unsaturated carbon atoms are present, These values by no means represent the maximum E.M.F., measurements being made when the continued rise became small.In this case also the addition of alcohol lowered the supertension, but this again might have been due to insufficient polarisation. With benzaldehyde quite a large fall of E.M.F. was noticed quite different from the case of copper. The product of the reaction contained in the case of platinum far more resin and less benzyl alcohol than in the case of the previous experiments. The gradual rise of cathodic potential for a constant current density probably indicated the slow formation of a metallic hydride. This might even become super- saturated with hydrogen, the smooth formation of the surface aiid the consequent greater surface tension preventing the ready liberation of the absorbed gas. Platinum, Polished, in Potassium Hydroxide.-These results differ only very slightly from those obtained with acid solutions.There was, however, a marked difference in the product of reduction. The resin formation was very small, but not entirely absent, and an almost theoretical yield of hydro- benzoin resulted. It is quite evident, therefore, that the constitution of the solvent has a very decided influence on reduction processes quite independent of the supertension. On the addition of alcohol the polarisation was allowed to continue for some time before making any measurements (2). This caused the E.M.F. to rise above that obtained without alcohol, but as the current density was increased the value gradually fell below that obtained in (3). This seems to prove that the polarisation takes place more slowlyLIBERATED FROM METALLIC SURFACES 55 - 0'0045 - 0.0163 - 0.0261 0'0461 0.1715 - (1) Current Density in Amperes per sq.cm. - 0-1 140 -0'Ogoo - 0.0687 - 00045 + 0.1694 - - - 0'0377 - o.Oo05 0.0007 O'oo09 0.0015 0.005 0.015 0.025 0.05 0'00 I 0'0 I 0'0249 0'0472 0.1129 0.2282 TABLE 111. Polished Platinum in Potassium Hydroxide. - 0'0524 - 0*0241 + 0'0219 + 0.- 0.203 I 0~2303 0.2508 0'2624 0.3078 0'3993 0'4567 0'5352 0.6038 - E.M.F. IN VOLTS. (3) 5 % KOH. 50 % Alcohol. 0'2505 0'281 I 0*2887 0.30 9 0'3493 0.4267 0'5203 0.5608 0'4677 - 0.1838 0'2193 0'2367 02448 0.2825 0.3225 0'3552 0.3852 0.4283 0.4931 0-1087 0'1217 0'1295 0'1325 0- I 862 - - - - - in alcoholic solutions than in those containing only dilute alkali. It is not unlikely that this phenomenon is due to the difference in viscosity of the two solutions.The measurements tabulated under (4) were made very slowly, to allow the E.M.F. to rise to its highest value. The value obtained still remained considerably below that without the aldehyde, and in nowise resembled the result obtained with copper in acid solutions. Platinum, PZatinised.-It has already been shown by one of us (luc. cit.) that freshly prepared platinum behaves quite differently from electrodes which have been in use for some time. I t became necessary, therefore, to decide whether this was due to a difference in the supertension. The electrodes were prepared and subjected to varying conditions. One, for example, was polarised for 12 hours at the anode, while a second was given a similar treatment at the cathode, but in each case the resulting E M.F.did not vary to any considerable extent. TABLE IV. Platirtised Platinum in Sulphuric Acid. E.M.F. IN VOLTS. I 0.0005 0.001g 0.003 0.005 0.025 0.05 0'00 I 0'002 0'0 I 0.0103 00148 0'0232 0-0273 0.0349 0.0890 - - - I - -56 THE POTENTIAL OF HYDROGEN % In the first set of experiments (2, 3,4) a cathode which had been in use for several experiments was used. It was not active, and no longer reduced benzaldehyde to any considerable extent. The maximum E.M.F. was reached f a r more rapidly than in the previous case, the rise on continuing the experiment with a constant current density being very small, If a hydride of platinum is formed its supertension is not much higher than that of platinum itself. Here the supertension phenomenon is not marked as in the case of polished platinum, due no doubt to the roughened surface of the metal.The addition of alcohol in this case (2, 3) caused a slight increase in the E.M.F. with the higher current densities. The