摘要:

118 ELECTRICAL THEORY OF ADBORPTTON The writer considers the double layer as consisting of a swface of rigidly fixed atoms under continuous bombardment of positively and negatively charged ions, any particular point on the rigid surface becoming in turn negative, neutral and positive, these conditions arisdg in any order. The observed contact difference is the average effect of these conditions. Where several kinds of atoms are present in the solution the average number of any one of them at the surface will depend on their concentbration, valency and mobility. The variation of contact Werence from negative to neutral and positive was observed with cotton and aluminium sulphate near the neutral point. These variations occurred during the same experiment, the readings being direct measurements of E.1I.F.s developed by filtration under pressure.This point would be covered by putting n2 = 1 and = 2 or 3 in Mukherjee’s equation No. 13.118 ELECTRICAL THEORY OF ADBORPTTON The writer considers the double layer as consisting of a swface of rigidly fixed atoms under continuous bombardment of positively and negatively charged ions, any particular point on the rigid surface becoming in turn negative, neutral and positive, these conditions arisdg in any order. The observed contact difference is the average effect of these conditions. Where several kinds of atoms are present in the solution the average number of any one of them at the surface will depend on their concentbration, valency and mobility. The variation of contact Werence from negative to neutral and positive was observed with cotton and aluminium sulphate near the neutral point.These variations occurred during the same experiment, the readings being direct measurements of E.1I.F.s developed by filtration under pressure. This point would be covered by putting n2 = 1 and = 2 or 3 in Mukherjee’s equation No. 13.118 ELECTRICAL THEORY OF ADBORPTTON The writer considers the double layer as consisting of a swface of rigidly fixed atoms under continuous bombardment of positively and negatively charged ions, any particular point on the rigid surface becoming in turn negative, neutral and positive, these conditions arisdg in any order. The observed contact difference is the average effect of these conditions. Where several kinds of atoms are present in the solution the average number of any one of them at the surface will depend on their concentbration, valency and mobility.The variation of contact Werence from negative to neutral and positive was observed with cotton and aluminium sulphate near the neutral point. These variations occurred during the same experiment, the readings being direct measurements of E.1I.F.s developed by filtration under pressure. This point would be covered by putting n2 = 1 and = 2 or 3 in Mukherjee’s equation No. 13.118 ELECTRICAL THEORY OF ADBORPTTON The writer considers the double layer as consisting of a swface of rigidly fixed atoms under continuous bombardment of positively and negatively charged ions, any particular point on the rigid surface becoming in turn negative, neutral and positive, these conditions arisdg in any order.The observed contact difference is the average effect of these conditions. Where several kinds of atoms are present in the solution the average number of any one of them at the surface will depend on their concentbration, valency and mobility. The variation of contact Werence from negative to neutral and positive was observed with cotton and aluminium sulphate near the neutral point. These variations occurred during the same experiment, the readings being direct measurements of E.1I.F.s developed by filtration under pressure. This point would be covered by putting n2 = 1 and = 2 or 3 in Mukherjee’s equation No. 13.118 ELECTRICAL THEORY OF ADBORPTTON The writer considers the double layer as consisting of a swface of rigidly fixed atoms under continuous bombardment of positively and negatively charged ions, any particular point on the rigid surface becoming in turn negative, neutral and positive, these conditions arisdg in any order.The observed contact difference is the average effect of these conditions. Where several kinds of atoms are present in the solution the average number of any one of them at the surface will depend on their concentbration, valency and mobility. The variation of contact Werence from negative to neutral and positive was observed with cotton and aluminium sulphate near the neutral point. These variations occurred during the same experiment, the readings being direct measurements of E.1I.F.s developed by filtration under pressure.This point would be covered by putting n2 = 1 and = 2 or 3 in Mukherjee’s equation No. 13.118 ELECTRICAL THEORY OF ADBORPTTON The writer considers the double layer as consisting of a swface of rigidly fixed atoms under continuous bombardment of positively and negatively charged ions, any particular point on the rigid surface becoming in turn negative, neutral and positive, these conditions arisdg in any order. The observed contact difference is the average effect of these conditions. Where several kinds of atoms are present in the solution the average number of any one of them at the surface will depend on their concentbration, valency and mobility. The variation of contact Werence from negative to neutral and positive was observed with cotton and aluminium sulphate near the neutral point.These variations occurred during the same experiment, the readings being direct measurements of E.1I.F.s developed by filtration under pressure. This point would be covered by putting n2 = 1 and = 2 or 3 in Mukherjee’s equation No. 13.118 ELECTRICAL THEORY OF ADBORPTTON The writer considers the double layer as consisting of a swface of rigidly fixed atoms under continuous bombardment of positively and negatively charged ions, any particular point on the rigid surface becoming in turn negative, neutral and positive, these conditions arisdg in any order. The observed contact difference is the average effect of these conditions. Where several kinds of atoms are present in the solution the average number of any one of them at the surface will depend on their concentbration, valency and mobility.The variation of contact Werence from negative to neutral and positive was observed with cotton and aluminium sulphate near the neutral point. These variations occurred during the same experiment, the readings being direct measurements of E.1I.F.s developed by filtration under pressure. This point would be covered by putting n2 = 1 and = 2 or 3 in Mukherjee’s equation No. 13.118 ELECTRICAL THEORY OF ADBORPTTON The writer considers the double layer as consisting of a swface of rigidly fixed atoms under continuous bombardment of positively and negatively charged ions, any particular point on the rigid surface becoming in turn negative, neutral and positive, these conditions arisdg in any order.The observed contact difference is the average effect of these conditions. Where several kinds of atoms are present in the solution the average number of any one of them at the surface will depend on their concentbration, valency and mobility. The variation of contact Werence from negative to neutral and positive was observed with cotton and aluminium sulphate near the neutral point. These variations occurred during the same experiment, the readings being direct measurements of E.1I.F.s developed by filtration under pressure. This point would be covered by putting n2 = 1 and = 2 or 3 in Mukherjee’s equation No. 13.118 ELECTRICAL THEORY OF ADBORPTTON The writer considers the double layer as consisting of a swface of rigidly fixed atoms under continuous bombardment of positively and negatively charged ions, any particular point on the rigid surface becoming in turn negative, neutral and positive, these conditions arisdg in any order.The observed contact difference is the average effect of these conditions. Where several kinds of atoms are present in the solution the average number of any one of them at the surface will depend on their concentbration, valency and mobility. The variation of contact Werence from negative to neutral and positive was observed with cotton and aluminium sulphate near the neutral point. These variations occurred during the same experiment, the readings being direct measurements of E.1I.F.s developed by filtration under pressure. This point would be covered by putting n2 = 1 and = 2 or 3 in Mukherjee’s equation No.13.118 ELECTRICAL THEORY OF ADBORPTTON The writer considers the double layer as consisting of a swface of rigidly fixed atoms under continuous bombardment of positively and negatively charged ions, any particular point on the rigid surface becoming in turn negative, neutral and positive, these conditions arisdg in any order. The observed contact difference is the average effect of these conditions. Where several kinds of atoms are present in the solution the average number of any one of them at the surface will depend on their concentbration, valency and mobility. The variation of contact Werence from negative to neutral and positive was observed with cotton and aluminium sulphate near the neutral point. These variations occurred during the same experiment, the readings being direct measurements of E.1I.F.s developed by filtration under pressure.This point would be covered by putting n2 = 1 and = 2 or 3 in Mukherjee’s equation No. 13.118 ELECTRICAL THEORY OF ADBORPTTON The writer considers the double layer as consisting of a swface of rigidly fixed atoms under continuous bombardment of positively and negatively charged ions, any particular point on the rigid surface becoming in turn negative, neutral and positive, these conditions arisdg in any order. The observed contact difference is the average effect of these conditions. Where several kinds of atoms are present in the solution the average number of any one of them at the surface will depend on their concentbration, valency and mobility.The variation of contact Werence from negative to neutral and positive was observed with cotton and aluminium sulphate near the neutral point. These variations occurred during the same experiment, the readings being direct measurements of E.1I.F.s developed by filtration under pressure. This point would be covered by putting n2 = 1 and = 2 or 3 in Mukherjee’s equation No. 13.118 ELECTRICAL THEORY OF ADBORPTTON The writer considers the double layer as consisting of a swface of rigidly fixed atoms under continuous bombardment of positively and negatively charged ions, any particular point on the rigid surface becoming in turn negative, neutral and positive, these conditions arisdg in any order. The observed contact difference is the average effect of these conditions.Where several kinds of atoms are present in the solution the average number of any one of them at the surface will depend on their concentbration, valency and mobility. The variation of contact Werence from negative to neutral and positive was observed with cotton and aluminium sulphate near the neutral point. These variations occurred during the same experiment, the readings being direct measurements of E.1I.F.s developed by filtration under pressure. This point would be covered by putting n2 = 1 and = 2 or 3 in Mukherjee’s equation No. 13. The Fwaday Society is not respoirsible for opittiotzs expressed before it by Authors or Speakers. Cransactione OF Cbe TO PROMOTE THE FOUNDED 1903. 8TUDY OF ELECTROCHEMISTRY, ELECTROMETALLURQY, CHEMICAL PHYSICS, METALLOQRAPHY.AND KINDRED 8UWECTS. VOL. XIX. MARCH, 1924. PART 3. THE ELECTRODEPOSITION OF MANGANESE.-PART I.. BY A. J. ALLMAND AND A. N. CAMPBELL. (A paper read befare THE FARADAY SOCIETY, Monday, November I 2th, I 923, SIR ROBERT ROBERTSON, K.B.E., F.R.S., PRESIDENT i~ the chi^.) ReceivedJuZy 3rd, I 92 3. I. h?tYodwto?y. The problem of the cathodic deposition of manganese from aqueous solution has received little attention in the past. Bunsen states that he obtained the metal by electrolysis of aqueous manganous chloride solutions, using the apparatus employed in the deposition of chromium. The metal was deposited as sheets I cm.2 in area; it was metallic looking, and oxidised almost as rapidly as potassium. If the current density were reduced, mangano-manganic oxide came down.He gives no further details. Moore2 states that metallic manganese may be deposited as such from a neqtral solution containing a large excess of ammonium sulphocyanide ; a pderful current is necessary. Smith and Frankel find that, if an excess of potassium sulphocyanide be present, the metal comes down in greyish- white compact form. Under the conditions specified, the current must be low. Van Arsdale and Meier 4 give results of some experiments conducted on the electrolysis of manganese sulphate solutions. These will be referred to in more detail below, as they were to a large extent repeated by the present authors, t o whom the paper was unknown when the work was started. Finally, The metallic deposit is inclined to oxidise rapidly.Pogg. Ann., 1854.91, 621. Chem. News., 1886,153, ZOQ. 9 your. Aturlytical Chem., 1889, 3, 386. 4 Trans. Am#. Electrochem. Soc., 1918, 3, 109. 5595 60 THE ELECTRODEPOSITION OF MANGANESE-PART I. Foerster quotes some results obtained by Grube, who states that very pure manganese can be obtained by electrolysis of a 6-7 N. MnCl, solution, separated by a diaphragm from the anode. The electrolyte is also 1.5 N. with respect to NH,C1 and 0.1 N. with respect to HCl. The current density is 20 amps./dm.2 The electrolyte is strongly stirred, and its temperature 30’. The current efficiency is said to be between 5 0 and 60 per cent., and the purity of the deposit 99.9 to IOO per cent. “There is a marked tendency for the deposit to sprout at the edges of the cathode, as a result of the high current densities used.This can, however, be over- come by arranging that the cathode fills the whole cross-section of the cell, and manganese is then obtained as a smooth microcrystalline deposit, which can be removed from the copper cathode in the form of coherent sheet.” In later papers Grube and Metzger2 refer to sheets of metal I mm. thick made by this method. No detailed account of Grube’s work has yet been published. For this reason, and apart from the interest of our own experiments, we have decided to publish the results of our work, though it may be stated at once that we have not yet succeeded in obtain- ing coherent metal sheet as Grube claims to have done. The problem then was the working out of the conditions for the successful electrodeposition of a highly electropositive metal from aqueous solution.We decided in the first instance to use as electrolytes the simple salts of manganese (sulphate, chloride). I t could be predicted in advance that the electrolysis would be favoured by a high metallic salt concentration and by a low hydrogen ion concentration. A high current density was also likely to be advantageous. With regard to temperature, the matter was more complex. To take extreme cases, if manganese were a metal like zinc, with high hydrogen over-voltage and small irreversible resistance to cathodic deposition, a low temperature would be better; if like iron, with small hydrogen over-voltage and large irreversible cathodic effects, then high temperatures would be better.We commenced on the supposition that it would behave more like iron than like zinc, a view which turned out to be only partly correct, as both high hydrogen over-voltage and high irreversible resistance to manganous ion discharge were found. 2. Experiments wifh &@hate Solutions. The electrolyte was prepared from crystallised manganese sulphate, which contained a very slight trace of iron as its only impurity. Unless otherwise stated, the solution used always contained 400 gms. per litre of the tetra- hydrate. SERIES I. The electrolysis bath was an ordinary beaker, lagged with asbestos cord, and kept at the required temperature by a gas burner. The cathode was of aluminium sheet, rectangular in shape, with an immersed area of 2 0 cm.2 On either side of it was hung a platinum foil anode wrapped in parchment paper.This precaution was taken to prevent the manganese dioxide formed in accordance with the equation Mn” + zH,O + 2 @ += MnO, + 4H’ Elektrochemie wdsseriger Losilngen, 1922, page 560. This information only became available when the greater part of the work had been completed. In the 1915 edition of the same book (p. 317), experiments are mentioned according to which the metal can be got by electrolysis of a very concentrated hot chloride solution with a current density of 10-20 amps./dm.z Zeitsch. Elektrochem., 1923, 29, 17, 100.THE ELECTRODEPOSITION OF MANGANESE-PART I. 56 I fouling the bath. No attempt was made at this stage to control the acidity of the bath, and no acid was present initially.Current was supplied by a battery of eight accumulators. The results of these preliminary experiments are summarised in Table I. The deposit from these experiments was very unsatisfactory, consisting largely of basic material. The ready disengagement of hydrogen on treat- ment with very dilute acid showed, however, metal to be present in every case but (8), and in experiments (10) and (11) this amount was estimated by collecting the hydrogen in a nitrometer. The '' current efficiencies " were determined, as throughout this paper, by drying the cathode with filter paper and weighing. Any value they have here is merely comparative, as, apart from the basic nature of the deposits, these also contained much water which could not be removed by the simple treatment described.In experiments (3) to (s), it will be noticed that the current continually falls off, owing to the increasing resistance produced by the basic film. In ex- periments (6) to ( I I), on the other hand, current density was kept constant, and the experiment stopped when the bath voltage had reached the limit available for the electrolysis. In (S), the manganese dioxide precipitated at the anodes was collected, washed, ignited to Mn,O,, and weighed. I t cor- responded to a practically IOO per cent. current efficiency for the process Mn" + 2H,O + 2 @ + MnO, + 4H' A single experiment was done under similar conditions, using a cathode of copper. Only basic deposit was obtained, probably owing to the low hydiogen over-voltage at this metal. TABLE I. 4'4 11-5 9'1 +'4'5 11.8 3 5'5 10 3 1.4 1 '4 I0 I 0 I0 I0 I0 - - I - 5'6 3 7'8 2-8 3 7'7 4.7 + 16 5.2 4 16 4'4 --3 16 5*2+ 16 5.8 4 16 N.- - - - 0.07 0.025 0.018 0'071 0.078 0.032 0'172 -- Nature of Deposit. Dull black ; soluble in HCI, giving hydrogen. Heavier than (I) ; very much fouled. Much basic deposit. Some basic deposit. Deposit badly fouled. Deposit impure. Metallic in places-fouled elsewhere. Entirely basic material. Deposit badly fouled. Contained 4.2 per cent. Contained 15.8 per cent. metal. metal. SERIES II. The above experiments having shown that the avoidance of formation of a basic precipitate at the cathode was the first thing to look to, we modified the conditions (u) by giving the bath a certain acidity in advance (8) by stirring the electrolyte vigorously with a glass stirrer.As further, experiments (I) and ( I I) had indicated that 55' was the most favourable temperature of those worked at, this was adhered to throughout, and the current density kept constant at 10 amps./dm*. VOL. XIX-T22562 THE ELECTRODEPOSITION OF MANGANESE-PART I. 22 23 24 Number of Experiment, O c. 73 55 95 30 113 27'5 I2 I3 I4 '5 16 17 18 I9 20 21 TABLE 11. Duration of Electrolysis in Minutes. 35 37 I7 32 31 47 34 33 87 182 Initial Acidity. Neutral Neutral o.oo6o N. 0'0065 0'01 0'01 0'013 0.05 0.05 0.05 Vol tnge , 5'1 + I2 4-8 + 11.4 5'0 9 6.5 5'8 5'3 -3 I2 5'1 3 9 4-6 --* 8.6 4'6 3 5'3 5'2 --3 5'9 4'6 - 9'7 True Current Efficiency. Per cent. ? 4.4 34'0 65'4 12.9 4'8 35'5 27'3 33'3 2'1 Percentage of Metal in Deposit. Not determined 3 3 33'6 68.6 13.0 3'5 51.0 79'1 73'6 4'1 It should be noted that the current efficiencies in this table, as subse- quently, are true current efficiencies reckoned on metallic manganese, and determined by estimating the volume of hydrogen evolved on treatment of the cathodic deposit with very dilute mineral acid.I t is clear that the experimental inodifications introduced have improved the purity of the deposit. The results are very fluctuating, but on the whole the higher initial acidities correspond to the better deposits. These were black and dense, resembling gas carbon, and showed a metallic lustre when cut with a knife. The irreproducible nature of the results is accounted for by the fact of the production of acid at the anode, together with the difficulty of securing stirring of the same efficiency in the different experiments.In (21) an attempt was made to keep the acidity of the electrolyte constant by running in caustic soda solution during the experiment. I t was, however, unsuccessful, the bath becoming fouled with rapidly oxidising manganous hydroxide. In the other experiments, the acidity was determined at fifteen- minute intervals, the calculated volume of the electrolyte removed, and re- placed by neutral manganous sulphate solution. This is clearly not a perfect solution of the difficulty. SERIES 111. To improve the efficiency of stirring in the immediate neighbourhood of the cathode, a rotating cathode was introduced, and used in subsequent experiments. I t consisted of an aluminium rod, the lower part of which was split to admit of the insertion of a glass fin.The separate glass stirrer was dispensed with. At the same time, the effect of lower tempera- tures was tried. Current density was as in Series II., the acidity being regulated near 0.05 N. as described above. TABLE 111. Number of Duration in Temperature. Current Purity of Experiment. I Minutes. I 1 1 Efficiency. 1 Metal. _.____ 4'9 5'6 5'7 Per cent, 5'3 17.1 I4.3 Per cent. 88-3 98'7 98.6 A comparison of the results of (19) and ( 2 0 ) with those of (22) indicates that the introduction of the rotating cathode has improved the purity of theTHE ELECTRODEPOSITION OF MANGANESE-PART I. 563 Current Efficiency. deposit. I t will be noticed also that, as a result of the smaller production of basic material, it is now possible to keep the voltage practically constant throughout the run.Equally striking is the effect of dropping the tempera- ture further-there is a marked increase in the purity of the metal, and the current efficiency, very small in ( 2 2 ) , is improved. The deposits from (23) and (24) were dead black in appearance on removal from the bath, showed glistening white streaks on being scraped, and could be burnished with emery paper to give an appearance like that ofipolished iron. The differences in current efficiency in the two experiments are attributable to lack of exact control over the acidity factor. SERIES IV. Using the same apparatus, the effects of varying current density and initial acidity, and of a further lowering of temperature were tried suc- cessively.Purity of Metal. TABLE IV. Pa cent. 17.6 23'3 22'9 13.2 4'1 15'5 22.0 27.4 26'3 52'5 57'8 2'2 Number of Experiment 25 26 27 28 29 30 31 32 33 34 35 36 Pa cent. 53'6 87'6 94'3 91.2 81'5 1'00 85.5 60 98'3 98.4 9*3 98-3 Temperature. 