fall of the voltage on adding benzaldehyde was great only where low-current densities were employed, and increased rapidly as the current density was made greater (4). In (5, 6) the same cathode was used, but was polarised for 12 hours at the anode. The E.M.F. differed only slightly in alcoholic solutions, but on the addition of benzaldehyde the voltage fell and did not rise again to the previous value, In another set of experiments, which are not recorded, the same electrode was employed, but KOH was substituted for H,SO,.The results were practically the same as in V. until benzaldehyde was added, and the E.M.F. fell as in experiment 6. Nickel.-The results obtained with this metal varied somewhat with different samples, In the first set of experiments the cathode was soft and readily soluble in nitric acid. As it dissolved the surface remained smooth and did not appear crystalline. In the second case the nickel employed was dull in appearance, and was almost insoluble in cold nitric acid. At higher temperatures it dissolved slowly, but still remained dull in appearance and crystalline. 5 % KOH. 50 % Alcohol (1) Current Density in Amperes per sq. cm. O'Ooo5 0*0007 O'ooo9 0.003 0'005 0.025 0.05 0.001 0'0 I TABLE V. Nickel in Potassium Hydroxide. E.M.F. IN VOLTS. 0'3723 0'3957 0'4131 0.4191 0'4699 0'4924 0.5203 "'5, 70 0'5377 - 0'4420 0.4637 0'4684 0'5210 0'5445 0'5754 0'6190 0.6626 I.Y 0.1641 0.2 87 0.291 I 0-3 I 30 0'3227 0'33 0 0.38986 0.2851 0*4.466 d 0.2499 0.2664 0'2795 0.2852 03368 0'3595 0.38 I 0*42& 0.4703 - 0.3232 0'3394 0'3454 0.4015 0'4202 0'4412 0.4683 0.4932 ('I) 5 % KOH. ;o %.Alcohol 6 6 % CtjH5CHO. 0.1985 0'2303 0'2353 0.2649 0.2768 0.3 182 0'2202 0'2915 0'3497 \ 11. (8) 5 ob KOH. 50 % Alcohol. ooooz gm. PtC14. 0.1741 0.2 105 0.2834 0.3438 0'3741 - - 0'3091 - In set I. with the soft nickel the E.M.F. was increased by the addition of alcohol (3), but this was due to the longer polarisation, the maximumLIBERATED FROM METALLIC SURFACES 57 supertension not being reached immediately. In (6) the addition of alcohol was not so marked as in the previous case, but the supertension remained persistently below those of set I.This result was not due to metallic impurity, for the analysis showed that each was almost pure and contained only a trace of copper. Further, the addition of platinum chloride (8) to the cathode solution had only a relatively small effect on the supertension. It 1.- Nickel. -- I 0 8 Supertension. Fig. 1. was due then to the structure of the metallic surface, and corresponds to platinum, which shows an even more remarkable change. In another experiment the E.M.F. was even lower than in the one recorded, and in this case the reduction took place very slowly, and reached a normal value only after polarisation for some hours. The product of reaction was then the same as in the previous cases, and consisted of almost pure hydrobenzoin.VOL. 111-T358 THE POTENTIAL OF HYDROGEN (1) Current Density in Amperes per sq. cm. 0*0005 0-0007 0.003 0.005 0'025 0.05 0'00 I 0'0 I TABLE VI. Tin in Sulphuric Acid. 0.5287 0.5624 0.6383 0.6722 0.72 I I 0.76 I I - - (3) 5 % HzS04- 0.6249 0.6587 0.7026 0.7220 0.8367 O'gooo 0'6459 0'7784 (4) 5 % HzS04. 06407 0.6627 0.6857 0.7426 0'807 I 0.8892 0.9882 0.7694 0.262 I 0.2948 0.3726 0'4107 0'4632 0'5080 - - (6) j % HzS04. 10 % Alcohol OOI gm. cuso4. 0.3856 0.4192 0'49 I 8 0.53 I 8 0'5945 0.6630 - - 0'4942 0.525 I 0'5709 0.645 I 0'6703 0.71 13 - 0'5938 0'4595 0,4708 0.4856 0.5301 0.5580 0*6041 0*7000 0'7976 The tin used in these experiments contained small traces of metallic impurities. The addition of alcohol made a small difference to the result, about the same as in alkaline solutions, and, as before, a maximum E.M.F.was not reached immediately. In experiment 2 a freshly cut electrode was used, but the resulting supertension was considerably below that obtained by Tafel, although the polarisation was continued for some considerable time. This electrode was left standing overnight in the acid solution, and during that time it had become very dull in appearance. The experiment was then repeated, and recorded in 3 and 4. The supertension had now risen very considerably, and was practically the same as that measured by Tafel. This was caused, apparently, by the formation of a pure metallic surface. It is an example of self-preparation, and is caused probably by inequalities of solution tension of different parts of the surface, i.e., by the formation of small couples and the consequent deposition of an even surface over the electrode.This action may be aided by the presence of atmospheric oxygen. The addition of impurities lowered the supertension considerably, and was greatest with platinum. This later metal is not found in commercial tin, but iron and copper are, and it became of interest, therefore, to determine the influence of these substances on a pure tin surface. In the case of copper small additions produced a very marked fall of E.M.F., but-with iron a slight increase was observed in the supertension when a decrease was expected. This extremely curious result illustrates the interesting behaviour of iron, and will be further demonstrated later.Small additions of nickel had the effect of lowering the supertension somewhat.LIBERATED FROM METALLIC SURFACES 59 TABLE VII. Tin in Potassium Hydroxide. E.M.F. IN VOLTS. I (1) Current Density in Amperes per sq. cm. O*OOo~ 0.005 0.025 0.05 0'00 I 0'0 I (a) 5 06 KOH. 0.6084 0.65 I 5 0'7775 08289 0.9035 0'9575 -I I I I 0'5403 0.5963 0.7316 0.788 I 0.8589 09130 0'2920 0'3393 0'4866 0.5281 0.4069 0.4364 0'4905 0.5273 0'6047 0.6401 0'6919 0.7386 0'2560 0.2783 0'3293 0'3657 0'7764 0.8717 In alkaline solutions the fall of E.M.F. was rather greater on the addition of alcohol than in acid solutions (3). The increase of I supertension on allow- I) Tin. in KOH and ALloohoL FY !a Supmenrim In V d t r ing the metal to stand overnight in contact with the electrolyte was not as great as in the previous case, but was still observed (2).The effect of60 THE POTENTIAL OF HYDROGEN metallic impurities was very marked, but not as great as in acid solutions In this case the addition of small quantities of iron did not increase the E.M.F., but lowered it slightly. The behaviour of benzaldehyde is very curious in this case. As the current increases the E.M.F. rises continually, and continues to rise slowly for a constant current density. When this latter reaches o'ooz amperes per sq. cm., and the E.M.F. 0.5 volt, the rise in the supertension becomes very rapid. This was caused probably by the formation of a resin on the cathode, and when the latter was removed it was found to be covered with a thin coating of a sticky substance.On the other hand, it may be due to the rapid Ill.--Magnesium. Fig. 8. 0 4 0 8 10 Supartenston In Volts removal of benzaldehyde round the cathode by reduction, leaving a neigh- bouring layer free from depolariser. In acid solutions the rise of E.M.F. in the presence of benzaldehyde was a more gradual process,4no very sudden rise being observed. In these latter solutions the resin formation takes place very readily, and is formed probably at the very lowest E.M.F. in sufficient quantity to raise the supertension to its maximum value soon after the start. The rise of E.M.F. due to films in other cases will be seen later. Magnesium in Potassium Hydroxide.-The electrode was made from an ingot of pure electrolytic magnesium.In this case it was difficult to obtain concordant results. With a perfectlyLIBERATED FROM METALLIC SURFACES 61 TABLE VIII. Magnesium in Potassium Hydroxide. 5 % KOH. Current Density in Amperes per sq. cm. 0.0005 0'0015 0.003 0.005 0'0 25 0.05 0'00 I 0'0 I 0.7340 0.7930 0.8 I 70 0.847 I 0.8655 0.8868 0.8950 0.93 I 8 5 % KOH. - - - - 0'7757 0'8022 0.8287 0.8571 ~~ ~ ~ clean magnesium electrode the readings given in experiment 2 were deter- mined. On long polarisation, however, much lower results were obtained (3) ; thus after the lapse of 2 hours the readings shown in (3) resulted. Very high values may, however, be obtained, and in one case the E.M.F. rose to 3 volts at I ampere, but immediately began to fall. This fall is by no nieans rapid, and takes place over a period of several hours.At present we cannot offer any complete explanation of this phenomenon. It is evident, however, that some resistance film is set up, but at present we cannot tell the exact nature of this. TABLE