30'5' C. 26'0 29.0 263 26'0 29.0 25'5 23.0 23 '5 5 '0 5 '0 22'0 Initial Acidity. 0.05 N 0'05 0.05 0 '05 005 0'10 0.075 0.025 0.025 0.025 0.025 0.025 Current densit] in amps./dmz. 20 15 12.5 7'5 5 20 20 I0 7'5 5 7'5 7'5 Duration in Minutes. Voltage. 8.0 6.6 5'9 5'0 4'5 7 '0 6.9 5'9 4'9 4'4 5'9 6.1 I- Small differences between the figures in this table have no significance, owing to the ill-regulated acidity. The following main points, however, emerge. An increase in current density will reduce the purity of the deposit, unless the acidity is correspondingly increased. A decrease in current density will lower the yield, unless the acidity is proportionately reduced.To high acidities, therefore, correspond good deposits and poor current efficiencies. Finally, a lowering of temperature allows of much better current efficiencies. The deposits from (35) and (36) were of excellent quality, and metallic in appearance. SERIES V. Hitherto not more than one gram of manganese had been produced in any one experiment. In order to make larger quantities, and at the same time to amve at a better solution of the fundamental question of regulation of acidity at the cathode, we proceeded to the design and trial of various fornis of apparatus in which the electrolyte of slightly acidified manganous sulphate was caused to flow continuously through the cell, entering at the cathode and leaving at the anode, being passed once more through the cell after its acidity had been corrected.The first forms used need not be described-they tailed because the rate of flow of electrolyte was too low, and basic material became precipitated on the cathode. A third modifica- tion allowed of far more rapid flow, and gave comparatively satisfactory results, a IOO per cent. pure metal being produced at 56.5 per cent. current564 THE ELECTRODEPOSITION OF MANGANESE-PART I. Current Density in amps./dm2. _~ I0 I0 efficiency, using an initial acidity of nearly o*IN, a temperature of 8" C., and a cathodic current density of KO amps./dm2. Unfortunately the rate of flow was so excessive that it was impracticable on a laboratory scale to test the acidity of the electrolyte and to correct the same by regulated additions of alkali in time for its return to the supply reservoir.Eventually a design of cell was adopted in which anolyte and catholyte were kept apart by a diaphragm, each flowing separately through the cell. The catholyte as before was a manganous sulphate soiution, with an added acidity of o*IN. The anolyte was the same manganous sulphate solution, but with an acidity of 0'2N. This increased acidity was used in the hope that hydrogen ion migration from anolyte to catholyte during electrolysis would compensate for hydrogen ion discharge at the cathode, and thus keep the acidity in the catholyte substantially constant, doing away with the necessity of correcting it by the addition of alkali. This hope was realised, it being found possible to carry out runs of fair duration with the minimum of alteration to the composition of the electrolyte. The cathode compartment consisted of a Soxhlet extraction thimble (7.5 cm. high and 3.3 cm.in diameter), the bottom of which fitted in a cup-shaped glass vessel, into which it was cemented by sodium silicate. A tube descended vertically from the centre of the base of the cup, and was provided with a tap. The extraction thimble was pierced at the bottom with a hole 5 mm. in diameter. Electrolyte introduced at the top of the cathode chamber would thus pass freely through at a rate essentially determined by the tap. A glass tube (4.5 cm. in diameter) was sealed on outside the vertical exit tube, a little distance below the base of the '' cup," so as to surround concentrically the extraction thimble. The annular space between this tube and the thimble constituted the anode chamber.The whole was surrounded by an inverted bottle with base removed, the outer annular space serving as a cooling bath. The rotating aluminium cathode and platinum sheet anodes were as before. The catholyte, introduced at the top of the thimble, passed through the cell at nine litres per hour. The anolyte was introduced by a narrow tube, passing down to the bottom of the anode compartment, and left by an overflow at the top. The results of two typical experiments only are recorded here. Its rate of passage was about one litre per hour.hrs. 12 2* TABLE V. per cent. I 6.3 47% IOO per cent. pure; but 6-3 26'5 1 xoopercent. pure; flaked 1 very loose. , excessively. O c. 37 1 6 Duration. Voltage. EkE$. l i ! Nature of Deposit. Although the electrolysis was easy to carry out, and furnished a pure metal, the main object of the experiments, i.e. the production of heavy coherent manganese deposits, was defeated. The drop in current efficiency in (38) compared with (37) is simply due to the fact that the yield was always estimated from the increase in weight of the cathode, and in (38) consider- able flaking took place after the lapse of about one hour, strips of metallic manganese being noticed floating in the electrolyte. This behaviour wasTHE ELECTRODEPOSITION OF MANGANESE-PART I. 565 invariably observed.Deposits up to a calculated thickness of 0.03 mm. were very smooth and adherent. At about 0-1 mm. they were already very loose, and a continuance of the electrolysis led to flaking. The same phenomenon was observed in a number of experiments (unrecorded) carried out at and above room temperature-it does not appear to be connected with the low temperatures used in (37) and (38). SERIES VI. At this stage in the work, we again modified the conditions of electrolysis. (I) We had no pump available for circulating the catholyte, and the cell just described could not therefore be left to itself for a long run. (2) I t seemed dsirable to avoid the use of manganese salt in the anode compartment, and thus the consequent formation of MnO,. (3) So-called " conducting salts " ( e g .sulphates of sodium, magnesium, and ammonium) are known to improve the nature of the deposit in a nickel-plating bath. Preliminary experiments with sodium sulphate had led to very bad deposits-we therefore settled on ammonium sulphate. The bath employed worked with a stationary electrolyte. I t consisted of a rectangular glass cell, containing the catholyte, in which were stood two porous pots containing the platinum anodes. The same rotating cathode was used as before. The catholyte contained per litre 300 grams of MnSO,, 4H20, IOO grams of (NH,),SO,(I.~N), and 2 - 5 grams H2SOl (o*ogN. ). By the regulated addition of strong H2S04, the acidity of the catholyte was kept as constant as possible. The results of two preliminary experiments with this apparatus are given in Table VI.Our reasons were as follows. The anolyte was 1a5N. (NH,),SO, + o*ogN. HTSO,. TABLE VI. Current hrs. I per cent. 39 14'0 I 0 4 15'8 100 per cent. pure; crys- talline. 40 13'5 I0 I 9 9 i The deposits, besides being pure, were adherent and markedly crystalline. At the commencement of (39) the catholyte was found to have become neutral before the first addition of sulphuric acid was made. No basic material, however, came down on the cathode. In view of this fact, and of the low current efficiency in (qo), we were induced to try experiments with the electrolyte as above, but without the addition of any acid Somewhat to our surprise, the results were satisfactory. The yield of metal rose, and the deposit remained pure, no precipitation of basic material taking place either in the electrolyte or on the cathode, although the liquid smelt of ammonia at the end of the run.The cause is of course the well-known action of ammonium salts in suppressing the ionisation of ammonium hydroxide. It is nevertheless somewhat remarkable that, using such a high current density, the metal deposit should remain pure. Experiments were then directed towards finding the optimum conditions for current density and temperature, the essential results being contained in Table VII.566 THE ELECTRODEPOSITION OF MANGANESE-PART I. Number of gxperiment. 41 42 43 44 45 46 48 47 49 remperature. O c. '4 15 '4 '3 30 30 30 51 7 TABLE VII. Current Densit in .IllPS./lL2. I 0 I0 'LO to 15 ' 5 20 10 20 Duration. Current 3tliciency 22'6 20% 25.8 40'7 35'6 6.6 40.1 5'0 - Nature of Deposit.100 per cent. pure ; crys- talline. 9 9 9 9 Much basic deposit. 100 per cent, pure ; crys- talline. 99 9 ) Some basic deposit, 100 per cent. pure ; loose and fine-grained. Save (44),* (48), and (49), all these runs yielded metal of excellent purity, and quite adherent. Thus (41) gave 5.1 grams of highly crystalline, very With regard to other points, much ozone is liberated at the anode when the electrolysis is carried out at room temperature and the small amount of manganese salt diffusing through the porous pots is oxidised to permanganate. At higher temperatures, ozone formation is slight, the diffusion of manganese sulphate increases, and there is considerable anodic MnOa formation. Some of the results given in Table VII.are rather puzzling and need further work. The best conditions of those investigated would, however, definitely appear to be a temperature of 30° and a cathodic current density of 10 amps. /dm.2. Using these optimum conditions, attempts were made to get heavier deposits of manganese, but without success. After a certain point had been reached, the deposits began to become loose and deterio- rated, just as was the case in the experiments of Series V. In view of the marked difference in the physical nature of the original deposit in the two cases, the reason for this behaviour is by no means clear. In order to investigate the cathodic potential during the manganese deposition, a few experiments were carried out using a stationary cathode, but stirring the catholyte vigorously.Provided the stirring were good, and the volume of the catholyte sufficiently large, a pure bright and coherent (but thin) deposit of metal was obtained, using 10 amps./dmz at the cathode. The current efficiencies were low. The cathode potentials were measured during and after deposition, using a normal calomel electrode and a Luggin capillary. w a q " deposit with a lustre like freshly cut bismuth. The following were the results obtained. (a) Temperature 14' C. (6) Temperature 18" C. E, = - 1.210 volt during deposition. Readings taken hourly during deposition up to a period of three hours. E, = - I -220 volt (remarkably constant). On cutting off current, E, fell to - 1.096 volt, from which value it altered very slowly.(c) A cathode covered with manganese some time previously was placed in the bath and gave a value of E, = - 0.796 volt. Current was passed for five minutes, when E, was found to be - 1.266 volt. On cutting off current, the potential fell to - 0.850 volt and remained sub- stantially unchanged for thirty minutes. Current passed for thirty minutes, when E, was - 1-322 volt. Current cut off, when the potential fell quickly to (6) Temperature 31" C.THE ELECTRODEPOSITION OF MANGANESE-PART I. 567 - 0.803 volt, and, on further stirring, to - 0,771 volt. Current was then passed for another thirty minutes, the value of E, reached being - 1.373 volt. On stopping the current, there was a rapid drop to - 0.858 volt, followed by a slower fall to - 0.832 volt, after which the rate of change became very slow.In all these cases, the potential of a manganese electrode prepared some time previously was practically the same in the compound electrolyte as in a solution simply containing the same amount of manganese sulphate, ie. E, = - 0.796 to - 0'798 volt. The figures thus afford clear evidence that a very considerable excess polarisation above the equilibrium value is necessary for the cathodic deposition of manganese, the metal behaving in this respect very like nickel. Increased hydrolysis makes it impossible to overcome this factor by increasing the temperature, as can be done in the case of the ferrous metals. The fact which renders possible the deposition of this very electropositiie metal is the high over-voltage it presents to hydrogen ion discharge.At a current density of 10 amps. /cm4. this amounts to 1.044 volt at 16" C., as determined in an electrolyte consisting of N/,oNaOH + N/l NaaOkl The potential of the reversible hydrogen electrode in the solution used for electrolysing out the manganese was found to be E, = - 0.166 volt at 18" C., and the cathodic potential necessary for hydrogen discharge under these conditions consequently - 1.210 volt. This is very close to the second of the values observed during the manganese electrodeposition, and is actually equal to the first of them. The reason for the simultaneous deposition of hydrogen and of manganese is thus clear. SERIES VII. We referred at the beginning of this paper to the work of van Arsdale and Meier.z These authors electrolysed a neutral molar manganous sul- phate solution at 23O C., varying the cathodic current density between about g to 43 amps./foot2 (I to 4-6 amps./dm2), and claimed current efficiencies of 73 - 89 per cent., the maximum figure being obtained at about 2 amps./dm2.In a second series of experiments they added in- creasing initial amounts of acid, working at a current density of 8 amps./ foot2, and found that the current efficiencies fell off rapidly and almost linearly, becoming zero at about 0.36 per cent. of H2S04 in the electrolyte. They state their deposits to have been dark grey and powdery in character, very readily oxidising in the air, but do not say how the degree of purity was established. The current efficiencies were determined by dissolving the cathodic deposit in dilute acid, and titrating.In view of our own results, it seemed highly improbable that anything but very impure metal was obtained under these conditions. We therefore repeated their work, using a stationary copper cathode and platinum anodes. The results are contained in Tables VIII. and IX. The current efficiencies are "ap- parent" values only, as in Table 1. The deposits were in all cases highly impure, as analysis figures and the high '' current efficiencies " in Table VIII. show. The effect of variations in current density and in acidity are as would be anticipated, and it would appear that van Arsdale and Meier could never have got anything but very impure material. Their conclusion that, at an acidity of about 0.36 per cent. H2S04, cathodic deposition entirely ceases is confirmed.Campbell, Tram. Cltent. SOL, 1923, 123, 0323. ' L O C . cit.568 THE ELECTRODEPOSITION OF MANGANESE-PART I- ____- 0.56 1-7 2.8 1'1 2'2 55 57 56 I& I I* 1 I TABLE VIII. e; a L? CI i? G O c. 17 17.5 IS 17'5 IS J M CI d P 2'6 3'3 3'7 4'1 4'9 TABLE IX. Per cent. H&04 0.1 13 0.196 0.319 Per cent. H2S04 0.132 0.270 0'407 Undetermined 0'0095 N 0'011 0.0245 0'0295 Y ." v) P 6 w >. .d ._ 2 a - Undetermined. 16'4 per cent. metal. 5-4 per cent. metal. 3'6 per cent. metal. Highly impure. I I 4 Y .- 0 a. a" c 0 x .- a a4 iPer cent./ 3. Exjerimen fs with Chloride SoZzitions. Unless otherwise stated, all experiments were carried out with solutions The salt containing 350 grams of the crystallised tetrahydrate per litre.was found to be free from detectable traces of iron. SERIES VIII. Preliminary experiments were carried out at an early stage in this work, before the disturbing effects of high temperatures were realised, under similar conditions to those described under Series I. for sulphate electrolysis. The results obtained were in many respects like those already given in Table I, and need not be detailed here. There were, however, certain striking differences. The deposits, although impure, were generally denser and more adherent that those from sulphate solutions, and showed no sign of white basic material, being, on the contrary, dead black in appearance. Further, the voltage, instead of rising gradually to very high figures owing to increased resistance at the cathode, remained constant throughout the runs at values, depending on current density and on temperatu're, varying between 2-6 and 4-2 volts, figures some 2 to 2-5 volts lower than the lowest rewrded with sulphate solutions under similar conditions.The fact that the impurity, whatever its nature, was conducting metallically, together with its colour, pointed to its being manganese dioxide. Subsequent work, already described,' made it clear that manganous chloride can readily be anodically oxidised at platinum anodes to manganese tetrachloride. This Campbell, Tratrs. C h m . Sot., 1923, 123, 892.THE ELECTRODEPOSITION OF MANGANESE-PART I. 569 being the case, the mechanism of manganese dioxide precipitation at the cathode is an obvious one. Although with the more dilute manganous chloride solutions used in the experiments here described, a large amount of MnO, is precipitated at the anodes as in the sulphate experiments, enough of the tetrachloride nevertheless is carried over to the cathode to be precipitated there in considerable quantity, as a consequence of the local impoverishment of hydrogen ions.SERIES IX. Some experiments were carried out with the continuous Aow apparatus described under Series V. Using the same acidities as with the sulphate solution, viz., catholyte 0-1 N and anolyte 0.2 N, and keeping the level of the catholyte above that of the anolyte to counteract any tendency for per- chloride to diffuse into the former, it was found that no deposit of any kind was obtained. The acidities in both the cell liquids were accordingly halved, and over short runs, smooth and brilliant deposits were got, at current efficiencies of almost 70 per cent.These deposits were, however, not quite pure, averaging 97-5 per cent. of metal. The apparatus was then somewhat modified, by substituting a porous pot with a rubber stopper in the bottom for the Soxhlet thimble cemented in its glass base, and doing away with the external water bath. It was hoped that the increased dia- phragm resistance would in large measure prevent the Mn"" ions passing into the catholyte, and a run of 24 hours was carried out. The results were disappointing. A pure metal was certainly got, but the current effi- ciency was low, and there was a very marked tendency to flaking after half an hour. SERIES X.We have carried out a few experiments on the lines indicated by him, in order better to compare his results with ours. We invariably found that when we used manganous chloride in our anode compartment as he does, tetrachloride of manganese is formed, resulting in, sooner or later, the deposition of manganese dioxide on the cathode. I t may be remarked that the nature of Grube's anodes is not stated. Ours, as always, were of platinum. If, on the other hand, we used an apparatus in which the catholyte was of comparatively large bulk, and of composition 6N. MnCl, + 1.5N. NH,CI, whilst the anodes were contained in small porous pots dipping in the catholyte, the anolyte being simply I . ~ N . NH,Cl, we were able to obtain pure metallic manganese, the conditions of temperature, current density, and stirring being as indicated by Grube.Thus our first experiment on these lines lasted for five and a half hours, and gave us IOO per cent. metal at a 55 per cent. current efficiency. The physical character of the deposit was, however, if anything, worse than that of the manganese prepared from sulphate solutions-nodular and very loose. I t may be noted in connection with these experiments that we did not employ the addition of HC1 recommended by Grube-our experiments under Series VI. appeared to us to show that it would serve no useful purpose. Further, we lay considerable stress on the necessity for a large bulk of catholyte or small cathodic current concentration. If this is not arranged for, it is quite likely that manganous hydroxide will be precipitated in the catholyte during the course of a long run.In the introductory section, the work of Grube is referred to.5 70 THE ELECTRODEPOSITION OF MANGANESE-PART I. 4. Attempts to improve ihe naiure of ih dejosii. A large number of experiments have been carried out with this aim in view, without, however, any striking success. (a) A variety of addition agents were tried, using the standard manganese sulphate and ammonium sulphate electrolyte. None produced any marked improvement, and in several cases (e.6 gum arabic, dextrine, gelatin) the deposit was made much less pure. (6) With the same electrolyte, an experiment was done, making use of a burnisher (an ebonite strip) pressed against the rotating cathode. The deposit obtained was smooth and dense, but the current efficiency was reduced to 8.6 per cent. (c) Other electrolytes were tried. The use of sulphocyanide (see Moore I and Smith and Franke12) gave a more coherent but less pure deposit. The use of a mixture of manganese and ammonium perchlorates also led to nothing, as the whole catholyte hydrolysed with great rapidity, becoming filled with manganous hydroxide. (d) I t was thought that the flaking described in Series V. might have been due to the presence of the trace of iron mentioned as being present in the manganous sulphate used. This was removed by the addition of ammonia (its absence being subsequently shown by the ferrocyanide test), but no improvement in the nature of the deposit was subsequently noted. (e) I t may be mentioned in conclusion that it is a matter of common knowledge that the simultaneous evolution of large amounts of hydrogen renders it very difficult to get good cathodic deposits of any metal. The work of Kohlschutter and has pupils6 connects this difficulty with the strains caused in the electrodeposited layers by the presence or tne hydrogen. Attempts were made to see whether such depolarisers as were found by Stager to be successful in the case of nickel could also be used in manganese electrodeposition. Hydrogen peroxide, potassium chlorate, nitrobenzene and cinnamic acid were all employed, but with negative results. 5. Summary. (I) The electrodeposition of manganese from aqueous solutions of its (2) The effects of changes in composition of electrolyte, current (3) Pure manganese in coherent form can be prepared in small quantity (4) Attempts to prepare larger amounts in coherent form were sulphate and chloride has been studied. density, temperature, and type of cell have been investigated. with a current efficiency of 40-50 per cent. unsuccessful. This work was commenced in the summer of I 92 I and finished about Further experiments are being carried out on some of Christmas, 1922. the points mentioned in the paper. University of London, King’s Co Zlege, June, 1923. Loc. cit. * LOC. cit. J Tried in view of the favourable results obtained by Mathers in depositing lead and Cf. Engemann. Kohlschiitter and VuilIeumier. Zeitsch. Elekfrocitem., 1918, 3, 300. Stager. other metals. Zeitsch. Elektrochem., 19x1, 17, gro. Kohlschiitter. Helv. China. Acta, 1920, 3, 584. Helv. Chim. Acta, 1920,3, 614.

ISSN:0014-7672    

DOI:10.1039/TF9241900559

出版商:RSC    

年代:1924

数据来源: RSC

摘要:

118 ELECTRICAL THEORY OF ADBORPTTON The writer considers the double layer as consisting of a swface of rigidly fixed atoms under continuous bombardment of positively and negatively charged ions, any particular point on the rigid surface becoming in turn negative, neutral and positive, these conditions arisdg in any order. The observed contact difference is the average effect of these conditions. Where several kinds of atoms are present in the solution the average number of any one of them at the surface will depend on their concentbration, valency and mobility. The variation of contact Werence from negative to neutral and positive was observed with cotton and aluminium sulphate near the neutral point. These variations occurred during the same experiment, the readings being direct measurements of E.1I.F.s developed by filtration under pressure.This point would be covered by putting n2 = 1 and = 2 or 3 in Mukherjee’s equation No. 13.118 ELECTRICAL THEORY OF ADBORPTTON The writer considers the double layer as consisting of a swface of rigidly fixed atoms under continuous bombardment of positively and negatively charged ions, any particular point on the rigid surface becoming in turn negative, neutral and positive, these conditions arisdg in any order. The observed contact difference is the average effect of these conditions. Where several kinds of atoms are present in the solution the average number of any one of them at the surface will depend on their concentbration, valency and mobility. The variation of contact Werence from negative to neutral and positive was observed with cotton and aluminium sulphate near the neutral point.These variations occurred during the same experiment, the readings being direct measurements of E.1I.F.s developed by filtration under pressure. This point would be covered by putting n2 = 1 and = 2 or 3 in Mukherjee’s equation No. 13.118 ELECTRICAL THEORY OF ADBORPTTON The writer considers the double layer as consisting of a swface of rigidly fixed atoms under continuous bombardment of positively and negatively charged ions, any particular point on the rigid surface becoming in turn negative, neutral and positive, these conditions arisdg in any order. The observed contact difference is the average effect of these conditions. Where several kinds of atoms are present in the solution the average number of any one of them at the surface will depend on their concentbration, valency and mobility.The variation of contact Werence from negative to neutral and positive was observed with cotton and aluminium sulphate near the neutral point. These variations occurred during the same experiment, the readings being direct measurements of E.1I.F.s developed by filtration under pressure. This point would be covered by putting n2 = 1 and = 2 or 3 in Mukherjee’s equation No. 13. DISCUSSION 5 7 1 Mr. W. R. Cooper said he was very glad that the authors had tackled this subject, concerning which so very little had been done. With regard to Table I., he could not follow from the context why the voltage should vary so much ; for example from 4-7 to 16.That was a very large varia- tion, considering that the current density was constant. Mr. D. J. Macnoughtan : Some time ago I made a few experiments upon the electrodeposition of manganese. The results obtained from manganese sulphate solution were unsatisfactory. The addition of am- monium sulphate appeared to be beneficial. This appears to be amply confirmed by the authors’ experiments and is an interesting parallel to the beneficial effect upon the deposition of nickel produced by the addition of ammonium sulphate to nickel sulphate solutions. A serious drawback to the method employed by the authors is the use of insoluble anodes which renders control of the acidity and metal content of the solution very difficult. Manganese metal of high purity is obtainable (made by the thermit process), and it is probable that with the addition of a small proportion of ammonium chloride (or other suitable chloride) to the solution, to minimise passivity, anodes of this metal would prove satis- factory and allow of continuous deposition.The view of Engemann referred to on page 570 that iron present as an impurity is the chief cause of the flaking of nickel deposits is disputed by a number of investigators to-day, and it is interesting to note that the authors found that flaking occurs in manganese deposits containing no iron. I t would be useful to know the hardness of the manganese deposits obtained, and also whether the brittleness referred to could be corrected by heat treatment. Dr. H. Borns : I have only a question which is not meant critically.The authors refer to Grube and Metzger and the &its.$ Efektrochm. as to the preparation of sheets of manganese, I mm. in thickness. Is that reference correct 3 I know the paper ; it concerns the anodic behaviour of manganese and the preparation of permanganate by means of alternating currents superposed upon direct currents. But there is no mention of the electrolytic deposition of manganese, I think. Mr. A. N. Campbell said it was stated in the paper by Foerster, in his reference to Grube and Metzger’s work, that the purity of the deposit of manganese was 99.9 to IOO per cent. and that sheets of metal I mm. thick were made by them by electrodeposition. Mr. H. J. T. Ellingham pointed out that not only had the authors found the flaking effect, which was characteristic of iron and nickel and similar metals, but there was a very curious drop of the potential to a certain value, notably baser than the equilibrium value, which remained constant for a reasonable period.That was often found as a characteristic of iron and similar metals, but could the authors give any suggestion as to the significance of it in the case of manganese. In the case of the deposition of these other metals such behaviour was usually attributed to the alloying of the metal with the hydrogen, and it was considered that this alloying is largely responsible for the flaking effect. Dr. J. N. Pring said it appeared to him that the important feature in the deposition of manganese was the high irreversible resistance, mentioned in the early part of the paper and that, of course, was a type of passivity.Judging from the analogy of other similar cases, it seemed that current5 7 2 THE ELECTRODEPOSITION OF MANGANESE density would play a very large part in the efficiency of the deposition. The results in the paper rather indicated that a low current density was favourable for the separation of manganese relatively to hydrogen and that appeared to be in line with a number of anabgous observations. At one time he himself was engaged in the investigation of the deposition of zinc fiom impure solutions, and he found that the influence of impurities such as iron and manganese could be got over by using a high current density, because in that case a very small amount of manganese or iron was de- posited relatively to the zinc, on account of the much slower reaction velocity in passing from the ionic condition to that of free metal.That was analogous to the large amount of work which had been done on this subject by Foerster.' Foerster did not actually measure the manganese, but he measured a number of other metals such as iron and nickel, and he also measured the polarisation under different conditions of current density and found that iron gave a very high polarisation and nickel still higher. I t would therefore be interesting if the results of the authors were looked into in the light of this earlier work of Foerster on other metals to see how the polarisation fits in with the values obtained. Mr. F. S . Spiers said the suggestion of Mr.Ellingham that exfoliation of iron and nickel might be due to occluded hydrogen was confirmed by some work which he did a few years ago. He found that in depositing nickel from a sulphate solution varying the acidity showed the existence of avery sharp critical value, and if the acidity was above that value there was always exfoliation. I t was also found that this exfoliation was due to some sort of stress set up in the metal which disappeared when the !metal was heated in vacuo to get rid of the hydrogen. I n copper deposition there were similar effects that could be ascribed to occluded hydrogen. I t was well known that the addition of certain colloidal substances to a copper bath of normal composition had the effect of greatly hardening the metal deposited, an effect of some technical application.Such deposits, however, were often not only hard but very brittle. When these deposits were heated to 200" or 250' C., preferably in oil, the occluded hydrogen appeared to be driven out, and while the deposits retained their toughness and hard- ness, they lost their brittleness. Had the authors found occluded hydrogen to be present in their manganese deposits? Mr. A. N. Campbell replying to the discussion said that the variation of voltage in Table I. was simply due to the impure nature of the deposit. I t had been covered more and more with basic salt which was a bad con- ductor and the voltage rose for that reason. With regard to the use of soluble anodes, if fairly pure manganese could be obtained he thought that a very good continuous process could be worked on these lines. If there were other electro-negative metals as impurities there would be the danger of the impurities going into solution subsequently depositing on the cathode and liberating hydrogen in great volume and so reducing the current efficiency. As to the hardness of the manganese, he had no actual figures ; all he could say was that it appeared to be very hard and very brittle.Regarding the effect of current density on efficiency, in the experiments described in the paper the current density was varied considerably and it was found that by increasing the current density the efficiency went up. A perfectly pure electrolyte was used and there was no question of preferential deposition of ions, except the hydrogen ion. How an impure commercial electrolyte would behave, he would not like to say. He imagined it would lZeits. Elekfrochem., 1911, 17, p. 877.DISCUSSION 573 be unsatisfactory because manganese is more electro-negative than any other common impurity. With regard to manganese dioxide forming on the cathode in the manganese chloride experiments that was really due to tetrachloride of manganese, which he had shown elsewhere to be formed at the anodes where there is plenty of acid, and that diffused to the cathode where it hydrolysed and so deposited manganese dioxide on the cathode. He believed that flaking was due to the hydrogen, as had been suggested. An endeavour had been made to overcome this difficulty by every possible means ; by very carefully purifying the electrolyte, testing for traces of iron etc., but it had not been got rid of in that way. The only constant factor was a vigorous hydrogen evolution and therefore it was thought that this must be the cause of flaking. As to hydrogen being the cause of brittle- ness he did not think that was the case because he had heated manganese in high vacuum at 300' C., and the manganese still remained brittle.

ISSN:0014-7672    

DOI:10.1039/TF9241900571

出版商:RSC    

年代:1924

数据来源: RSC

摘要:

118 ELECTRICAL THEORY OF ADBORPTTON The writer considers the double layer as consisting of a swface of rigidly fixed atoms under continuous bombardment of positively and negatively charged ions, any particular point on the rigid surface becoming in turn negative, neutral and positive, these conditions arisdg in any order. The observed contact difference is the average effect of these conditions. Where several kinds of atoms are present in the solution the average number of any one of them at the surface will depend on their concentbration, valency and mobility. The variation of contact Werence from negative to neutral and positive was observed with cotton and aluminium sulphate near the neutral point. These variations occurred during the same experiment, the readings being direct measurements of E.1I.F.s developed by filtration under pressure.This point would be covered by putting n2 = 1 and = 2 or 3 in Mukherjee’s equation No. 13.118 ELECTRICAL THEORY OF ADBORPTTON The writer considers the double layer as consisting of a swface of rigidly fixed atoms under continuous bombardment of positively and negatively charged ions, any particular point on the rigid surface becoming in turn negative, neutral and positive, these conditions arisdg in any order. The observed contact difference is the average effect of these conditions. Where several kinds of atoms are present in the solution the average number of any one of them at the surface will depend on their concentbration, valency and mobility. The variation of contact Werence from negative to neutral and positive was observed with cotton and aluminium sulphate near the neutral point.These variations occurred during the same experiment, the readings being direct measurements of E.1I.F.s developed by filtration under pressure. This point would be covered by putting n2 = 1 and = 2 or 3 in Mukherjee’s equation No. 13.118 ELECTRICAL THEORY OF ADBORPTTON The writer considers the double layer as consisting of a swface of rigidly fixed atoms under continuous bombardment of positively and negatively charged ions, any particular point on the rigid surface becoming in turn negative, neutral and positive, these conditions arisdg in any order. The observed contact difference is the average effect of these conditions. Where several kinds of atoms are present in the solution the average number of any one of them at the surface will depend on their concentbration, valency and mobility.The variation of contact Werence from negative to neutral and positive was observed with cotton and aluminium sulphate near the neutral point. These variations occurred during the same experiment, the readings being direct measurements of E.1I.F.s developed by filtration under pressure. This point would be covered by putting n2 = 1 and = 2 or 3 in Mukherjee’s equation No. 13.118 ELECTRICAL THEORY OF ADBORPTTON The writer considers the double layer as consisting of a swface of rigidly fixed atoms under continuous bombardment of positively and negatively charged ions, any particular point on the rigid surface becoming in turn negative, neutral and positive, these conditions arisdg in any order.The observed contact difference is the average effect of these conditions. Where several kinds of atoms are present in the solution the average number of any one of them at the surface will depend on their concentbration, valency and mobility. The variation of contact Werence from negative to neutral and positive was observed with cotton and aluminium sulphate near the neutral point. These variations occurred during the same experiment, the readings being direct measurements of E.1I.F.s developed by filtration under pressure. This point would be covered by putting n2 = 1 and = 2 or 3 in Mukherjee’s equation No. 13.118 ELECTRICAL THEORY OF ADBORPTTON The writer considers the double layer as consisting of a swface of rigidly fixed atoms under continuous bombardment of positively and negatively charged ions, any particular point on the rigid surface becoming in turn negative, neutral and positive, these conditions arisdg in any order.The observed contact difference is the average effect of these conditions. Where several kinds of atoms are present in the solution the average number of any one of them at the surface will depend on their concentbration, valency and mobility. The variation of contact Werence from negative to neutral and positive was observed with cotton and aluminium sulphate near the neutral point. These variations occurred during the same experiment, the readings being direct measurements of E.1I.F.s developed by filtration under pressure.This point would be covered by putting n2 = 1 and = 2 or 3 in Mukherjee’s equation No. 13.118 ELECTRICAL THEORY OF ADBORPTTON The writer considers the double layer as consisting of a swface of rigidly fixed atoms under continuous bombardment of positively and negatively charged ions, any particular point on the rigid surface becoming in turn negative, neutral and positive, these conditions arisdg in any order. The observed contact difference is the average effect of these conditions. Where several kinds of atoms are present in the solution the average number of any one of them at the surface will depend on their concentbration, valency and mobility. The variation of contact Werence from negative to neutral and positive was observed with cotton and aluminium sulphate near the neutral point.These variations occurred during the same experiment, the readings being direct measurements of E.1I.F.s developed by filtration under pressure. This point would be covered by putting n2 = 1 and = 2 or 3 in Mukherjee’s equation No. 13.118 ELECTRICAL THEORY OF ADBORPTTON The writer considers the double layer as consisting of a swface of rigidly fixed atoms under continuous bombardment of positively and negatively charged ions, any particular point on the rigid surface becoming in turn negative, neutral and positive, these conditions arisdg in any order. The observed contact difference is the average effect of these conditions. Where several kinds of atoms are present in the solution the average number of any one of them at the surface will depend on their concentbration, valency and mobility.The variation of contact Werence from negative to neutral and positive was observed with cotton and aluminium sulphate near the neutral point. These variations occurred during the same experiment, the readings being direct measurements of E.1I.F.s developed by filtration under pressure. This point would be covered by putting n2 = 1 and = 2 or 3 in Mukherjee’s equation No. 13.118 ELECTRICAL THEORY OF ADBORPTTON The writer considers the double layer as consisting of a swface of rigidly fixed atoms under continuous bombardment of positively and negatively charged ions, any particular point on the rigid surface becoming in turn negative, neutral and positive, these conditions arisdg in any order.The observed contact difference is the average effect of these conditions. Where several kinds of atoms are present in the solution the average number of any one of them at the surface will depend on their concentbration, valency and mobility. The variation of contact Werence from negative to neutral and positive was observed with cotton and aluminium sulphate near the neutral point. These variations occurred during the same experiment, the readings being direct measurements of E.1I.F.s developed by filtration under pressure. This point would be covered by putting n2 = 1 and = 2 or 3 in Mukherjee’s equation No. 13.118 ELECTRICAL THEORY OF ADBORPTTON The writer considers the double layer as consisting of a swface of rigidly fixed atoms under continuous bombardment of positively and negatively charged ions, any particular point on the rigid surface becoming in turn negative, neutral and positive, these conditions arisdg in any order.The observed contact difference is the average effect of these conditions. Where several kinds of atoms are present in the solution the average number of any one of them at the surface will depend on their concentbration, valency and mobility. The variation of contact Werence from negative to neutral and positive was observed with cotton and aluminium sulphate near the neutral point. These variations occurred during the same experiment, the readings being direct measurements of E.1I.F.s developed by filtration under pressure. This point would be covered by putting n2 = 1 and = 2 or 3 in Mukherjee’s equation No.13.118 ELECTRICAL THEORY OF ADBORPTTON The writer considers the double layer as consisting of a swface of rigidly fixed atoms under continuous bombardment of positively and negatively charged ions, any particular point on the rigid surface becoming in turn negative, neutral and positive, these conditions arisdg in any order. The observed contact difference is the average effect of these conditions. Where several kinds of atoms are present in the solution the average number of any one of them at the surface will depend on their concentbration, valency and mobility. The variation of contact Werence from negative to neutral and positive was observed with cotton and aluminium sulphate near the neutral point. These variations occurred during the same experiment, the readings being direct measurements of E.1I.F.s developed by filtration under pressure.This point would be covered by putting n2 = 1 and = 2 or 3 in Mukherjee’s equation No. 13. THE CATHODIC BEHAVIOUR OF ALLOYS. PART I. IRON-NICKEL ALLOYS. BY S. GLASSTONE, M.Sc., Ph.D. ( A Paper vend before THE FARADAY SOCIETY, Monday, November 1 2 f 4 1923, SIR ROBERT ROBERTSON, PRESIDENT, K.B.E., F.R.S., itz the Ckir.) ReceivedJuGy I 3th, I 9 2 3. Up to the present very little work appears to have been done on the measurement of polarisation voltage at electrodes composed of metallic alloys, and therefore a systematic investigation in this field has been undertaken with the hope of throwing some light on the problem of over- voltage and allied phenomena.In working with alloys that are prepared from a fused mixture of the metals, the structure and no doubt the pro- perties of the alloy as an electrode will depend upon the method of cooling the melt ; further, the surface skin will probably be in a different condition from the rest of the alloy. In order to avoid difficulties introduced in this way, it is proposed to deposit the alloys electrolytically from solutions containing salts of both of the metals in varying proportions; by this method a series of mixtures should be obtained varying in composition from that of one pure metal to that of the other. The object of this work is to investigate the cathodic behaviour of these alloys, as well as the electrolytically deposited pure metals for the sake of comparison, along three lines : (I) to measure the cathodic potential during deposition from a solution of mixed salts, ( 2 ) to determine the potential required for hydrogen liberation in sulphuric acid and sodium hydroxide solutions, Le., ‘‘ over- voltage,” and (3) to investigate the action of depolarisers.Schlotter has shown that the reduction efficiency of any particular cathode in potassium chlorate solution is greatly influenced by depositing upon it traces of another metal in amounts insufficient to cover the surface of the cathode completely. Thus the system electrode metal-deposited metal-hydrogen might be more or less efficient for the purposes of reduction than the original electrode system before metal deposition. Since an alloy at which hydrogen is being evolved as a result of electrolysis, resembles to a very great extent the electrode system used by Schlotter, it appeared that a systematic investigation of the cathodic behaviour in the presence of de- polarisers would be of considerable interest.In the present paper the results obtained with a series of iron-nickel alloys are described and discussed. These alloys were deposited from solutions containing varying proportions of ferrous and nickel sulphates ; the total concentration of metal was always two equivalents per litre, and in addition contained free sulphuric acid to the extent of 0.05 N. The cathode consisted of a small sheet of lead which had been completely Zeitsch. Ehkirochem., 1921, 27, 394. 574PART I. IRON-NICKEL ALLOYS 0.85 0.68 0-60 0'50 0'39 5 75 0'75 0.79 078 0.74 0'63 0'72 0'70 0*63 0'55 0.65 0.62 0.52 0'48 0.51 0'55 0'50 0'39 0'39 v44 0'44 waxed over with the exception of an area of one sq.cm. which was left clean and bright ; the anode was of carbon, which was thought to be pre- ferable to platinum, since the latter might have dissolved and been de- posited on the cathode. With a current density of 0.045 amps./cm.2 smooth and bright adherent deposits were obtained in about one and a half hours; the presence of ferric salt, however, caused the deposit to be dark and powdery. The electrolytic bath was kept at room temperature (about 18') and was of sufficiently large volume to leave its composition practi- cally unchanged as the result of a comparatively small cathodic deposition.I n order to determine the composition of the alloy, which was only slowly soluble in dilute sulphuric acid, the deposition was made under identical conditions except that a hard gas carbon electrode replaced the one of lead; the alloy was dissolved off the carbon by means of a mixture of dilute nitric and hydrochloric acids, and the ratio of nickel to iron deter- mined as follows. Ammonium chloride and hydroxide were added to the solution and the mixture boiled and filtered ; the filtrate was analysed for nickel by the volumetric method using potassium cyanide whilst the ferric hydroxide precipitate was dissolved in dilute sulphuric acid, reduced and estimated by means of standard permanganate solution. In addition to deposition from mixed solutions, pure iron and nickel were also deposited electrolytically in order to have a complete series varying from 100 per cent.of one metal to IOO per cent. of the other. In the following table (I.) are given the compositions of the various solutions and cathodic deposits, together with the numbers which will be applied to them throughout the rest of this paper. TABLE I. No. I. 11. 111. IV. V. VI. VII. Atomic Ratio Fe/Ni. Solut1an. r m p i t . Fe only IOO per cent. Fe 4.0 5'1 1'5 2.3 0.67 1.2 0.25 0'47 0'020 0.14 Ni only 100 per cent. Ni Potentitzd during Catlrodic Dtposition.-Whilst the alloys were being deposited, the potential of the cathode at various current densities was measured against that of a normal calomel electrode, using saturated potassium chloride solution as intermediate liquid.The jet of the tube connecting the experimental electrode with the standard electrode pressed tightly against the surface of the former. The results obtained are given in Table 11. TABLE 11. C.D. 0'12 0.06 0.03 0.015 0.005 I. 1.11 0'95 0.82 0.70 0.67 11. 1 111. I IV. 1 v. I VI. i I-1-1- VII. 0'94 0.88 0'73 0'60 0.66 ~ All current densities are given in amps. per sq. cm. and potentials in volts on the Sutton, '' Volumetric Analysis,'' 8th ed., p. 252. hydrogen scale. The negative sign has been omitted throughout.s 76 THE CATHODIC BEHAVIOUR OF ALLOYS Fig. I , Curve A shows the relationship of the concentration of the nickel- iron solution to that of the deposited alloy, whilst Curve B gives the earresponding deposition potentials at a current density of 0.12 amps.per sq. cm. The deviations from the latter curve are believed to be due to the inability to reproduce the same conditions exactly in every case, since the potentials measured by the l' direct " method depend to some extent upon the state of the electrode surface (see communication to the Chemical Society entitled : l1 Intermittent Current Electrolysis, Part II."). Potentiah for HydrogeB EvoCuiion.-The lead electrodes which had been coated with alloy at a C.D. of o-oqg amps./cm.s, were used as cathodes in an electrolytic cell containing either N-sodium hydroxide or N-sulphuric acid, the anode being of platinum foil in every case. The potent$ of the cathode was measured during polarisation at various current densities, the standard electrode being Hg/HgO N-NaOH in the alkaline solution, and - 1'3 - 1'1 - 1.0 - 0.9 - 0.8 - 0.7 P) 2 e U (D Y d B 2: ." o 10 20 30 40 50 60 70 80 go 100 Atomic per cent of Ni in solid.FIG. I. Hg/HgSO, N-H2S04 in the acid solution. Connection between the test and standard electrodes was made by means of a tube ending in a fine jet ; both this and the connecting vessel contained the same liquid as was used in the electrolytic cell and in the standard electrode. The method of measurement was to put on the polarising current at a C.D. of 0.12 amps./cm.Z for five minutes and then to measure the potential at this and at other current densities at one minute intervals. Owing to the un- certainty as to the correct method of measuring overvoltage, the measured potentials of the cathode with current flowing are recorded in Table 111.; the values obtained may include a potential due to the resistance of a gas film at the surface of the electrode.PART I. Eiectrolyte : N-NaOH. C.D. 1 I. I 11. -I I-- 0'12 1-57 1'21 006 0.03 1-04 Electrolyte : N-H,SO,. 0.12 0.71 0'36 0.06 0.67 0.30 0.015 0.60 0.25 0.03 1 0'63 1 0'28 IRONNICKEL ALLOYS 5 7 7 TABLE 111. 111. I -08 1-05 1.03 1-01 0.36 0.30 0'28 0.26 IV. 1'12 1-07 1.04 1'00 0'37 0'30 0.28 0'26 V. 1.11 1.08 1.04 1'02 0.41 0'37 0'34 0.29 VI. 1 VII. 1-22 1.38 1-08 1.30 s o 4 1-27 1.18 I 1-34 Negative signs have been omitted throughout. Fig. 2, Curve A gives the potentials for hydrogen evolution in sodium hydroxide solution at a current density of 0-1 z amps. per sq. cm. as a function of the composition of the electrode material, whilst Curve B gives the cor- responding potentials with N-sulphuric acid as electrolyte.The remarks made concerning deviations from Curve B in Fig. I apply here too. - 1.6 B z -1'5 I z c -1.4 .- - 0-8 - 0.7 - 0'6 - 03 - 0.4 - 0.3 d I 2 z C ." o 10 20 30 40 50 60 70 80 go 100 Atomic per cent. of Ni in electrode. FIQ. 2. Cufhdic Behviwr in fh Presence of DcpoZan>ers.--In these experi- A. Saturated aqueous solution of potassium chloride and chlorate. I3. Mixture of IOO C.C. zN-NaOH (aqueous), 100 C.C. alcohol, and C. Equal volumes of zN-NaOH and alcohol saturated with nitro- D. Mixture of 30 C.C. 6N-H,S04 and 7 0 C.C. alcohol saturated with ments four different solutions were used I o grams benzaldehyde. benzene. ni trobenzene.5 78 THE CATHODIC BEHAVIOUR OF ALLOYS In the first place the efficiency of the various electrodes for reduction pro- cesses was determined. The experiments were carried out in a small piece of apparatus resembling the Hofmann apparatus for the electrolysis of water, in which the volume of hydrogen liberated at the cathode could be measured from time to time.By measuring the volume of gas liberated at the same time in the same electrolyte without the depolariser, the effici- ency of the reduction process could be determined at a particular current density. The anode used in these experiments consisted of a short platinum wire fused into glass, and diffusion between the anodic and cathodic portions of the apparatus was obviated by the introduction of a plug of asbestos.It is possible that the various electrodes may exert different catalytic effects on the substances produced by the reduction process; no attempt has yet been made to investigate this point further, but the fact that the potentials recorded during the process of electrolytic reduction (see below) were all of the same order of magnitude indicates that for a given de- polariser and electrolyte, the reduction products were always the same. In general the reduction eficiency of an electrode was greater at the beginning of the experiment, when the electrode was fresh from its de- position which always occurred with simultaneous hydrogen evolution. The rate of reduction soon settled down to a constant value, and it was during this time that the readings, from which the figures in Table IV.were cal- culated, were taken. This alteration in the reduction efficiency corresponds with the observations by R Russl that the efficiency of an electrode is increased by previous polarisation, but that this increase falls off after use. Since both electrolytic iron and nickel are known to contain sorbed hydrogen, it is possible that the latter acts as a positive catalyst for the reduction process, but its gradual removal by the depolariser causes the catalytic effect to disappear. In the table below are recorded the reduction efficiencies of the vari- ous electrodes in the presence of different depolarisers at definite current densities. TABLE IV. Dopol. A B C D C.D. 0.03 0015 0.03 I. 1 11. 1 111. I IV. I-I-I- 86 78 Practically 100 per cent.for all C.D. 1 v. j VI. 1 VXI. l-I-I----- per cent.lPer centlper cent, 1 Nqooapprecitye reduction 44 79 I 73 I 58 '8 up to 0'15. In the second portion of the investigation of the effect of depolarisers, the potentials of the electrodes were measured in the presence of the former, at various current densities. The electrolytic cell was fitted up in the usual way with a normal calomel electrode as standard; saturated potassium chloride solution was used as intermediate liquid. The polaris- ing current was put on for two minutes at a C.D. of 0.12 amps./cm.%, and then the potentials wkre measured at one minute intervals. I t was found that a very slight movement of the jet of the connecting tube of the standard electrode caused a large change in the measured potential, consequently the results obtained cannot be regarded a.s definite but the general slope 1 Zeitsch.physikal. Chcm., goo,^, 641.PART I. IRON-NICKEL ALLOYS 579 of the current density-potential curve, as well as its actual position is of some interest. The curves are represented in Figs. 3 to 6 below, and are self-explanatory. Discussion ofResuZts.-An examination of Tables I. and 11. and Fig. I shows that much lower potentials are required for the deposition of mixtures of iron and nickel than for either of the pure metals, and that the deposits contain relatively more iron than do the solutions from which they were deposited. That the mixtures should be deposited at less negative po- tentials is to be expected; iron and nickel probably form a continuous series of mixed crystals in which the free energy of each metal is less than it is in the pure state.The solution pressure will thus be diminished and so a less negative potential will be required for the deposition of the alloy than for either of the pure metals. We should expect that when the liquid solution contains excess of iron that the solid solution deposited should contain relatively more nickel, since the concentration of nickel in the solid is much less than that of iron and so its deposition potential would be depressed to a very much greater extent. Similarly when the solution contains a large proportion of nickel then the solid should contain relatively more iron, but since the deposition of nickel takes place at less negative potentials than that of iron, we should expect this increase to be less marked than in the previous case where the solution contains iron in excess.The actual results obtained (Table I.) are, however, not in agreement with this theory, and so it is evident that other factors, in addition to those mentioned, are operative; two suggestions are put forward which may account for the anomalous behaviour. In general the cathodic polarisation which results when a metal is being deposited may be attributed to the slowness of the reaction Me" + 2 0 + Me. I f we regard nickel atoms or ions as a positive catalyst for the reaction Fe" + 2 @ + Fey and iron as either an inert substance or as a negative catalyst for the reaction Ni" + 2 0 3 Ni, then it is to be expected that the deposits will contain relatively more iron than do the solutions.Foerster has suggested that the retardation of the process Me" + 2 0 3 Me is due to the hydrogen, which is deposited simultaneously, acting as a negative catalyst ; if the pre- sence of small amounts of nickel in the iron causes a considerable diminu- tion in the amount of sorbed hydrogen during electrodeposition, then the reduction in the amount of the negative catalyst will enable the process Fe" + 2 0 4 Fe to proceed at a much more rapid rate. If on the other hand we assume that iron has practically no effect on the sorption of hydrogen by electrolytic nickel, the speed of the process Ni" + 2 0 3 Ni will be unaltered and we should expect all the deposits to contain a relative excess of iron over that in the solution.Kremann and his co-workers a have shown that at increased temperatures the electrolytic deposits contain more nickel than do those deposited at lower temperatures. At high temperatures the sorption of hydrogen by electrolytic iron is diminished to a very great extents and consequently the further effect of any dissolved nickel can only be relatively small; hence at increased temperatures there will be a tendency for the deposits to contain relatively more nickel. I t appears that an investigation of the amounts of hydrogen sorbed by the electrolytically deposited mixtures of iron and nickel at various temperatures Zeitsch. Elektrochem, 1916, 22, 96. See Foerster, Elektrochemle Waseriger LGsungen, 1922, p. 378. Mmratsh.9 1913,341 I757 ; 19% 359 731.THE CATHODIC BEHAVIOUR OF ALLOYS would be of some interest, and it is hoped to be able to undertake this work at a later date.According to the views of Tammannl on the properties of metallic solid solutions we should expect the more noble metal, nickel, to exert a " protective action '' on the iron, making the latter more electronegative and so more easily deposited. I t is possible that this factor may influence the relative amounts of iron and nickel in the cathodic deposit, but it will hardly explain the fact that solutions containing very little nickel give de- posits containing relatively more iron. Perhaps the most striking results of the experiments described above are those shown in Table III. and Fig. 2 : it is seen that hydrogen evolution takes place at nickel-iron alloy electrodes at much lower potentials than at either pure iron or nickel.Since the cause of overvoltage is still not definitely known, it is not yet possible to offer a satisfactory explanation for this interesting result; it can only be said that the presence of iron and nickel together, prevents the accumulation of the hydrogen charge which is the immediate, if not the ultimate, cause of overvoltage. Further investiga- tion with other alloys may throw more light on the problem. Ridea12 has shown that a parallelism exists between hydrogen overvoltage and the catalytic activity of a metal for reduction processes involving gaseous hydrogen, and although this view has been criticised in the First Report of the Committee on Contact Cataly~is,~ it certainly appears to be a fact that the low overvoltage metals are the best catalysts.In view of the results described above, an examination of the catalytic activity of finely divided solid solutions of iron and nickel would be of great interest. The results obtained in the presence of depolarisers show clearly that the reduction efficiency of a particular electrode is not altogether dependent upon the hydrogen overvoltage, but that the electrode itself may exert a catalytic effect. In the case of the reduction of potassium chlorate, either iron is a positive catalyst or nickel is a negative catalyst, since reduction occurs to some extent at electrodes II., 111. and IV., although the over- voltage at these is much lower than at a pure nickel electrode where no reduction takes place.With alkaline benzaldehyde as depolariser, it is obvious that nickel is a positive catalyst, since the percentage of hydrogen available for reduction purposes at this electrode is greater than at one of iron which h a a higher overvoltage; further the alloys with their low over- voltage are as efficient as iron, due no doubt to the catalytic effect of the nickel. For the reduction of alkaline nitrobenzene iron is a powerful positive catalyst, since the alloys are very much better for reduction purposes than is nickel with its much higher overvoltage. All the electrodes appear to be equally efficient in the reduction of acid nitrobenzene, since all the hydrogen produced at current densities up to 0.15 amps./cm.2 is available for reduction purposes ; differences between the various electrodes are, however, indicated by the curves in Fig 6.Since the current density-potential curves cannot be regarded as exactly reproducible, it will only be necessary to discuss them briefly. The position and slope of these curves depends upon three factors : (i) the rate of the reaction 2H +- H,, (ii) the rate of the reduction process, and (iii) diffusion of the depolariser, if the liquid is not vigorously stirred. When factor (i) is very large, that is the metal has a low overvoltage, or (ii) small, 1 Cf. Zeitsch. w g . Chrm., 1919, 107, I et seq. a J. Amer. Chem. Sac., 1920,*, 94. *See Bancroft, J. Ind. Eng. Chem., 1922, 14,644.PART I. IRON-NICKEL ALLOYS 581 then as a rule very little reduction will take place, and the curve obtained will be the normal curve for hydrogen evolution. If however (i) is small, and (ii) is fairly large, then either all or part of the hydrogen will be available for reduction purposes and the cathode potential will be lowered ; the greater the value of (ii) the lower will be the potential for a given 0'12 - d 0.015 I I I - 0.9 - 1'0 - 1'1 - 1'2 volt Cathode potential.FIG. 3.-Depolariser A. current density. At high current densities when the rate of reduction is great, the concentration of depolariser in the region of the electrode will be considerably diminished, since fresh depolariser is not able to diffuse in as fast as it is removed; in this case the potential will rise greatly with increasing current density. In the case of alkaline nitrobenzene as depolariser (Fig. 5 ) the cathodic potentials of iron and of the alloys - 1'0 - 1'1 - 1'2 volt Cathode potential. FIG. 4.-Depolariser B. increase much more rapidly with increasing current density than do those of nickel, at which electrode the amount of reduction is comparatively small. When several electrodes have the same overvoltage, that is factor (i) is constant, and the rate of the reduction reaction (ii) is also constant, then factor (iii) must also be constant, and the P.D.-C.D. Curves should all5 82 THE CATHODIC BEHAVIOUR OF ALLOYS 0'12 C1 E 2 a 2 0.06 g 0'02 0.015 - = - - 0'12 =. '1- E k 2 0-06 4 0.03 o*org - - - I 1 IDISCUSSION 583 ComZusWn. An examination of the cathodic behaviour of iron-nickel alloys from several standpoints has brought to light a number of new facts concerning their electrode deposition, overvoltage and action in the presence of de- polarisers. Suggestions have been offered to account for some of the observed facts, but a fuller discussion of their significance in connection with the theories of overvoltage, delayed cathodic deposition and of cathodic reduction, must be left until further information is available. University College, Exeter.