IX. Iron ira Potassium Hydroxide. (1) Current Density in Amperes per sq. cm. 0.0005 o*oo I 5 0.003 0.005 0.025 0.05 0'00 I 0'01 E.M.F. IN VOLTS, (2) 5 % KOH. 0.1480 0'1741 0.2232 0.2496 0'2947 0'3795 0.4470 0'1925 - I I 0.1615 0.2234 0.2368 0.2761 0.4188 0.6869 0'9947 0'2000 0'1992 02407 0.2646 0.3030 0'3335 0-3782 0.476 I 0'4348 0'2131 0.29% 0.348 I 0'4727 0.5146 0.5287 I I The chemical nature of the products of reduction varied very consider- ably even with the same iron cathode, and depended very largely on the previous treatment of the metal.Thus a piece of soft sheet iron containing a little carbon was almost incapable of reducing benzaldehyde, but on being allowed to stand in caustic potash solution for some days the reaction pro- ceeded quite normally, hydrobenzoin being the product. This same electrode was heated in a Bunsen burner, and on then being62 THE POTENTIAL OF HYDROGEN treated with nitric acid it dissolved very slowly. On being used as a cathode large quantities of resin were produced, which was not the case in the pre- vious experiments. The first measurements were made with sheet iron which had been heated in a Bunsen burner and allowed to stand in potassium hydroxide solution to prevent rusting (2). The E.M.F. was low throughout. In the next experiment (3) the solution contained an addition of alcohol, and the E.M.F.rose to that of lead; on allowing the current to pass for some time these values began to decrease. The current was cut off and the supertension was redetermined after standing for 12 hours in potassium IV.- Iron Fig. 4 Supertension in Volts. hydroxide solution, and a low value was then obtained (4). On the addition of benzaldehyde (5) the E.M.F. rose again to the high value, but fell off as the experiment proceeded. These results show conclusively that iron exists in two forms or becomes changed on the surface. I t also demonstrates the varying properties of iron. Thus a sample of zinc containing traces of iron is incapable of completely reducing arsenious oxide, due presumably to the low supertension of the iron impurity.A little iron added to a solution con- taining a tin cathode increases the supertension slightly. The iron in this case cannot have a supertension lower than that of tin. Further, the intro- duction of iron salts in small quantities to an electrolytic reduction processLIBERATED FROM METALLIC SURFACES 63 (1) Current Density in Amperes per sq. cm. has very little effect on the reaction, but when added to the Marsh-Berzelius apparatus for estimating arsenic, a marked falling off of reducing efficiency is at once apparent. TABLE X. Aluminium in Sulphuric Acid. (3) 5 % H a 0 4 - O~Ooo5 0.0007 0'00 15 0.003 0.005 0.025 0.05 0'00 I 0'0 I 0'4942 0.5 I 25 0.5283 0.5464 0.5894 0.6537 - 0'6155 0.6917 0 O? 0 0 001 h C - m 6 e 3 Q.00 E.M.F. IN VOLTS. (7) 5 % H 8 0 4 .50 % Alcohol. 0'4564 0-47 I I 0.4841 0'4995 0.5165 0'5404 0'5618 0'65 I 4 0.5894 V.--.Aluminium. FIE. 6 SupenenuDn.64 I THE POTENTIAL OF HYDROGEN I I Alumiizium in Sulphuric Acid.-This metal behaves like iron. A new electrode invariably causes the production of tarry matter with benzaldehyde, but at other times behaves like a metal of low supertension. As in the case of iron, the supertension rose rapidly, but began to decrease at the higher current densities (3). On repetition with the same electrode a similar phenomenon was observed, as was also the case on exposing the metal overnight (3). A new electrode was now used, and the measurements were redetermined with the addition of alcohol; the result differed only slightly from the previous ones (4).A new solution was now made contain- ing benzaldehyde, and the E.M.F. rose to the high value given in 6. The same electrode was carefully washed and allowed to stand overnight and the E.M.F. redetermined (7). In this case the supertension was low, and on the addition of benzaldehyde no repetition of the previous phenomenon was shown (8). The resin produced was also small in quantity, but was the main product in thc previous case. The question now arises as to the cause of this fall in the E.M.F. In experiment 5, therefore, platinum chloride was introduced into the cathode solution, but relatively large amounts were required to produce any considerable change in the supertension. It is probable, therefore, that this action is caused by a change of the metallic surface of the cathode and iiot to added impurities.Ziizc in Potassium Hydroxide.