ISSN:0014-7672    

DOI:10.1039/TF9241900574

出版商:RSC    

年代:1924

数据来源: RSC

摘要:

118 ELECTRICAL THEORY OF ADBORPTTON The writer considers the double layer as consisting of a swface of rigidly fixed atoms under continuous bombardment of positively and negatively charged ions, any particular point on the rigid surface becoming in turn negative, neutral and positive, these conditions arisdg in any order. The observed contact difference is the average effect of these conditions. Where several kinds of atoms are present in the solution the average number of any one of them at the surface will depend on their concentbration, valency and mobility. The variation of contact Werence from negative to neutral and positive was observed with cotton and aluminium sulphate near the neutral point. These variations occurred during the same experiment, the readings being direct measurements of E.1I.F.s developed by filtration under pressure.This point would be covered by putting n2 = 1 and = 2 or 3 in Mukherjee’s equation No. 13.118 ELECTRICAL THEORY OF ADBORPTTON The writer considers the double layer as consisting of a swface of rigidly fixed atoms under continuous bombardment of positively and negatively charged ions, any particular point on the rigid surface becoming in turn negative, neutral and positive, these conditions arisdg in any order. The observed contact difference is the average effect of these conditions. Where several kinds of atoms are present in the solution the average number of any one of them at the surface will depend on their concentbration, valency and mobility. The variation of contact Werence from negative to neutral and positive was observed with cotton and aluminium sulphate near the neutral point.These variations occurred during the same experiment, the readings being direct measurements of E.1I.F.s developed by filtration under pressure. This point would be covered by putting n2 = 1 and = 2 or 3 in Mukherjee’s equation No. 13.118 ELECTRICAL THEORY OF ADBORPTTON The writer considers the double layer as consisting of a swface of rigidly fixed atoms under continuous bombardment of positively and negatively charged ions, any particular point on the rigid surface becoming in turn negative, neutral and positive, these conditions arisdg in any order. The observed contact difference is the average effect of these conditions. Where several kinds of atoms are present in the solution the average number of any one of them at the surface will depend on their concentbration, valency and mobility.The variation of contact Werence from negative to neutral and positive was observed with cotton and aluminium sulphate near the neutral point. These variations occurred during the same experiment, the readings being direct measurements of E.1I.F.s developed by filtration under pressure. This point would be covered by putting n2 = 1 and = 2 or 3 in Mukherjee’s equation No. 13. DISCUSSION 583 Mr. W. R. Cooper said he was sorry the author was not present as there were one or two points he wished to raise. First, with regard to the method of finding the composition of the alloys. This was found by depositing a second sample on carbon for analysis, but he felt uncertain whether one would be justified in assuming that the same alloy would be obtained in the two cases.He should have thought it would have been more satisfactory to take the analyses of the alloys as used and found on the lead ; the lead could be determined and allowed for, and the composition of the alloy found in the usual way. The footnote to Table 11. reads rather curiously : ‘‘ All current densities are given in amps. per sq, cm. and potentials in volts on the hydrogen scale,” as though there was more than one kind of volt. As to the char- acter of the curve obtained, we were accustomed to curves of this kind connecting the physical properties of an alloy with its composition, but he did not know whether we should necessarily expect the same sort of curve when it came to overvoltage, and he had been wondering whether it was simply a question of the character of the surface obtained.They all knew that overvoltage varied a great deal with the character of the surface ; possibly the curve meant that there was a maximum in roughness corresponding to the minimum of the curve. The other possibility seemed to be a variation in the rate of evolution of hydrogen owing to the amount of hydrogen absorbed. Hydrogen was absorbed very markedly in the case of iron ; he did not know to what extent it was absorbed by nickel-perhaps not so much, and therefore possibly in between there would not be such a high absorption. The overvoltage might then vary and there might be a minimum obtained in that way.He did not know why the author had taken such a high current density; he should have thought it would have been better to have worked with a low current density so as to avoid other possible effects. Mr. D. J. Macnaughton was inclined to think that the troubles encountered when using alloys made by ordinary metallurgical methods would not be greater than those experienced with alloys made by the electrochemical methods employed by the author. In Table 1. the metal content of the iron and nickel, obtained electro- lytically is stated to be IOO per cent. Such a degree of purity is doubtful, as apart from a certain considerable percentage of hydrogen, at least traces of other elements would almost certainly be present.5 g4 THE CATHODIC BEHAVIOUR OF ALLOYS The composition of the alloys determined by analysis represents the average composition of the deposit and may differ quite considerably from the composition of the surface layers. Thus during deposition using, as the author does, an insoluble anode, it is doubtful whether the hydrogen ion concentration would remain constant.This, according to the views of the author on page 579, would affect the relative proportions of iron and nickel being deposited. The crystalline structure of deposited alloys may be extremely fine or comparatively coarse according to the conditions of deposition. Such differences would considerably affect the cathodic behaviour of the deposited alloys. I t thus appears to be doubtful whether alloys prepared by deposition, as described, are superior, for the purpose in view, to alloys prepared by casting at definite temperatures, removal of the external skin by machining and subjecting if necessary to a suitable heat treatment to obtain a definite structure.The study of these electrolytic deposits is interesting because alloys or mixtures prepared metallurgically differ from alloys or mixtures prepared by the chemical or electrolytic precipitation of solutions of several metals. Those differences were the object of the researches of Tammann on isomeric alloys from liquid or solid solutions, published in 1918 in the Gottinger Nuchrichten, a publication hardly accessible over here now. One of the important points for the cathode potential and the nature of the electrolytic deposits is the relative rate of the diffusion of the two metals. The author does not appear particularly to refer to diffusion.His reference to Tammann’s paper in the Zits. f: anorg. Chm., may possibly imply a mis- understanding. I t is, I think, in accord with Tammann’s view that the deposits from solutions containing very little nickel should contain relatively more iron. Dr. J. N. Pring said the author’s results were in close accordance with the theory of Foerster in his paper in the 2eits.fiir Elekfrockrn. (1911, 17, 877), in which he found that nickel has a higher overvoltage for separation than iron ; although it had a lower potential it had got a higher overvoltage, which would be a function of the current density. This fact should account for the results of Dr.Glasstone’s work in which iron is found to be separated in relatively larger amounts than nickel. These overvoltage or passivity considerations might give a more direct explanation than the theory put forward according to which the nickel atom was supposed to have a positive catalytic effect in the separation of iron. Perhaps the point could be settled by the method employed by Foerster of using an oscillograph for measuring the potential during deposition. Such a method should also be of particular value in the measurements made with depolarisers as in the work of Le Blanc and of Reichinstein. I n this work by means of an oscillograph, a determination was made in the case of other metals and electrolytes of the three separate factors referred to in the present paper as affecting the nature of the curves shown in Fig.6. Dr. S. Glasstone (communicated reply) : I t is possible that Mr. Cooper’s suggestion that the surface of the nickel-iron alloys was rougher than that of the pure metals may account for part of the lowering in overvoltage; observation by the naked eye shows, however, that nickel and nickel-rich alloys have very smooth surfaces and yet the overvoltage is lower than at iron where the electrodeposited surface is very rough. Also it has been found that a rough iron rich alloy may have almost the same overvoltage as a smooth nickel rich one. Dr. H. Borns: I do not quite agree with the last speaker.DISCUSSION 585 From the results of Kremann and his co-workers, it is seen that the composition of an alloy does not vary very much if the material upon which it is deposited is altered ; considering the difficulties that would arise in the analysis if large proportions of lead were present, it was considered that a smaller error would be involved by depositing the alloy on carbon.A number of previous workers have shown that the composition of an alloy varies during the period of deposition, and consequently the analysis of a deposit of appreciable thickness would not give the composition of the sur- face layer. In the present work the deposits could not have been much more than 0.005 cm. thick, and hence the difference in composition be- tween the surface and the whole of the alloy could not have been very great. Further, Figs. x and 2 show that an error of a few per cent. in the determin- ation of the composition of the alloy would not affect the general nature of the results. According to the present author’s interpretation of Tammann’s views and formulae one would expect nickel in an iron rich mixture to exert only a very small protective action on the iron atoms. This may be completely counter-balanced by the large diminution in the free energy and solution pressure of the nickel, and hence the electro-deposited alloy would not necessarily contain relatively more iron than the solution.

ISSN:0014-7672    

DOI:10.1039/TF9241900583

出版商:RSC    

年代:1924

数据来源: RSC

摘要:

118 ELECTRICAL THEORY OF ADBORPTTON The writer considers the double layer as consisting of a swface of rigidly fixed atoms under continuous bombardment of positively and negatively charged ions, any particular point on the rigid surface becoming in turn negative, neutral and positive, these conditions arisdg in any order. The observed contact difference is the average effect of these conditions. Where several kinds of atoms are present in the solution the average number of any one of them at the surface will depend on their concentbration, valency and mobility. The variation of contact Werence from negative to neutral and positive was observed with cotton and aluminium sulphate near the neutral point. These variations occurred during the same experiment, the readings being direct measurements of E.1I.F.s developed by filtration under pressure.This point would be covered by putting n2 = 1 and = 2 or 3 in Mukherjee’s equation No. 13.118 ELECTRICAL THEORY OF ADBORPTTON The writer considers the double layer as consisting of a swface of rigidly fixed atoms under continuous bombardment of positively and negatively charged ions, any particular point on the rigid surface becoming in turn negative, neutral and positive, these conditions arisdg in any order. The observed contact difference is the average effect of these conditions. Where several kinds of atoms are present in the solution the average number of any one of them at the surface will depend on their concentbration, valency and mobility. The variation of contact Werence from negative to neutral and positive was observed with cotton and aluminium sulphate near the neutral point.These variations occurred during the same experiment, the readings being direct measurements of E.1I.F.s developed by filtration under pressure. This point would be covered by putting n2 = 1 and = 2 or 3 in Mukherjee’s equation No. 13.118 ELECTRICAL THEORY OF ADBORPTTON The writer considers the double layer as consisting of a swface of rigidly fixed atoms under continuous bombardment of positively and negatively charged ions, any particular point on the rigid surface becoming in turn negative, neutral and positive, these conditions arisdg in any order. The observed contact difference is the average effect of these conditions. Where several kinds of atoms are present in the solution the average number of any one of them at the surface will depend on their concentbration, valency and mobility.The variation of contact Werence from negative to neutral and positive was observed with cotton and aluminium sulphate near the neutral point. These variations occurred during the same experiment, the readings being direct measurements of E.1I.F.s developed by filtration under pressure. This point would be covered by putting n2 = 1 and = 2 or 3 in Mukherjee’s equation No. 13.118 ELECTRICAL THEORY OF ADBORPTTON The writer considers the double layer as consisting of a swface of rigidly fixed atoms under continuous bombardment of positively and negatively charged ions, any particular point on the rigid surface becoming in turn negative, neutral and positive, these conditions arisdg in any order.The observed contact difference is the average effect of these conditions. Where several kinds of atoms are present in the solution the average number of any one of them at the surface will depend on their concentbration, valency and mobility. The variation of contact Werence from negative to neutral and positive was observed with cotton and aluminium sulphate near the neutral point. These variations occurred during the same experiment, the readings being direct measurements of E.1I.F.s developed by filtration under pressure. This point would be covered by putting n2 = 1 and = 2 or 3 in Mukherjee’s equation No. 13.118 ELECTRICAL THEORY OF ADBORPTTON The writer considers the double layer as consisting of a swface of rigidly fixed atoms under continuous bombardment of positively and negatively charged ions, any particular point on the rigid surface becoming in turn negative, neutral and positive, these conditions arisdg in any order.The observed contact difference is the average effect of these conditions. Where several kinds of atoms are present in the solution the average number of any one of them at the surface will depend on their concentbration, valency and mobility. The variation of contact Werence from negative to neutral and positive was observed with cotton and aluminium sulphate near the neutral point. These variations occurred during the same experiment, the readings being direct measurements of E.1I.F.s developed by filtration under pressure.This point would be covered by putting n2 = 1 and = 2 or 3 in Mukherjee’s equation No. 13.118 ELECTRICAL THEORY OF ADBORPTTON The writer considers the double layer as consisting of a swface of rigidly fixed atoms under continuous bombardment of positively and negatively charged ions, any particular point on the rigid surface becoming in turn negative, neutral and positive, these conditions arisdg in any order. The observed contact difference is the average effect of these conditions. Where several kinds of atoms are present in the solution the average number of any one of them at the surface will depend on their concentbration, valency and mobility. The variation of contact Werence from negative to neutral and positive was observed with cotton and aluminium sulphate near the neutral point.These variations occurred during the same experiment, the readings being direct measurements of E.1I.F.s developed by filtration under pressure. This point would be covered by putting n2 = 1 and = 2 or 3 in Mukherjee’s equation No. 13.118 ELECTRICAL THEORY OF ADBORPTTON The writer considers the double layer as consisting of a swface of rigidly fixed atoms under continuous bombardment of positively and negatively charged ions, any particular point on the rigid surface becoming in turn negative, neutral and positive, these conditions arisdg in any order. The observed contact difference is the average effect of these conditions. Where several kinds of atoms are present in the solution the average number of any one of them at the surface will depend on their concentbration, valency and mobility.The variation of contact Werence from negative to neutral and positive was observed with cotton and aluminium sulphate near the neutral point. These variations occurred during the same experiment, the readings being direct measurements of E.1I.F.s developed by filtration under pressure. This point would be covered by putting n2 = 1 and = 2 or 3 in Mukherjee’s equation No. 13.118 ELECTRICAL THEORY OF ADBORPTTON The writer considers the double layer as consisting of a swface of rigidly fixed atoms under continuous bombardment of positively and negatively charged ions, any particular point on the rigid surface becoming in turn negative, neutral and positive, these conditions arisdg in any order.The observed contact difference is the average effect of these conditions. Where several kinds of atoms are present in the solution the average number of any one of them at the surface will depend on their concentbration, valency and mobility. The variation of contact Werence from negative to neutral and positive was observed with cotton and aluminium sulphate near the neutral point. These variations occurred during the same experiment, the readings being direct measurements of E.1I.F.s developed by filtration under pressure. This point would be covered by putting n2 = 1 and = 2 or 3 in Mukherjee’s equation No. 13.118 ELECTRICAL THEORY OF ADBORPTTON The writer considers the double layer as consisting of a swface of rigidly fixed atoms under continuous bombardment of positively and negatively charged ions, any particular point on the rigid surface becoming in turn negative, neutral and positive, these conditions arisdg in any order.The observed contact difference is the average effect of these conditions. Where several kinds of atoms are present in the solution the average number of any one of them at the surface will depend on their concentbration, valency and mobility. The variation of contact Werence from negative to neutral and positive was observed with cotton and aluminium sulphate near the neutral point. These variations occurred during the same experiment, the readings being direct measurements of E.1I.F.s developed by filtration under pressure. This point would be covered by putting n2 = 1 and = 2 or 3 in Mukherjee’s equation No.13.118 ELECTRICAL THEORY OF ADBORPTTON The writer considers the double layer as consisting of a swface of rigidly fixed atoms under continuous bombardment of positively and negatively charged ions, any particular point on the rigid surface becoming in turn negative, neutral and positive, these conditions arisdg in any order. The observed contact difference is the average effect of these conditions. Where several kinds of atoms are present in the solution the average number of any one of them at the surface will depend on their concentbration, valency and mobility. The variation of contact Werence from negative to neutral and positive was observed with cotton and aluminium sulphate near the neutral point. These variations occurred during the same experiment, the readings being direct measurements of E.1I.F.s developed by filtration under pressure.This point would be covered by putting n2 = 1 and = 2 or 3 in Mukherjee’s equation No. 13.118 ELECTRICAL THEORY OF ADBORPTTON The writer considers the double layer as consisting of a swface of rigidly fixed atoms under continuous bombardment of positively and negatively charged ions, any particular point on the rigid surface becoming in turn negative, neutral and positive, these conditions arisdg in any order. The observed contact difference is the average effect of these conditions. Where several kinds of atoms are present in the solution the average number of any one of them at the surface will depend on their concentbration, valency and mobility.The variation of contact Werence from negative to neutral and positive was observed with cotton and aluminium sulphate near the neutral point. These variations occurred during the same experiment, the readings being direct measurements of E.1I.F.s developed by filtration under pressure. This point would be covered by putting n2 = 1 and = 2 or 3 in Mukherjee’s equation No. 13.118 ELECTRICAL THEORY OF ADBORPTTON The writer considers the double layer as consisting of a swface of rigidly fixed atoms under continuous bombardment of positively and negatively charged ions, any particular point on the rigid surface becoming in turn negative, neutral and positive, these conditions arisdg in any order. The observed contact difference is the average effect of these conditions.Where several kinds of atoms are present in the solution the average number of any one of them at the surface will depend on their concentbration, valency and mobility. The variation of contact Werence from negative to neutral and positive was observed with cotton and aluminium sulphate near the neutral point. These variations occurred during the same experiment, the readings being direct measurements of E.1I.F.s developed by filtration under pressure. This point would be covered by putting n2 = 1 and = 2 or 3 in Mukherjee’s equation No. 13.118 ELECTRICAL THEORY OF ADBORPTTON The writer considers the double layer as consisting of a swface of rigidly fixed atoms under continuous bombardment of positively and negatively charged ions, any particular point on the rigid surface becoming in turn negative, neutral and positive, these conditions arisdg in any order.The observed contact difference is the average effect of these conditions. Where several kinds of atoms are present in the solution the average number of any one of them at the surface will depend on their concentbration, valency and mobility. The variation of contact Werence from negative to neutral and positive was observed with cotton and aluminium sulphate near the neutral point. These variations occurred during the same experiment, the readings being direct measurements of E.1I.F.s developed by filtration under pressure. This point would be covered by putting n2 = 1 and = 2 or 3 in Mukherjee’s equation No. 13.118 ELECTRICAL THEORY OF ADBORPTTON The writer considers the double layer as consisting of a swface of rigidly fixed atoms under continuous bombardment of positively and negatively charged ions, any particular point on the rigid surface becoming in turn negative, neutral and positive, these conditions arisdg in any order.The observed contact difference is the average effect of these conditions. Where several kinds of atoms are present in the solution the average number of any one of them at the surface will depend on their concentbration, valency and mobility. The variation of contact Werence from negative to neutral and positive was observed with cotton and aluminium sulphate near the neutral point. These variations occurred during the same experiment, the readings being direct measurements of E.1I.F.s developed by filtration under pressure.This point would be covered by putting n2 = 1 and = 2 or 3 in Mukherjee’s equation No. 13.118 ELECTRICAL THEORY OF ADBORPTTON The writer considers the double layer as consisting of a swface of rigidly fixed atoms under continuous bombardment of positively and negatively charged ions, any particular point on the rigid surface becoming in turn negative, neutral and positive, these conditions arisdg in any order. The observed contact difference is the average effect of these conditions. Where several kinds of atoms are present in the solution the average number of any one of them at the surface will depend on their concentbration, valency and mobility. The variation of contact Werence from negative to neutral and positive was observed with cotton and aluminium sulphate near the neutral point. These variations occurred during the same experiment, the readings being direct measurements of E.1I.F.s developed by filtration under pressure.This point would be covered by putting n2 = 1 and = 2 or 3 in Mukherjee’s equation No. 13. THE VOLUMES OCCUPIED BY THE SOLUTE ATOMS IN CER- TAIN METALLIC SOLID SOLUTIONS AND THEIR CON- SEQUENT HARDENING EFFECTS. BY A. L. NORBURY, M.Sc., University College, Swansea. (A Pufer read &fore THE FARADAY SOCIETY, Noday, November I 2th, 1923. SIR ROBERT ROBERTSON, K.B.E., F.R.S., PRESIDENT, in the chair.) Received Spfe7nbcr I of&, I 9 2 3. Objecf of Research and Contents. The object of the present research was to determine the densities of certain copper a-solid solution alloys in order to calculate the volumes that the solute atoms were occupying in each case, the ultimate object being to compare the values so obtained with the hardness values of the same alloys-the latter having been determined by the author in a previous research.' In order to obtain accurate values, the effects of annealing temperature and cold-work on the density of copper were first studied.The densities of the copper a-solid solutions were then determined, and the results calculated in a certain manner in order to estimate the '' atomic volumes ?? occupied by the solute atoms in each case. The results obtained were then compared with the hardness values, and a certain relationship was brought out.I t was also found that the solute atoms were not occupying their normal atomic volumes, but that in each case a contraction or expansion had taken place-the amount of contraction or expansion apparently increasing as the ' I chemical affinity I' of the solute for the solvent increased. The paper is divided as follows :- I. Method of Determining Densities. 2. Effect of Annealing Temperature and Cold-Work on the Density 3. Density of Commercial Copper. 4. Densities of Certain Copper a-Solid Solutions. 5. Calculation of '' Mean Atomic Volumes " and Comparison of latter 6. Similar Results calculated from Data of Previous Workers. 7. Suggested Explanation of Hardening Effects of Elements in Solid Solution. 8. Contraction or Expansion of Elements in Solid Solution.9. Summary and Conclusions. of Cathode Copper. with Hardness Data. I. Method of Determining the Densities. Most of the specimens weighed 40 to 5 0 grms. ; they were weighed in air and in distilled water. The surfaces of the specimens were " finished 'I with a fine-cut file, all sharp edges and irregularities where bubbles might 586 yoimz. Inst. Metais, 1923, No. I., Vol. 29, pp. 407-444.SOLUTE ATOMS I N METALLIC SOLID SOLUTIONS 587 tend to lodge being removed. As long as this condition was satisfied, it was not found necessary to emery paper or polish the specimens (t$ section 2). For the weighing in water the specimens were suspended by means of a 0.01 inch diam. platinum wire. I t was found that errors arose from surface-tension effects between the wire and the surface of the water.A compensating " platinum wire-of exactly the same weight and length, and dipping into water to exactly the same depth as the suspending wire- was therefore used on the other arm of the balance. The water in the two vessels was levelled by means of a siphon tube filled with water. In making a weighing in water the final balance was obtained by making the pointer swing to the left and noting its point of rest, then making the pointer swing the same number of divisions to the right and again noting its point of rest, equidistant points of rest from the balance pointer's zero indicating that the correct weights had been applied. Besides eliminating surface-tension effects, this method automatically eliminates calculations correcting for the weights in air and in water of the suspending platinum wire. The densities were calculated from the formula :- m A = -(Qt - A) + X W (where m = weight in air, w = weight of water displaced, Qt = density of water at P, X = density of air at 16" = o*oo121.) TABLE DENSITIES OF CATHODE AND COMYERCIAL COPPER SPECIMENS AS ANNEALED AHD AFTER COLD-HAMMERING.~ Specimen. Annealing Temperature and Time. 950" for 2 hours 850" for 2 hours 99 Y9 750" for 2 hours 9 9 9 9 9 9 $ 9 550" fix 66 hours 550" for 2 hours 99 99 400" for 4 hours 9 9 9 9 goo" for 2 hours 9 9 9 9 9 9 9 9 knsity (Corrected; as Annealed. 8'853 8*918 8918 8.878 - - - 8'897 8-918 8908 8'923 8'924 Reduction in Thickness by Cold-hammering. Per Cent. 75 do 60 85 40 50 95 90 - - - - 66 50 do Densit (Corrected) After &ammering.588 SOLUTE ATOMS IN METALLIC SOLID SOLUTIONS 2.Efeci of Annealing Temperaiurc and Cold- Work on the Dznsiiy of The specimens tested were cut from those used in the previous research (la. tit.). The cathode copper specimens were melted under molten barium chloride (to exclude oxygen), were subsequently cold-hammered (about 50 per cent. reduction in thickness), and were then annealed at the tempera- tures stated in Table I. and Figure I. The densities of the specimens as annealed are given in Table I., and plotted in the left-hand margin of Fig. I. I t is thought that the decrease in density with increase in annealing temperature is due to the expansion of gas contained in minute “blow- holes ” in the specimens, although the specimens were quite sound and free from blow-holes in the ordinary sense-none being visible at roo diameters.Cathode Copper. 2 -+ .n I It 4 ,x a a“ ..- 20 30 40 50 60 70 80 90 Percentage reduction in thickness by cold-hammering. FIG. 1.-Densities of cathode copper specimens as annealed (X) and after cold- hammering (.). The effect of cold-hammering on the densities of the various cathode copper specimens will be seen from Fig. I and Table I. With from o to 50 per cent. reduction in thickness the specimens become denser-due pre- sumably to the closing up of the minute blow-holes. With from 50 per cent. reduction onwards the densities of the specimens are very nearly constant at 8.924. A relationship very similar to that shown in Fig. I is shown by Johnson 1 for copper after various amounts of cold-roZling.In the case of cold-drawing, Alkins,2 Kalbaum and others, have shown that the density decreases. 1 Johnson, Journ. Inst. Metals, No. I, 1920, Vol. 23, p. 474. *Alkins, Journ. Inst. Metals, No. I, 1920, Vol. 23, p. 411.AND CONSEQUENT HARDENING EFFECTS 589 3. Dcnsify of Commrcia 2 Copgcr. In Table I. are also shown the densities of three specimens of commercial copper, from three different manufacturers. I t will be seen that this less pure copper has an appreciably lower density than cathode copper (viz. 8.904 as against 8.924). DENSITIES AND TABLE 11. MEAN ATOMIC VOLUMBS " OF COPPER Q-SOLID SOLUTIONS. Wei ht Per t e n t . Added Element. A1 1.92 A1 3-82 A1 5-84 A1 7-02 Si 0.93 Si 1.71 Si 2.29 Si 2.67 Si 2-71 Si 3-83 Mn 2-40 Mn 7.20 Mn 15-37 Mn 31-08 Ni 4'52 Ni 9-20 Ni 14.93 Zn 4-06 Z n 7-86 Zn 11$6 Zn 15'58 Ag 1'44 Ag 2'75 Ag 4'24 Ag 5'53 Sn 1.00 Sn 2.03 Sn 2-96 Sn 3.88 Sn 5'75 ~- Density (Corrected).As Annealed. 8.617 8.320 8'05 I 7'%P 8-81 L 8.643 8'577 8-574 8.391 8.796 8'564 8.710 8,126 7.810 8'913 8'917 8.824 8.884 8.816 8'776 8.688 8'810 8-887 8 - 8 9 8'837 8-927 8.903 8'884 8'873 8.921 After Hammering 50 Per Cent. Reduction. 