-It is of the greatest importance that the supertensioii of this metal should be determined. It is almost invariably used for the detection and estimation of small quantities of arsenic, but it gives results which vary very considerably when different samples are employed. TABLE XI. Zinc in Potassium Hydroxide. I E.M.F. IN VOLTS. Current Density in Amperes per sq. cm. 5 % KOH. 50 % Alcohol. 5 % KOH. 50 oh Alcohol. 0.01 gm. Fe$O.+ 0.0003 0-0007 om03 0-007 0.017 0.033 0'002 0'70 I 6 0'8649 0.9142 0.98 I 8 1.0800 1.1620 0.7548 0'6449 0.7102 0.8763 0.9472 1.0348 1.1033 - 5 % KOH. 50 % Alcohol. 0.01 gm. CuSO.+ 0'5752 0.6 136 0.69 I 7 0.7197 0.7641 0.83 I 5 - (5) 5 %, KOH. 50 %, Alcohol. ooooz gm.PtC14. 0.4827 0'5135 0.58 I I 0.6204 0.7123 0.8327 - The supertension of this metal in a pure state is very high (2), and the addition of metallic impurities lowers this value very considerably (3, 4, 5). Thus a small quantity of copper or platinum causes a fall in potential equal to about 0.3 volt (4). The addition of iron had no marked effect, but this might have been due to the precipitation of the hydroxide of iron. In any case iron can take the part of a metal of high supertension. Cadmium in Potassium Hydroxide.-As cadmium is related to zinc, it is probable that the two metals will be closely allied. It is of greatest import- ance that its relation to zinc should be determined, for it has been shownLIBERATED FROM METALLIC SURFACES 0.025 0 02 0 015 > YI C d 0 01 El a 0 0 006 Supertension.VI.-Zinc. Fig 6 TABLE XII. Cadmium in Potassium Hydroxide. I E.M.F. IN VOLTS. Current Density in Amperes per sq. cm. 5 % KOH. 0.0005 0.001g 0.003 0.005 0.025 0'05 0'001 0'0 1 0'5227 0'5569 0'5744 0'6232 0.7252 0'7702 0'8244 0.8845 0~1090 0.2137 0.2342 0.2596 02850 0.5810 0.6961 0.3854 (5) 5 % KOH. 50 % Alcohol. omoooz gm. PtCl.+ 0'3343 0,3850 - - 0.4693 0'4964 0'5425 0'5862 5 % HZS04. 0,4320 0'4323 0.5149 0'5325 0'5539 0.5895 0,6304 -66 THE POTENTIAL OF HYDROGEN quite recently that when the latter metal is behaving anomalously as a reducing agent a coat of cadmium will adjust matters. It has been noticed, however (Law and Chapman, Analyst), that zinc must be very carefully coated with cadmium before the full reducing efficiency of the former metal is recovered.In experiment 2, therefore, cadmium was deposited on 'platinum to the thickness of 0.5 mm. and the E.M.F. determined. The surface was crystalline and the whole mass fairly pliable. I t will be seen from this that the supertension is considerably lower than that of zinc. This, however, is due to the influence of the platinum. It may be assumed that this latter metal is not completely covered, and the action is the same as that of adding platinum chloride to the cathode solution. That the whole mass is behaving as a metal of low supertension is quite obvious from the experiment, in which the resin formation was small (4). These experi- ments throw a great deal of light on the method of manipulating the Marsh- Berzelius test for small traces of arsenic.It is quite evident that all particles of low supertension must be entirely covered before complete reduction can take place. The addition of alcohol caused a considerable fall of potential, but this began to rise immediately to a value which on long polarisation was equal to that obtained before. Platinum behaves the same as in other cases. Cadmium in SulPJzuric Acid.-A fresh electrode was prepared, and the E.M.F. was even lower than in the previous case, but thc coat was far more crystalline, which accounts for the discrepancy. Experiment 6 gives the result for deposited cadmium which is practically the same as with platinum alone, although apparently a complete envelope had been prepared. In the next experiment a sheet of pure cadmium, wasused, and the super- tension was practically the same as that of pure zinc.From this it may be inferred that the reducing power of both metals in a pure state is identical. It is difficult, however, to obtain commercial zinc free from metallic impuri- ties, while the salts of cadmium may be easily purified by crystallisation. For all reduction purposes, therefore, it is useless to waste time in obtaining pure zinc, for this metal may be readily coated, either chemically or electrically, with pure cadmium, and more concordant results obtained. The metal, like others showing a high supertension, is very readily affected by small traces of platinum. Lead i n Potassium Hydroxide.-These results agree closely with those given by Tafel for sulphuric acid solutions.The electrodes were very carefully prepared from pure lead by alternately oxidising and reducing the metal; they were then allowed to stand in contact with the alkali for some hours. If the readings, however, were taken immediately after preparation, the results were much lower than expected. This agrees with the results for tin. The introduction of alcohol lowers the potential somewhat, Platinum has a fairly marked effect, but its action was by no means permanent. The action of benzaldehyde was also fairly marked. With commercial lead prepared in the above manner the differences between the results obtained immediately after the introduction into the alkali and those on long standing in the cathode cell were more noticeable than above. The results agreed closely with those obtained by Tafel in acid solution. Lead in Sutphuric Acid-Commercial lead was used in this case, and the results were taken immediately after the introduction into the cathode solution. The differences were regularly between 0'2 and 0.3 volt.LIBERATED FROM METALLIC SURFACES 67 TABLE XIII. Lead in Sulphuric Acid. E.M.F. IN VOLTS. I 5 % H W 4 . 9 % Alcohol. Current Density inAmperes 1 5 % H$o4. 1 per sq. inch. I I 0.0005 O'Ooo9 0.0015 0.003 0'005 005 0'00 I 0'01 0.5701 0.6028 0~6078 0.6297 0'6633 0.6873 0'7038 0'7427 0.5260 0.5825 0'6137 0'6621 0'7361 0'5585 0'5637 0'6340 -I 0.4059 0'4234 0'4259 0'4377 0'4469 0'4545 0.4669 0'5394 It will be seen immediately that these readings are much lower than those obtained by Tafel for pure lead. On being left to stand overnight in acid solution these values increased and approached those of the pure metal. Amalgamated Metals.-Amalgamated lead (2, 3) gave values slightly below those of mercury, but this may have been due to the absence of the curious increase of potential observed with the latter metal. With zinc, however, TABLE XIV. Amalgamated It1 etals. (1) Current Densitj in Amperes per sq. cm. 0*0005 0.003 0.005 0'025 0.05 0'001 0'01 (3) Lead. 5 % KOH. 50 % Alcohol. 0'7127 0*9101 0'9544 1.0270 1-0687 0'8055 1'0012 (8) Lead. 5 % Hzs04. 50 % Alcohol. 0'7070 0.7485 0'7997 0~8210 0.8756 0.9420 1.0327 E.M.F. IN VOLTS. (4) Zinc. 5 % KOH. 50 % Alcohol. 0.9365 0'9621 0.0851 0'9973 1.1632 1'2495 I (6) Zinc. 5 % KOH. 5 % Alcohol. 1'002 gm. PtCld. 0.6145 0.6521 0.8082 0'9031 1.1085 O.gg80 - (8) Zinc. 5 % H&04. 50 % Alcohol. 0.8626 0'9536 0.9748 1'0045 1.2194 1'37% - the values were quite as high as those of mercury alone, this combination behaving in all respects like mercury (4, 6). The action of platinum is very curious in the case of lead. The introduction of 0*0002 gram of platinum chloride produced a slight increase of E.M.F. in potassium hydroxide solutions. With 0'001 gram a very small fall was obaerv d, but with 0.01 gram a very considerable change68 THE POTENTIAL OF HYDROGEN was produced, a result very little higher than that with platinised platinum being registered. In acid solutions the introduction of 0~002 platinum chloride produced an immediate fall of potential amounting to 0.1 volt. The same quantity of platinum chloride produced the same drop for zinc in both acid and alkaline solutions (5). Very small quantities of platinum chloride had, however, scarcely any effect on the supertension. Mercury.-The results obtained with this substance were much higher than was expected. This is probably due to the formation of a hydrogen film. Even on largely increasing the surface of the metal this film per- sisted. The result was practically the same as that of amalgamated zinc in potassium hydroxide (experiment 4). Violent agitation would break it down altogether.