8'611 8-3 I I 8'043 7'904 8'799 8*642 8.605 8.589 cracked 8'819 8'574 8.712 - - 8.940 8'945 8.939 8'879 8'850 8.777 8.743 8*928 8'955 8.974 8'974 8'927 8.914 8.926 8'922 8-92 I Atomic Per Cent. Addad Element. 4'39 A1 833 A1 12-71 A1 15*05 Si 2.06 si 3'75 Si 4-80 si 5'79 Si 5-87 Si 8-19 Mn 2-77 Mn 8-25 Mn 1.7'38 Mn 34-39 Ni 4-88 Ni 9-89 Ni 15*@ Zn 3'96 Zn 7-66 Zn 1136 Zn 15'1g Ag 0.85 At3 1-64 Ag 2-55 Ag 3'33 Sn 0.54 Sn 1.10 Sn 1'60 Sn 2.11 Sn 3'16 Mean Atomic Weight.02*00 w 4 9 58'97 58'12 62'88 62.28 61 'go 61-56 61-54 60.78 63'36 62.88 62-14 60.72 63'36 63.1 I 62.82 63-65 63'73 63'80 63'88 63-98 64'35 64'73 65-09 63-91 64-50 64'77 65'38 64-21 '' Mean Atomic Volume.' As Annealed. 7-19 7-28 7'33 7'35 7'14 7-15 7-16 7-18 7'18 7-25 7.20 7'34 7'65 7'78 7-11 7'20 7.1 2 7'17 7-22 7-27 7'35 7'24 7'29 7'38 7'16 7-20 7'24 7-29 7'37 - After Hammering io Per Cent. Reduction. 7-19 7'27 7-33 7'36 7-15 7.15 7-16 7'15 7-17 - 7-18 7'33 - - 7-09 7-36 7'04 7'17 7.22 7 '37 7'31 7*=7 7-18 7.21 7'25 7-16 7-19 7'23 7-27 7'33 4. Densities of Certain Copper a-Solid Sotufions. The experiments already described showed that in order to obtain a correct value for the density of copper, it was necessary to close up any minute blowholes by cold-hammering.With 50 per cent. reduction in thickness and onwards fairly constant density values were obtained. In order to compare the densities of the copper a-solid solutions it was decided to hammer them all 60 per cent. reduction in thickness and to keep the size and shape of the specimens as much the same as possible.590 SOLUTE ATOMS IN METALLIC SOLID SOLUTIONS The alloys were melted under barium chloride, cooled in crucible, cold- hammered to about 50 per cent. reduction, and then annealed at suitable temperatures between 600’ and 950° ; their preparation, etc., is described more fully in the previous paper.’ The results for each alloy, “as annealed ” and “ after hammering 60 per cent.,” were obtained with dzyerent pieces cut from the same specimen.They are shown in Table II.,2 and are plotted in Fig. 2. If the densities of these alloys before and after hammering are compared, it will be seen that in a number of cases there is no difference in density. It seems probable, therefore, that the added elements have eliminated the gas which caused unsoundness in the cathode copper specimens. Judging by the different results obtained for cathode and commercial copper, oxygen must have a relatively large effect on the density, and its presence in small amounts probably accounts for some of the irregularities. ..I P B 8’5 8’4 8’3 S’Z 8’1 7’9 I z 3 4 5 6 7 8 g x o r r 1 2 1 3 ~ 4 1 5 Weight per cent.added clement. FIG. 2.-Densities of copper .-solid solutions. The above sources of error, viz., unsoundness and oxygen, would in each The highest value obtained for case tend to giva too low a density value. each alloy has therefore been taken for plotting in Figs. 2 and 36. 5. CuZcuZation of 4 L Mean Atomic VoZumes,” and Comparz3on ‘with Hardness Data. I n comparing the atomic volumes of the elements one is comparing the volumes occupied by equal numbers of atoms of each element (viz., by one gramme atomic weight). I t was desired to compare the volumes occupied by equal numbers of atoms of the copper a-solid solutions in a similar way, in order to calculate the apparent volumes occupied by the solute atoms in each case. The 1 Loc. cit. The densities obtained for the aluminium-copper and the zinc-copper alloys are very similar to those given by Reader, yourn.Inst. Metals, VoI. XVIII., No. 2,1922, p. 322, and Bamford, Ibid., Vol. XVII., No. I, p. 212.cu. ATOMIC PER CENT ADDED ELEMENT FIG. 3a-Log. a + n hardness values plotted against atomic composition, copper caolid eolutions.592 SOLUTE ATOMS IN METALLIC SOLID SOLUTIONS “ mean atomic volumes ” of the solid solutions were therefore calculated as follows :- In a binary alloy of metals A and B. Mean atomic weight = atomic weight A x per cent, A in alloy + atomic weight B x per cent. B in alloy. “ Mean atomic volume ” = The results calculated in this way are shown in the last two columns of Table 11. The “ mean atomic volumes ” from Table 11. are plotted in Fig.36 against atomic composition. of the same alloys, as deter- mined in the previous research,2 are plotted in Fig. 3a for comparison. I t will be seen that the hardness curves (Fig. 3a) are very similar to the “mean atomic volume” curves (Fig. 36). In fact, if one plots dzyerence between hardness of I atomic per cent. solid solution and that of copper, against difirence between “ mean atomic volume ” of I atomic per cent. solid solution and that of copper (as has been done in Fig. 4), there is a simple relationship. Mean atomic weight density of alloy * The hardness values 7’3 7‘2 7’1 I 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 Atomic per cent. added element. FIG. 3b.--“ Mean atomic volume ” values plotted against atomic composition, Copper a-solid solutions. There is one exception, however, and that is in the case of the silicon- copper solid solutions.This exception will be referred to again later. 6. SimiZar ResuZts CaZcuZufed from Data of Previous Workers. Goebel’s data for the densities of certain lead a-solid solutions have been calculated as (‘ mean atomic volumes,” and are plotted in this form in Fig. 56 4 for comparison with his hardness results in Fig. 5a. The “ mean atomic volume ” differences are plotted against hardness differences in Fig. 6 in the same manner as the results in Fig. 4. I t will be seen that the relationship is very similar to that in Fig. 4, but again there is one exception-the sodium-lead alloys in this case. 1 From the initial slopes of the curves in Fig. 3a. the following increases in the values of log.a + t l , for I atomic per cent. added element have been estimated :-Zn 0.003, N i o 00 $5, A1 0-012, Si 0.016, Mn 0’025, Ag 0’053, Sn wogg. These values have been used in Fig. 4. 3 L O C . cif. 3Goebe1, Zeitschrift filr Mctallkunde, Vol. 14, Sept.-Dec., 1922. 4 This method of plotting shows the limit of solubility of sodium in lead very clearly (cf. Fig. 56).AND CONSEQUENT HARDENING EFFECTS I 2 3 4 5 6 Difference in size of solute and solvent atoms. FIG. +-Hardening effects plotted against difference in size of solute and' solvent atoms (copper a-solid solutions). ~ ~~ I z 3 4 5 6 7 8 g 1 o 1 1 1 ~ 1 3 1 4 1 5 Atomic per cent. added element. FIG. 5a.-Brine11 hardness numbers plotted against atomic composition. Lead a-solid solutions (Goebel's data). : 18'4 3 18'3 cs) 18'2 -g 18.1 ; 18'0 Y 8 E 17'9 17.8 I 2 3 4 5 6 7 8 9 10 11 1 2 1 3 1 4 1 5 Atomic per cent.added element. FIG. 5b.--" Mean atomic volumes " plotted against atomic composition. Lead a-solid solutions (calculated from Goebel's data), VOL. XIX-T23 5 93594 SOLUTE ATOMS IN METALLIC SOLID SOLUTIONS -No I I I I I I 2 3 4 5 6 I - - . . . Difference in size of solute and solvent atoms. FIG. 6.-Hardening effects plotted against difference in size ot solute and solvent atoms. (Lead a-solid solutions, from Goebel’s data,) I 2 3 4 5 6 7 8 9 Atomic per cent. added element. FIG. 7a.-Ultimate stresses plotted against atomic composition, aluminium (Data from 8th and 10th Alloys Research Reports.) =-solid solutions. In Fig. 76 ‘‘ mean atomic volumes ” calculated from data in the eighth 1 I n Fig.7a the an2 tenth2 Alloys Research Reports have been plotted.3 1 Carpenter and Edwards, PPOC. Itrst. Mech. Eng., 1907, pp. 229 and 242. 1 Rosenhain and Archbutt, Proc. Inst. Mech. Eng., 1912, pp. 356 and 385. 3 The ( 6 mean atomic volumes ” have been plotted with the values of the ordinates decre&ng as they ascend, in Fig. 76, in order to bring out the similarity to Fig. 7a more clearly.AND CONSEQUENT HARDENING EFFECTS 595 ultimate stresses of the same alloys are shown. of results show the same general type of relationship. Here again the two sets 7. Suggested Exjlnnation of Hurdening Efecis of EZenzenfs i72 Solid So Zu fion. In view of the relationship brought out by the foregoing results, it is interesting to recall that a large part of Roberts-Austen’s work was directed at trying to prove a relationship between the atomic volumes of the added elements and their effects on the mechanical properties of pure metals.Many suggestions have since been put forward to explain the increased hardness of solid solutions, the most recent, notably, being those of Jeffries and Archer’z who ‘( have attributed the hardness of solid solutions largely to increased inter-atomic forces” and to interference with slip; and of 99’ 9-92 -6 993 P zl 996 5 997 - 994 *i 995 ld 998 9’99 10’00 X I 2 3 4 5 6 7 8 9 Atomic per cent. added element. FIG. 7b.--“ Mean atomic volumes ” plotted against atomic composition, aluminium a-solid solutions. (Data calculated rrom 8th and 10th Alloys Research Reports.) Rosenhaiq3 who states, “ since the amount of distortion which the intro- duction of a given ‘dissolved’ atom produces in the space-lattice of the solvent metal governs both the limiting solubility of the dissolved metal and the degree of hardening produced in the solvent metal, we should expect to find that the hardening effect of one metal upon another in the form of a solid solution should, to a first approximation be inversely pro- portional to its solid solubility.” Although the above are in general agree- ment with the foregoing results, neither of these theories fully explains them.I t is suggested that two types of solid solution must be assumed: (u) atomic solid solutions, and (b) molecular solid solutions. The ex- It may be assumed that the Brine11 hardness curves of these alloys would be very Jeffries and Archer, Chem.and Met. Eng., Feb. 8th, 1922, p. 249. similar to those shown in Fig. 7a. 3 Rosenhain, Proc. Roy. SOL, Series A, Vol. gg, No. 2698.596 SOLUTE ATOMS IN METALLIC SOLID SOLUTIONS ceptions in Figs. 4 and 6 (viz., Si in Cu and Na in Pb) being of the second type- In the first type, atoms of solute replace atoms of solvent and distort the latter‘s space-lattice 1 according as they differ in size from the solvent atoms. I t will be seen from Fig. 4, however, that this effect is not a simple linear one. Considering the portion of the curve between the intercept and the positions of Zn and Ni,2 the indication seems to be that the space- lattice of copper can suffer a certain amount of distortion before the hard- ness is materially affe~ted.~ Further distortion of the space-lattice (viz., between the position of Zn and Ag, respectively, on the curve) causes an almost linear increase in the hardening effect.With still further distortion (viz., Ag to Sn) the hardening effect tends to approach a maximum. The greater the distortion the greater the hardening effect. Similar remarks apply to the curve shown in Fig. 6 . I t is possible that the “ chemical affinity ” effects discussed in Section 8 are also influencing the results. Certain exceptional cases (uiz., Si in Cu and Na in Pb., cf: Figs. 4 and 6) do not, however, fall in line with the preceding general explanation, and it is therefore suggested that in these cases ‘‘ molecular solid solutions ” are formed.These exceptional elements have lower atomic weights than any of the other solute elements investigated, and the “ chemical affinity ” between solute and solvent atoms is probably particularly ~ t r o n g . ~ I t is therefore suggested that each atom of these elements has pulled one or more of the surrounding solvent atoms out of the latter’s space-lattice, to form a molecule of inter-metallic compound having a new space-lattice. In the above-mentioned cases, however, these molecules are able to exist dispersed singly throughout the solvent’s space-lattice. The relative interference with slip and consequent hardening would be much greater in this type of solid solution. With regard to the complicated nature of the hardness curves shown in Fig. 3a (viz., bending upwards and then tending to reach a maximum), this is probably explained by space-lattice considerations, the resistance to slip increasing more rapidly than the distortion at first, then, after a certain point, new possible planes of slip tending to become available.With regard to the relationship between hardness and solubility suggested by Rosenhain (Zuc. lit.), the hardness results shown in Fig. 3a indicate that the relationship is not such a simple one. Manganese, for instance, which is believed to form continuous solid solutions with copper, has a relatively large hardening effect. Other factors have apparently to be taken into account. They do not, however, cause a new space-lattice to be formed, as Bain (Chem. and Met. Eng., 1922, April 5th, p.655, Abstract) has shown by X-ray analysis in the case of copper containing 30 per cent. zinc, and in the case of certain other solid solutions. 2 Nickel differs from the other solutes plotted in Fig. 4 in SO far as its atoms distort the copper space-lattice by being smaller than the copper atoms. SThis possibility bears some relationship to the fact that small amounts of cold-work do not increase the hardness of copper when the latter is expressed as log. a + n values. 4It will also be seen from Figs. 3b and 5b that the c c mean atomic volume ” curves for these elements are much more curved than those of the others.AND CONSEQUENT HARDENING EFFECT.S Normal Atomic Volume of Solute. 597 Contraction or Expansion. TABLE 111. APPARENT ATOMIC VOLUMES OF SOLUTE ELEMENTS IN CERTAIN DILUTE SOLID SOLUTIONS (Cf.FIG. 8). 7'6 6.6 10.3 9'2 10'0 16'5 11'2 Solute and Solvent. + 1.3 - 0.3 - 0.3 - 1.8 - 3'4 - 3-6 - 0'2 Mn in Copper . . ' . Ni , , , , . . . . Al ,, ,, . . . . Sn ,, ,, . . . . Si ,, , , . . . . ;; :; $ 9 ' - * , , - . . . 22'0 12'8 13'3 16'5 21'2 Na in Lead . . . . Cd ,, ,, . . . . H g ,, ,, . . . . Sn , , , , . . . . Bi ,, ,, . . . . - 3'7 4- 0.9 - 0.5 - 2'2 - 1'2 Zn in Aluminium . . . c u 9, 9 , . . . Apparent Atomic Volume of Solute. I - 8.9 6.3 7'9 8.2 12.8 7'4 10'1 18'3 11.6 14.2 16.0 19.1 9'9 5'2 9 '2 7'1 + 0.7 - 1'9 8. Contraction or Expansion of EZements in SoZid Sobtions. If the initial slopes of the ' L mean atomic volume " curves shown in Figs. gb, gb, 7b are taken, it can be calculated by extrapolation that the solute atoms are (when present in very dilute solid solution) occupying spaces corresponding to the atomic volumes shown in Table III.In this table their normal atomic volumes are also given, also the differences between the two. The question of calculating what expansion or contraction has occurred is difficult owing to the different space-lattices of the elements. For instance, the metal zinc crystallising in a hexagonal space-lattice has an atomic volume of 9.2, and it is calculated that the zinc atoms when present in small quantities in solution in copper and distributed in the more closely packed cen tre-faced-cube lattice of copper have an apparent atomic volume of 8.0. The question would be simpler were it known what volume the zinc atoms would occupy if the metal crystallised in the more closely packed centre-faced-cube lattice as do the copper atoms.The above factor constitutes an unknown variable, but apart from this the results seem to show a certain relationship. The solute elements which have least "chemical affinity" for copper show the smallest amounts of contraction on entering into solution. Those which have greater affinity show a much larger contraction (and in the case of manganese an expansion). The above is shown in Fig. 8, where the differences between the observed and the normal atomic volumes of the solutes are plotted against the positions the solutes occupy, with respect to copper in the Pehodic Table. In a previous paper1 the relative effects of equi-atomic percentages of various elements in solid solution in increasing the electrical resistivity of 1 Norbury, Trans. Faraday SOC., 1921, VoI.XVII., NO. I, p. 251.598 SOLUTE ATOMS IN METALLIC SOLID SOLUTIONS copper were plotted against their positions with respect to copper in the Periodic Table in a similar manner. I t was suggested (hi. tit.) that the results could be explained as being large or small according as the solute element was near to or far from the solvent in the Periodic Table, that is according to the amount of “ chemical affinity ” between the two. I t is thought that something of the same nature is influencing the results shown in Fig. 8, the apparent contraction or expansion of the solute being greater as the chemical affinity is greater. The same sort of result has been arrived at by Bainl from X-ray analysis of certain solid solutions.“It is rather an interesting fact that in the above solid solutions (Zn and Sn in Cu) the lattice is always stretched somewhat less than would be expected from a proportional increase in lattice size computed from atomic volume considerations. This indicates a weak but perfectly definite attraction between unlike atoms. . . . It is also apparent that the tin and copper atoms pack more closely-considering the volume of the tin atom- than do zinc and copper.” He says :- . THE PERIODIC TABLE Group 7 Group8 Group1 GroupZCroupSGraup4 4.0 I Mn Ni CuAg Zn A1 SnSi Na Cd Hg PbSn Bi plotted against their positions in the Periodic Table with respect to the solvent element. 8.-Differences between observed and normal atomic volumes of elements in dilute solid solution 9.Sormnauy and Condusions. I. In density determinations errors due to surface-tension effects be- tween the surface of the water and the suspending wire may be eliminated by the use of a compensating platinum wire. 2. Cold-hammering has the effect of closing up minute blow-holes in copper. Very severe cold-hammering sets up stresses and strains in the metal which probably cause local increases and decreases in density. Otherwise cold-hammering does not affect the density. 3. Commercial copper has a distinctly lower density than cathode copper. 4. For theoretical purposes there are certain advantages in calculating density results as ‘‘ mean atomic volumes.” 5. When an element is distributed in solid solution as single atoms replacing single atoms of the solvent in the space-lattice of the latter, the hardening effect is, in general, proportional to the difference in size of the solute and solvent atoms.The above relationship does not, however, hold in certain exceptional cases (viz., Si in Cu and Na in Pb), which appear to arise when the solute has an exceptionally strong “chemical affinity” for the solvent. In such cases it is suggested that the solute 1 Bain, Chem. atad Met. Elog., Jan. 3rd, 1923, p. 22.AND CONSEQUENT HARDENING EFFECTS 599 exists in solid solution in the form of molecules of an inter-metallic com- pound having a different space-lattice from that of the solvent. The interference with slip and consequent hardening being relatively much greater in this type of solid solution. 6. When an element forms a solid solution with another element, there is a certain contraction or expansion which seems to be large or small according to whether the " chemical affinity " between the elements is large or small. The author wishes to acknowledge his indebtedness to Professor C. A. Edwards, D.Sc., for facilities for carrying out the present work, and for his interest and encouragement. He also expresses his thanks to the Al58-• c 3'7 3'8 3'9 Hardness (as annealed) before hammering. (Brine11 hardness expressed a s Log. a + 12.) FIG. A.-The effect of cold-hammering in increasing the hardness of the copper or-solid solutions. Royal Society for a Government Grant and to the Institute of Metals for permission to reproduce Fig. 3a. APPENDIX. Tk E'ect oJ Cold-hammering in Increasing tAe Wardnesses of the Coppw SoZid Solutions. From the theoretical point of view it was thought interesting to ascertain whether cold-hammering would increase the hardness of each of the solid solutions to the same extent. The solid solutions were therefore reduced first 60 per cent. in thickness and later go per cent. in thickness and their hardnesses measured after each reduction.600 SOLUTE ATOMS IN METALLIC SOLID SOLUTIONS I t was difficult to hammer all specimens to exactly the same amount and this factor makes the results somewhat erratic; they seem, however, to be sufficiently accurate to show that the nickel-copper solid solutions have hardened relatively less than any of the other solid solutions, which appear to have hardened to equal extents. I n Fig. A their original hardnesses (as annealed) are plotted against their hardnesses after hammering 90 per cent. reduction in thickness. The hardness units employed are not the same, but this does not affect the point in question. The 60 per cent. reduction series were more erratic, but they also showed the lesser hardening of the nickel-copper alloys quite clearly. The above is significant when one recalls the fact that the nickel-copper solutions are the only ones which owe their increased hardness to the presence of a solute element having a smaller atomic volume than copper.