ISSN:0014-7672    

DOI:10.1039/TF9070300050

出版商:RSC    

年代:1907

数据来源: RSC

摘要:

68 THE POTENTIAL OF HYDROGEN DISCUSSION. Mr. J. G. R. Rhodin dwelt on the importance in many metallurgical industries, e.g., the manufacture of Muntz’s metal, of having exact measure- ments of hydrogen potentials. With regard to’the behaviour of copper, that metal in the presence of a strong reducing agent dissolved as cuprous and otherwise in the cupric state. He had noticed that most of the irregularities that had been observed in the dissolution of metals were caused by variations in the barometric pressures. That being the case, in experiments like those in the Paper the pressure should be carefully measured and adjusted. Put crudely, it was obvious that if the pressure were low more hydrogen could get off, and therefore if concordant results were to be expected, they must be reduced to standard temperatures and pressures.The cumulative effect of impurities in metals was of interest in connection with their dissolution velocities. Thus, in the case of nickel in a manganese bronze, the dissolution velocity curve was exactly similar to that of a dissolved salt. The case of aluminium and copper alloys was an exception ; here compounds’ seemed to be formed, and the dissolution velocity did not go on increasing. Dr. H. Borns asked why passivity of the kathode should be explained on the assumption of an oxide film. The authors also spoke of noticing the formation of resistance films. We knew that such films might be invisible. Mr. Rhodin had once more referred to his observation that hydrogen was more easily generated when the barometric pressure was reduced.Dr. Borns did not wish to raise any priority question, but the fact was mentioned, he remembered, by Guthe some years ago, and Guthe did, of course, not claim the observation as original. Dr. A. C. Cumming wanted to know the opinion of the authors on the influence of impurities, The effect of adding a trace of platinum solution appeared to be that the electrode then behaved more like a platinum electrode. If one metal was deposited on another, one would expect the electromotive force to be decided by the metal of lower supertension. Mr. N. T. M. Wilsmore thought the authors’ results were of special interest in proving that the connection supposed by Nernst to exist between supertension and reducing power did not really exist at all.He asked why in the cadmium experiment a piece of platinum coated with cadmium had been used and not a sheet of pure cadmium. Mr. H. D. Law (partly communicuted): It is quite possible that the barometric pressure had some effect on the (( supertension” measured in these experiments, but in any case this could not be very considerable.LIBERATED FROM METALLIC SURFACES 69 It was certainly not of the same order of magnitude as the variations measured in some cases, and it was as a consequence an unnecessary precaution to take it into account. In no single case could anything like a fixed value of supertension be assigned to any single metal, this unsatisfactory state being a characteristic of the kathode itself and not due to such external influences as the one above mentioned. The film formation is one of the main causes of these variable results, and it is not at all unlikely that oxides or hydroxides play an important part even at the kathode.Thus in the case of aluminium the electrolytic solution tension is high enough to cause the pure metal to react with water to form hydrogen and aluminium hydroxide, and this latter can set up a resistance which would account for the curious behaviour of this metal. The latter reaction would proceed best in presence of atmospheric oxygen, and it was noticed that whenever the electrode was removed from the porous pot to introduce a depolariser or other substance, the high value of the supertension was observed on restarting the current. It has already been shown by one of us that the reducing power of such a substance as zinc depends to a large extent on the purity of the metal. To overcome this difficulty cadmium was deposited on the impure zinc surface, but it was found that different samples required varying quantities of cadmium to overcome the bad effects of the first impurity.It was decided, therefore, to try the covering power of cadmium on platinum, with the extraordinary result above recorded. Afterwards, however, pure cadmium was tried, and the result agreed with that recorded by Tafel. In general it might be stated that the bad effects of an impurity can be overcome only by completely surrounding the original metallic surface with a coherent deposit of some such substance as cadmium. The effect of an impurity on a metallic surface depends always on the relative supertension of the coupled metals.I t is governed invariably by the metal of lowest supertension. The subsequent values of the determined E.M.F. depends on the quantity of added impurity and the ease with which this latter diffuses into the original metallic surface. At the present moment it is impossible to state what is the connection between supertension and chemical reaction even from a qualitative standpoint. In general, however, the most reaction takes place with the highest supertension. This does not, however, explain the behaviour of copper and platinised platinum and many other anomalous results. Until this field is greatly extended many curious facts will have to remain as chemical mysteries. 68 THE POTENTIAL OF HYDROGEN DISCUSSION.Mr. J. G. R. Rhodin dwelt on the importance in many metallurgical industries, e.g., the manufacture of Muntz’s metal, of having exact measure- ments of hydrogen potentials. With regard to’the behaviour of copper, that metal in the presence of a strong reducing agent dissolved as cuprous and otherwise in the cupric state. He had noticed that most of the irregularities that had been observed in the dissolution of metals were caused by variations in the barometric pressures. That being the case, in experiments like those in the Paper the pressure should be carefully measured and adjusted. Put crudely, it was obvious that if the pressure were low more hydrogen could get off, and therefore if concordant results were to be expected, they must be reduced to standard temperatures and pressures. The cumulative effect of impurities in metals was of interest in connection with their dissolution velocities.Thus, in the case of nickel in a manganese bronze, the dissolution velocity curve was exactly similar to that of a dissolved salt. The case of aluminium and copper alloys was an exception ; here compounds’ seemed to be formed, and the dissolution velocity did not go on increasing. Dr. H. Borns asked why passivity of the kathode should be explained on the assumption of an oxide film. The authors also spoke of noticing the formation of resistance films. We knew that such films might be invisible. Mr. Rhodin had once more referred to his observation that hydrogen was more easily generated when the barometric pressure was reduced.Dr. Borns did not wish to raise any priority question, but the fact was mentioned, he remembered, by Guthe some years ago, and Guthe did, of course, not claim the observation as original. Dr. A. C. Cumming wanted to know the opinion of the authors on the influence of impurities, The effect of adding a trace of platinum solution appeared to be that the electrode then behaved more like a platinum electrode. If one metal was deposited on another, one would expect the electromotive force to be decided by the metal of lower supertension. Mr. N. T. M. Wilsmore thought the authors’ results were of special interest in proving that the connection supposed by Nernst to exist between supertension and reducing power did not really exist at all. He asked why in the cadmium experiment a piece of platinum coated with cadmium had been used and not a sheet of pure cadmium.Mr. H. D. Law (partly communicuted): It is quite possible that the barometric pressure had some effect on the (( supertension” measured in these experiments, but in any case this could not be very considerable.LIBERATED FROM METALLIC SURFACES 69 It was certainly not of the same order of magnitude as the variations measured in some cases, and it was as a consequence an unnecessary precaution to take it into account. In no single case could anything like a fixed value of supertension be assigned to any single metal, this unsatisfactory state being a characteristic of the kathode itself and not due to such external influences as the one above mentioned.The film formation is one of the main causes of these variable results, and it is not at all unlikely that oxides or hydroxides play an important part even at the kathode. Thus in the case of aluminium the electrolytic solution tension is high enough to cause the pure metal to react with water to form hydrogen and aluminium hydroxide, and this latter can set up a resistance which would account for the curious behaviour of this metal. The latter reaction would proceed best in presence of atmospheric oxygen, and it was noticed that whenever the electrode was removed from the porous pot to introduce a depolariser or other substance, the high value of the supertension was observed on restarting the current. It has already been shown by one of us that the reducing power of such a substance as zinc depends to a large extent on the purity of the metal.To overcome this difficulty cadmium was deposited on the impure zinc surface, but it was found that different samples required varying quantities of cadmium to overcome the bad effects of the first impurity. It was decided, therefore, to try the covering power of cadmium on platinum, with the extraordinary result above recorded. Afterwards, however, pure cadmium was tried, and the result agreed with that recorded by Tafel. In general it might be stated that the bad effects of an impurity can be overcome only by completely surrounding the original metallic surface with a coherent deposit of some such substance as cadmium. The effect of an impurity on a metallic surface depends always on the relative supertension of the coupled metals. I t is governed invariably by the metal of lowest supertension. The subsequent values of the determined E.M.F. depends on the quantity of added impurity and the ease with which this latter diffuses into the original metallic surface. At the present moment it is impossible to state what is the connection between supertension and chemical reaction even from a qualitative standpoint. In general, however, the most reaction takes place with the highest supertension. This does not, however, explain the behaviour of copper and platinised platinum and many other anomalous results. Until this field is greatly extended many curious facts will have to remain as chemical mysteries.

ISSN:0014-7672    

DOI:10.1039/TF9070300068

出版商:RSC    

年代:1907

数据来源: RSC