ISSN:0014-7672    

DOI:10.1039/TF9241900586

出版商:RSC    

年代:1924

数据来源: RSC

摘要:

118 ELECTRICAL THEORY OF ADBORPTTON The writer considers the double layer as consisting of a swface of rigidly fixed atoms under continuous bombardment of positively and negatively charged ions, any particular point on the rigid surface becoming in turn negative, neutral and positive, these conditions arisdg in any order. The observed contact difference is the average effect of these conditions. Where several kinds of atoms are present in the solution the average number of any one of them at the surface will depend on their concentbration, valency and mobility. The variation of contact Werence from negative to neutral and positive was observed with cotton and aluminium sulphate near the neutral point. These variations occurred during the same experiment, the readings being direct measurements of E.1I.F.s developed by filtration under pressure.This point would be covered by putting n2 = 1 and = 2 or 3 in Mukherjee’s equation No. 13.118 ELECTRICAL THEORY OF ADBORPTTON The writer considers the double layer as consisting of a swface of rigidly fixed atoms under continuous bombardment of positively and negatively charged ions, any particular point on the rigid surface becoming in turn negative, neutral and positive, these conditions arisdg in any order. The observed contact difference is the average effect of these conditions. Where several kinds of atoms are present in the solution the average number of any one of them at the surface will depend on their concentbration, valency and mobility. The variation of contact Werence from negative to neutral and positive was observed with cotton and aluminium sulphate near the neutral point.These variations occurred during the same experiment, the readings being direct measurements of E.1I.F.s developed by filtration under pressure. This point would be covered by putting n2 = 1 and = 2 or 3 in Mukherjee’s equation No. 13.118 ELECTRICAL THEORY OF ADBORPTTON The writer considers the double layer as consisting of a swface of rigidly fixed atoms under continuous bombardment of positively and negatively charged ions, any particular point on the rigid surface becoming in turn negative, neutral and positive, these conditions arisdg in any order. The observed contact difference is the average effect of these conditions. Where several kinds of atoms are present in the solution the average number of any one of them at the surface will depend on their concentbration, valency and mobility.The variation of contact Werence from negative to neutral and positive was observed with cotton and aluminium sulphate near the neutral point. These variations occurred during the same experiment, the readings being direct measurements of E.1I.F.s developed by filtration under pressure. This point would be covered by putting n2 = 1 and = 2 or 3 in Mukherjee’s equation No. 13.118 ELECTRICAL THEORY OF ADBORPTTON The writer considers the double layer as consisting of a swface of rigidly fixed atoms under continuous bombardment of positively and negatively charged ions, any particular point on the rigid surface becoming in turn negative, neutral and positive, these conditions arisdg in any order.The observed contact difference is the average effect of these conditions. Where several kinds of atoms are present in the solution the average number of any one of them at the surface will depend on their concentbration, valency and mobility. The variation of contact Werence from negative to neutral and positive was observed with cotton and aluminium sulphate near the neutral point. These variations occurred during the same experiment, the readings being direct measurements of E.1I.F.s developed by filtration under pressure. This point would be covered by putting n2 = 1 and = 2 or 3 in Mukherjee’s equation No. 13.118 ELECTRICAL THEORY OF ADBORPTTON The writer considers the double layer as consisting of a swface of rigidly fixed atoms under continuous bombardment of positively and negatively charged ions, any particular point on the rigid surface becoming in turn negative, neutral and positive, these conditions arisdg in any order.The observed contact difference is the average effect of these conditions. Where several kinds of atoms are present in the solution the average number of any one of them at the surface will depend on their concentbration, valency and mobility. The variation of contact Werence from negative to neutral and positive was observed with cotton and aluminium sulphate near the neutral point. These variations occurred during the same experiment, the readings being direct measurements of E.1I.F.s developed by filtration under pressure.This point would be covered by putting n2 = 1 and = 2 or 3 in Mukherjee’s equation No. 13.118 ELECTRICAL THEORY OF ADBORPTTON The writer considers the double layer as consisting of a swface of rigidly fixed atoms under continuous bombardment of positively and negatively charged ions, any particular point on the rigid surface becoming in turn negative, neutral and positive, these conditions arisdg in any order. The observed contact difference is the average effect of these conditions. Where several kinds of atoms are present in the solution the average number of any one of them at the surface will depend on their concentbration, valency and mobility. The variation of contact Werence from negative to neutral and positive was observed with cotton and aluminium sulphate near the neutral point.These variations occurred during the same experiment, the readings being direct measurements of E.1I.F.s developed by filtration under pressure. This point would be covered by putting n2 = 1 and = 2 or 3 in Mukherjee’s equation No. 13.118 ELECTRICAL THEORY OF ADBORPTTON The writer considers the double layer as consisting of a swface of rigidly fixed atoms under continuous bombardment of positively and negatively charged ions, any particular point on the rigid surface becoming in turn negative, neutral and positive, these conditions arisdg in any order. The observed contact difference is the average effect of these conditions. Where several kinds of atoms are present in the solution the average number of any one of them at the surface will depend on their concentbration, valency and mobility.The variation of contact Werence from negative to neutral and positive was observed with cotton and aluminium sulphate near the neutral point. These variations occurred during the same experiment, the readings being direct measurements of E.1I.F.s developed by filtration under pressure. This point would be covered by putting n2 = 1 and = 2 or 3 in Mukherjee’s equation No. 13.118 ELECTRICAL THEORY OF ADBORPTTON The writer considers the double layer as consisting of a swface of rigidly fixed atoms under continuous bombardment of positively and negatively charged ions, any particular point on the rigid surface becoming in turn negative, neutral and positive, these conditions arisdg in any order.The observed contact difference is the average effect of these conditions. Where several kinds of atoms are present in the solution the average number of any one of them at the surface will depend on their concentbration, valency and mobility. The variation of contact Werence from negative to neutral and positive was observed with cotton and aluminium sulphate near the neutral point. These variations occurred during the same experiment, the readings being direct measurements of E.1I.F.s developed by filtration under pressure. This point would be covered by putting n2 = 1 and = 2 or 3 in Mukherjee’s equation No. 13.118 ELECTRICAL THEORY OF ADBORPTTON The writer considers the double layer as consisting of a swface of rigidly fixed atoms under continuous bombardment of positively and negatively charged ions, any particular point on the rigid surface becoming in turn negative, neutral and positive, these conditions arisdg in any order.The observed contact difference is the average effect of these conditions. Where several kinds of atoms are present in the solution the average number of any one of them at the surface will depend on their concentbration, valency and mobility. The variation of contact Werence from negative to neutral and positive was observed with cotton and aluminium sulphate near the neutral point. These variations occurred during the same experiment, the readings being direct measurements of E.1I.F.s developed by filtration under pressure. This point would be covered by putting n2 = 1 and = 2 or 3 in Mukherjee’s equation No.13.118 ELECTRICAL THEORY OF ADBORPTTON The writer considers the double layer as consisting of a swface of rigidly fixed atoms under continuous bombardment of positively and negatively charged ions, any particular point on the rigid surface becoming in turn negative, neutral and positive, these conditions arisdg in any order. The observed contact difference is the average effect of these conditions. Where several kinds of atoms are present in the solution the average number of any one of them at the surface will depend on their concentbration, valency and mobility. The variation of contact Werence from negative to neutral and positive was observed with cotton and aluminium sulphate near the neutral point. These variations occurred during the same experiment, the readings being direct measurements of E.1I.F.s developed by filtration under pressure.This point would be covered by putting n2 = 1 and = 2 or 3 in Mukherjee’s equation No. 13. THE CATALYTIC DECOMPOSITION OF HYDROGEN PEROXIDE SOLUTION BY BLOOD CHARCOAL. BY J. B. FIRTH, D.Sc., F.I.C., AND F. S. WATSON, M.Sc. ( A Paper read before THE FARADAY SOCIETY, Monday, November 12fh, 1923, SIR ROBERT ROBERTSON, K.B.E., F.R.S., PRESIDENT, in the Chair.) Received October 3 rd, I 9 2 3. I t has been shown already by many investigators that hydrogen peroxide is readily decomposed by a great variety of substances with the liberation of oxygen. Fillipi showed that all powders decompose hydrogen peroxide, the velocity of decomposition being proportional to the exposed surface of the added powder and independent of the concentration of the hydrogen peroxide.Lemoine found that charcoals obtained in the de- composition of wood, coconut and sugar are efficient catalysts of hydrogen peroxide ; the catalytic effect being apparently correlative with their sorptive power for gases. Clayton showed that hydrogen peroxide does not vola- tilise appreciably from its aqueous solutions at 50" to 60" C., and the rate of decomposition is not affected by stirring. Rideal and Thomas showed that hydrogen peroxide is decomposed by Fuller's earth. The catalytic activity in the decomposition of the hydrogen peroxide is not dependent on the adsorptive power ; it would appear that the iron content of the Fuller's earth may be the governing factor.On applying the equation for a uni- molecular surface reaction the velocity coefficient is constant for a given sample of Fuller's earth. The authors5 have shown that ordinary pure sugar carbon gives rise to only very slight decomposition of hydrogen peroxide in aqueous solution but the rate and extent of the decomposition increases considerably as the activity of the carbon is increased, and also with rise of temperature. The activity of the carbon gradually decays during the reaction. the authors describe experiments with carbons from other carbohydrates in which it is shown that the rate of de- composition varies considerably with the different carbons, but the funda- mental difference is in the initial activity. The activity falls rapidly after the first few minutes, becoming ultimately very slight, even though the solution contains a fair proportion of undecomposed hydrogen peroxide. The object of the present investigation was to study the decomposition of hydrogen peroxide solution by blood charcoal, before and after activation treatment. Whereas in previous experiments relatively pure carbons were used, in the present case the charcoal contains appreciable quantities of impurity, mair,ly iron.Experiments were therefore carried out in order to determine as far as possible the influence of this impurity on the rate of decomposition of the hydrogen peroxide. Arch. Farm. sperim., 1907, 6, 363. 3 Trans. Faraday SOC., 1916, 11, 164. 5 Trans. Clacm. SOC., 1923, 123, 1750. In a further paper Comfit. rend., 1916, 162, 725.your. SOC. Chem. Ind., 1g23,W, 37IT. 4 Trans. Chem. SOC., 1922,121,2119. 601602 THE CATALY'TIC DECOMPOSlTION OF HYDROGEN Exferimenta 1. The blood charcoal as supplied gave on ignition 8-82 per cent. of ash which consisted almost entirely of iron oxide. The charcoal was digested for several days with hot aqua regia, after which treatment the ash content was 7-30 per cent. It was then subjected to a prolonged treatment with aqua regia, for about a month, the ash content becoming 6-32 per cent. A quantity of this charcoal was then further treated with bromine for several days, after which the ash content was 5.79 per cent. I n each case all soluble material was extracted by washing with boiling dis- tilled water until the filtrate showed no indication of iron with potassium sulphocyanide, nor of halogen with silver nitrate solution.A sample of the purified charcoal was taken after each of the four purification processes given above. The charcoal was thoroughly dried by heating in an air oven at 1 2 0 O C. and the catalytic activity determined in each case. The experimental details are exactly as described in our first paper (Zooc. cit.). The hydrogen peroxide solution used contained 240-5 C.C. of available oxygen per 25 C.C. of solution. 0.~5 grams of the charcoal and 25 C.C. of the hydrogen peroxide solution were used in each experi- ment, and the temperature of experiment was 18" C. The reaction velocity coefficients are calculated from the equation for a unimolecular surface reaction dx/dt = K(a - x) to the Naperian base and the minute as unit of time.The volumes are, in all cases, corrected to N.T.P. and are ex- pressed in C.C. The volumes of oxygen liberated were determined at intervals ranging from 30 seconds to half an hour according to the rate and stage of the reaction. The results obtained are given in Table I. TABLE r. Time in Minutes. 0'5 3 6 24 51 66 10.5 I I2 I22 (1) Charcoal as Received. Volume of Oxygen in C.C. K X 103. 12'80 I I 'go 8-93 7.16 4'49 10'10 2-12 - - - (2) Charcoal Purified by Digestion with Aqua Regia for Several Days. Volume of Oxygen in C.C. 3'2 6.1 14'8 24.0 39'5 60.7 100.g 111.8 110'2 112'2 ____ K X I$. I 1.60 9-16 7-60 6.49 5-26 4'63 4'03 2-60 2-23 11'20 (3) Charcoal Purified by Digestion with Aqua Regia for a Month.Volume of Oxygen in C.C. 2.8 5'2 12'0 21'0 35'4 98.2 IL 5.1 127.7 58'5 122'2 __ __ K x 103. - . 10'20 9'40 7-40 6.62 5 '04 4'47 2 '93 2.6 I 5-76 3'78 (4) Charcoal Purified by Bromine Treatment. Volume of Oxygen in C.C. _ _ _ ~ 1 '9 4'8 10'0 16% 30.0 52'5 92 -1 102'6 126.8 132'3 K x I@?. 6.80 8-70 6-16 5 '23 4-82 4'45 4-11 3'66 3-10 2-84 From the above table it would appear that the initial rate of decomposi- tion is decreased by continued treatment with aqua regia and subsequent bromine treatment ; whilst on the other hand the total decomposition with- in the period of observation is appreciably increased by this treatment. The decomposition by all three purified charcoals is approximately double that of the original charcoal, after the reaction has proceeded sixty-six minutes.In the case of the charcoal treated with bromine, the maximum rate is not developed in the first half minute as in the other cases. I t seems possible that the decrease in the initial velocity with the purifiedPEROXIDE SOLUTION BY BLOOD CHARCOAL 603 Charcoal Activated by Method 111. charcoals might be due to the retention of a trace of the purifying agent, either free or combined with the iron, for although no halogen could be detected in the final washings during the purification of the charcoal, a slight trace of halogen was detected in the resulting ash and this was most pronounced after the bromine treatment. I n all subsequent experiments the charcoal was purified by digesting the original charcoal for about a month with aqua regia, as previously de- scribed.The resulting purified charcoal was then activated by one of the following methods :- I. The finely divided charcoal was introduced into a quartz flask and heated in a vacuum for two hours at 600' C., allowed to cool in a vacuum and then 0.25 gram of the charcoal rapidly weighed out. 11. As in I. except that the temperature of activation was gooo C. 111. A quantity of charcoal activated as in I., was treated with K/IO iodine solution in chloroform, in the proportion of 25 C.C. of the solution per gram of charcoal, for twenty-four hours. The charcoal was then filtered off, transferred to a silica dish and gently heated until practically the whole of the iodine had been volatilised. The charcoal was then shaken several times with an alcoholic solution of potassium hydroxide and boiled with distilled water until, on filtering, the filtrate showed no opalescence with silver nitrate solution.The resulting charcoal was finally heated in a vacuum in a quartz flask at about 600' C. for two hours and 0-25 gram of the charcoal rapidly weighed out on cooling to room temperature. IV. Finely divided charcoal was first treated as in I1 then as described in III., except that the final heating in a vacuum was carried out at gooo C. The results obtained for the four charcoals activated as described above are given in Table 11. 2 5 C.C. of the hydrogen peroxide solution contained 242'5 C.C. of available oxygen and 0.25 gram of charcoal was used in each case. Charcoal Activated by Method IV.TABLE 11. Vo!ume of Oxygen in C.C. Charcoal Activated 1 bv Method I. K X I d Time I - Volume of Oxygen in C.C. in K X 103. 0'5 3 6 24 30 60 I I2 I 22'6 39'5 87'4 129.2 157'7 173'3 175.0 175'7 25'5 42'3 100.6 149'3 165'1 173'5 176.1 177'8 85 '00 77-20 64-70 5 5'05 37'94 22'70 18-50 9'33 96'40 83'30 77'60 69'21 41-33 22-75 18-66 9'56 Charcoal Activated by Method 11. l- I Volume of Oxygen in C.C. K x IS. 162'60 147.20 102'80 70.15 41'51 22-77 18-81 9'60 81'0 126.8 148.2 I 66% 174.8 177'4 180.8 258.80 176.60 107'13 68'36 42.13 23.09 19.04 9-91 The object of the next series of experiments was to determine the in- fluence of the presence of the iron in the blood charcoal on the catalytic activity. I n order to do this an '' artificial blood charcoal " was prepared from cane sugar as follows : To a solution of cane sugar, containing a weighed amount of sugar, a definite amount of ferric chloride was added.The iron was then precipitated by the addition of excess of ammonium hydroxide. The resulting liquid was then evaporated to dryness, the liquid604 THE CATALYTIC DECOMPOSITION OF HYDROGEN Time in Minutes. 0.5 3 6 24 30 60 I I2 being repeatedly stirred and the product carbonised at as low a temperature as possible. The resulting charcoal was then digested with hot aqua regia for several days, then filtered off and boiled with distilled water until, on filtering, the filtrate gave no test for iron with potassium sulphocyanide and no opalescence with silver nitrate. Two samples of such charcoals were prepared. No. I containing 1-46 per cent.of ash and No. 2 yielding 9.10 per cent. ash. The catalytic activity of these charcoals was then determined as de- scribed in previous experiments ; first after drying at 1 2 0 ~ C. and then activated by methods I. or 11. as given above. The strength of the hydro- gen peroxide solution was equivalent to 243.4 C.C. of available oxygen. The results obtained are given in Table 111. TABLE 111. No. I Artificial Blood Charcoal. - Dried at I?OO C. Activatedat 6oo°C Oxygen in C.C. Oxygen in C.C. Volume of Volume of 3'7 7'9 7'0 11.3 13-1 18-1 19.2 24-6 26-5 33'7 36'7 44'7 39'7 49'5 4 9'2 66.4 3'8 7'7 16.2 24'4 36% 57'5 65 '4 90.1 No. z Artificial Blood Charcoal. - 8'7 18.1 27'9 43 'I 683 77'1 100.3 Dried at 120° C. Activated at 6ooo C. Activated at gooo C.Volume of 1 Volumeof 1 Volume of Oxygen in C.C. Oxygen in C.C. Oxygen in C.C. I I i 5 mins. 1 0 9 9 20 9 9 40 7 9 60 97 12 hours 2'0 3'6 5'7 8 2 9'7 20'6 49'0 79'0 188.0 203.1 204.3 206.4 213'6 148.5 Thc Acfivity of fhe Solid Impurities in fhe Active ChrcoaZ. In order to obtain some information as to the extent of the activity which might be attributable to the presence of the solid impurities, as such, present in the charcoals, the catalytic decomposition produced by the following sub- stances was determined :- I. The ash obtained by calcining a quantity of activated blood charcoal. 2. The ash from No. z artificial blood charcoal. 3. The ash from I after reduction by heating in a current of coal gas. 4. A sample of pu e reduced iron. 19 each case 0.25 gram of the substance was treated with 25 cc.of the The results are given in Table IV. hydrogen peroxide solution (= 245 C.C. oxygen). TABLE IV. Volume of Oxygen in C.C. Time. I I- 2'3 4'1 6.0 8.8 20'4 10'2 2 *I 3'6 5'8 8.3 9'6 21'2 1'4 2 '4 4'4 6-8 8'2 IS'SPEROXIDE SOLUTION BY BLOOD CHARCOAL 605 3 M 6 . Mins. 69.3 123.3 34.3 75.6 17'0 40'7 5.2 17-5 I t will be observed from the above results that the activity of thesesub- stances is relatively very slight. In Experiments 3 and 4 some of the iron is simultaneously converted to oxide. I t must not be overlooked, however, that the action of the isolated substance may be somewhat moddied when distributed throughout the mass of the charcoal. In Experiment I the bulk of the ash was 0.4 c.c., whereas the bulk of the charcoal containing this amount of ash was approximately 4 C.C.Decay in Aciivify of t h Charcoal. I t has been shown in previous papers (Zococ. cit.) and also in the present paper that the activity of charcoal in contact with hydrogen peroxide solu- tion rapidly falls off and finally disappears, even though a fair proportion of the hydrogen peroxide in solution remains undecomposed. The object of the following series of experiments was to determine whether the decay in activity was permanent or temporary. A quantity of finely divided charcoal was activated at 600' C., by Method III., previously de- scribed. The activity of the charcoal was then determined as in previous experiments. When the velocity of the reaction had become relatively slow ( i d . after about thirty minutes) the charcoal was filtered off and dried at 120' C.A quantity of the charcoal thus recovered was weighed out and the experiment repeated. The process was again repeated until the charcoal had been successively treated four times with hydrogen peroxide solution. The weights of charcoal used were 0.5, 0.4, 0.3 and 0.2 gram, and the volumes of hydrogen peroxide solution 5 0 c.c., 40 c.c., 30 c.c., and 2 0 C.C. respectively. The strength of the hydrogen peroxide solution was equivalent to 240% C.C. of available oxygen per 25 C.C. of solution. The results are given in Table V. and for the purpose of comparison the volumes are calculated for 0.25 gram of charcoal and 25 C.C. of the solution, in each case. TABLE V. 6 I2 24 Mins. Mins. Mins. M%. ------ 146'6 164'8 173'2 174'6 C.C.oxygen 118.9 142-7 161.3 1728 ,, ,, 60.8 96.0 153'0 167.5 ,, ,, 33'4 62-3 99.8 121-2 ,, ,, Time in Minutes. ! (I 0.5 I 6 IZ 24 Min. Mia. Mfns. Mins. Mins. Mins. ------- (I) 25 C.C. of solution 42.4 68.5 121.9 143.2 165.1 171.6 (2) Further 25 C.C. of solution added . 20'2 27.4 60'5 95.1 132'7 158'5 (3) Further 25 C.C. of solution added . - 4.9 16.1 30.2 61.4 92.3 First treatment Second ,, Third ,, Fourth ,, 30 Mins. 175.2 C.C. oxygen 160'2 ,, ,, 101.6 ,, ,, 0.5 Min. 41'2 2 1.6 The same problem was examined by a further series of experiments, in a different manner. When the velocity of reaction had become very slow, as in the previous experiments, instead of filtering off the charcoal, the con- centration of the solution was increased by the addition, from time to time, of further quantities of the original solution.TABLE VI.606 THE CATALYTIC DECOMPOSITION OF HYDROGEN (1) 443'6 (2) 221'8 (3) 110.9 (4) 55'5 ( 5 ) 27'8 (6) 13'9 0 - 2 5 gram of charcoal activated as before by Method 111. was treated with 25 C.C. of the hydrogen peroxide solution. 28'7 47'0 - 20.7 - 113 - 9'4 - 3'9 24'4 37'2 fizjuence of the Concentration of the SoZufion on the Activity of the Chrcoa 1. Charcoal activated by heating to 600' C. (Method I.), was treated with In all cases 0.25 gram of The results are given hydrogen peroxide solution of varying strengths. the charcoal was used, with 25 C.C. of the solution. in Table VII. Purified Charcoal ~ Dried at :zoo C. I TABLE VII. C.C. of Available of solution used. 1-1- 3 Mins.- 99'4 74'7 47'0 23.0 17'1 7'7 - 6 Mins. 154.8 1'4'7 68'4 33'7 21.9 11.4 12 Mins. $0 Mins. 339'7 170.4 101'4 51'6 - - C.C. oxygen evolved 9 9 9 9 9 9 ? I $ 9 9 9 9 ) 9 9 9 9 9 1 9 7 9 9 Temperature E'ecfs. The decomposition of hydrogen peroxide into water and oxygen is as- sociated with the liberation of approximately 23,000 calories per gram- molecule undergoing decomposition. Under the conditions of the experiments herein described, the influence of this heat liberated will be determined by the rate of decomposition of the hydrogen peroxide. When the velocity of the reaction is slow, the heat will be sufficiently rapidly dis- persed through the walls of the containing vessel to have very little influence on the temperature of the reaction. On the other hand, where the decom- position is very rapid, the temperature will rise appreciably.I t has already been shown in a previous paper (Cot. cit.) that the rate of decomposition of the hydrogen peroxide by charcoal rapidly increases with rise of tempera- ture, thus if the initial activity were sufficient to effect an appreciable rise in the temperature of the solution, then such a change would favour more rapid decomposition. The temperature effects were therefore approxi- mately determined for the various charcoals used. A simple glass calori- meter, consisting of a wide boiling tube fitted into a wide-necked flask, was TABLE VIII. Time in Minutes. I 3 6 24 30 60 I2 I 8.2 18-25 18.6 18-7 18.7 18.6 18-45 Temperature Readings. Activated by Method 1. I- - 21'2 24'3 26.2 25'5 23-6 22.7 18.4 Activated by Method 11.22'0 25.8 27.2 26.4 24'3 23 '35 18.5 Activated by Method 111. 22.9 27.1 28.7 27'4 25.0 23-85 18.6 Activated by Method IV. -I 24% 29'5 29'1 27-8 25 '3 24.2 I 8.8PEROXIDE SOLUTION BY BLOOD CHARCOAL 607 used. 25 C.C. of hydrogen peroxide containing 242.0 C.C. of available oxygen, were introduced into the boiling tube and 0.2 5 gram of the charcoal added. The initial temperature was 18” C. in each case and temperature readings were taken at intervals ranging from thirty seconds to ten minutes, according to the stage of the reaction. A summary of the results is given in Table VIII. Discussion of Resu Zts. The results given in Table I. show that blood charcoal, which has been dried at I 20’ C. is capable of bringing about the decomposition of hydrogen peroxide solution.In the case of charcoal not subjected to further purifica- tion, the rate of decomposition gradually diminishes until, after about fifty minutes, the rate becomes practically zero, whilst only about 25 per cent. of the hydrogen peroxide has undergone decomposition. In the case of the purified charcoals (Table I., 2, 3 and 4) although the initial activity is somewhat retarded, the rate of decomposition diminishes more gradually, the action being considerably prolonged. The reaction becomes very slow after two hours, whilst about 50 per cent. of the hydrogen peroxide has been decomposed. The effect therefore of the purification treatment ap- pears to have been to prolong the activity of the catalyst. From the typical results given in Table II., obtained after the charcoal has been activated, it is apparent that the activity of the charcoal is con- siderably increased by heating in a vacuum to 600’ C.and gooo C., and the activity is still further increased by previous sorption and removal of iodine. I t is important to notice that the effect of the various methods of activation is confined to the initial reaction. During the first thirty seconds, the percentage of hydrogen peroxide decomposed by the charcoals, activated by Methods I., II., 111. and IV. respectively, was 9-32, 9.68, 17.1 and 25-13, whilst after twelve minutes, the values are very similar in all cases. This is more clearly shown by a comparison of the velocity coefficients ; the values are widely different for the respective charcoals during the first six minutes, after which period they become very similar.In all four cases 74.5 per cent. of the hydrogen peroxide has been decom- posed in sixty minutes, after which period, the rate of decomposition has become relatively, very slow. The velocity coefficient has fallen after sixty minutes, to a value similar to the initial value for the unactivated charcoals. I t has already been pointed out (Rideal and Thomas Zoc. 02.) that the presence of iron in Fuller’s earth gives rise to a decomposition of hydrogen peroxide, in which the decomposition is ultimately almost complete and the velocity coefficient constant. In the present case, the decomposition is not complete within a reasonable period and the velocity coefficient con- tinuously diminishes over a very wide range.The fact that the iron con- tent of the purified charcoal and of No. 2 “Artificial Blood Charcoal ” is approximately the same as that of the Fuller’s earth, is interesting. I t should be noticed, however, that whilst in the case of Fuller’s earth the iron is present as oxide, in the case of blood charcoal the iron will, initially, be mainly metallic iron. The results given in Table III. show that the introduction of iron into pure sugar charcoal considerably increases the catalytic activity. In a previous paper (Zoc. a?.) it is shown that one gram of pure sugar carbon, activated by Method I., when treated with 25 C.C. of hydrogen peroxide solution, containing 336-8 C.C. of available oxygen, liberates 3-38 C.C. of oxygen in six minutes and 10.06 C.C.in thirty-three minutes, whereas similar charcoals containing added iron, to the extent of 1-46 and 9-10608 THE CATALYTIC DECOMPOSITION OF HYDROGEN per cent. ash, liberate 24.6 C.C. and 27.9 C.C. after six minutes, and 49.5 C.C. and 77.1 C.C. after thirty minutes using only 0-25 gram of the charcoal and hydrogen peroxide solution, containing 243-4 C.C. of available oxygen per 25 C.C. The results of experiment 3 (Table I.) and of charcoals activated by Methods I. and IV. are shown graphically in Fig. I by continuous lines 240 220 200 180 I 60 2 *: 140 c( 5 % P > 120 3 Io0 0 80 60 40 20 Fio. I. and are numbered I., 11. and 111. respectively, whilst the results for “artificial blood charcoal ” No. 2 activated by Methods I.and 11. are shown by broken lines and numbered IV. and V. respectively. The results given in Table V. show that the activity of the charcoal is considerably diminished by contact with hydrogen peroxide solution ; the activity is however partially recovered by drying at 120’ C. ; after four ex- periments it has become very small. Combining these results with those given in Table VI., it would appear that the extent of the decompositionPEROXIDE SOLUTION BY BLOOD CHARCOAL 609 is determined not only by the activity of the charcoal but by the concentra- tion of the solution. A concentration is ultimately reached at which the charcoal ceases to have any effect. From the experiments herein described, it does not appear that the extent of the decomposition is determined by the initiul activity, because from the results given in Table II., charcoals of widely different initial activities approach similar values after about thirty minutes.I t would ap- pear from the results, that the catalytic activity of the charcoal is of two types, one, a very rapid activity which decays after a few minutes, and a much slower activity which persists for a much longer period; both forms are capable of being increased. For convenience these two types of activity will be referred to as a and p activity respectively. Thus in the case of carbons not activated (Table I.) this a activity is absent. The slower /3 activity may persist for a comparatively short time or for a long period (Table I., I and 4). Hence from the data available, it is the /3 activity which determines the limit of decomposition for a given strength of solution: .provided the a activity is insufficient to bring about complete de- composition.The rate of decomposition is greatly influenced by the strength of the hydrogen peroxide solution as shown in Table VIL The extent of the decomposition will be determined by the rate of decay of the fl activity. With the weaker solutions the percentage decomposition is much greater than in the stronger solutions. Time of contact with the solution seems to be an important factor and if the rate of the decomposition is sufficiently rapid, the whole of the hydrogen peroxide may be decomposed before the charcoal becomes passive. In Table VL it is shown that in a solution in which the rate of de- composition has become very slow, if the concentration be increased by the addition of a further quantity of the original solution, the activity is increased.I t would appear therefore that the activity is determined by both the charcoal and the strength of the solution. A weakly active charcoal requires a strong solution of the peroxide for decomposition to take place whilst a more highly active charcoal is capable of decomposing weaker solutions. From Table VIII. it will be observed that during the initial stages of the reaction in which highly active charcoals are used, the temperature rapidly rises and since the rate of decomposition of the hydrogen peroxide increases with rise of temperature, this thermal effect tends to maintain a higher re- action velocity in the later stages than would occur if the temperature had remained constant throughout.The initial rate of the reaction is deter- mined by the u activity and this in turn, therefore, determines the thermal effects, so that the rate of reaction after the first few minutes is determined not only by the p activity of the charcoal but also by the thermal changes produced by the a activity. As the velocity of reaction diminishes the temperature gradually approaches the original temperature owing to loss of heat through the walls of the containing vessel. I t is important to note that if it were not for the initial rapid thermal changes, the demarcation between the a and /3 activity would be even more pronounced than is actually indicated by the results, i.c.the rise in temperature tends to counter- balance the decrease in activity of the charcoal. In considering the influence of the iron impurity present in the charcoal, it does not seem possible to explain the difference in the activity between sugar charcoal and blood charcoal on the iron content alone; a similar quantity of iron dispersed through Fuller’s earth has a very much lower activity and, also, the activity of the ash of the charcoal itself is very low.610 DECOMPOSITION OF HYDROGEN PEROXIDE SOLUTION It would appear, therefore, that the iron present in addition to increasing slightly the activity of the charcoal by virtue of its presence, greatly increases the activity of the carbon itself. It has previously been shown by one of us that the introduction of substances, either of a permanent or temporary character, into a carbohydrate prior to carbonisation, greatly increases the sorption activity of the resulting charcoal.I t is suggested that in the present case the original iron acts in a similar manner, that is, as a spacing agent whereby a less complex carbon molecule is produced than is the case when no such material is present. The results obtained for artificial blood charcoals, No. I and No. 2, show a wide difference in activity which cannot be solely accounted for by the difference of iron content alone and an ex- planation such as is given above seems reasonable. SUMMARY. I . Ordinary blood charcoal previously heated to 120' C., shows moderate catalytic activity in the decomposition of hydrogen peroxide soh tion. 2 . The catalytic activity of blood charcoal is considerably increased by previous heating in a vacuum at 600' C. and gooo C. and is still further increased by previous sorption of iodine from solution from which the iodine has been subsequently completely removed. 3. The activity of an activated charcoal consists of two types, one which is termed a activity which is very rapid, but ceases after a few minutes, and a second termed p activity which may persist for several hours. Both types may be increased by activation methods. I n ordinary blood charcoal a activity is absent. 4. The introduction of iron into sugar solution prior to carbonisation considerably increases the activity of the charcoal, by an amount greater than can be accounted for by the iron alone and it is suggested that the iron acts as a spacing agent thereby increasing the activity of the carbon itself. 5. The velocity of decomposition varies with the concentration of the hydrogen peroxide solution. 6. The proportion of hydrogen peroxide decomposed is determined by both the activity of the charcoal and the concentration of the hydrogen per- oxide solution. 7. In the case of the highly active carbons, the heat generated by the rapid decomposition of the hydrogen peroxide raises the temperature of the reaction, temporarily, by several degrees which further facilitates the de- composition of the hydrogen peroxide and thereby maintains the velocity of reaction at a higher level, until this heat has been dissipated. Firth, your. Sot. Chenz. I t ~ d . , 1923, G, 242T-244T. Chemistry Dejt., University CoZZege, Notti?Zghm.

ISSN:0014-7672    

DOI:10.1039/TF9241900601

出版商:RSC    

年代:1924

数据来源: RSC

摘要:

118 ELECTRICAL THEORY OF ADBORPTTON The writer considers the double layer as consisting of a swface of rigidly fixed atoms under continuous bombardment of positively and negatively charged ions, any particular point on the rigid surface becoming in turn negative, neutral and positive, these conditions arisdg in any order. The observed contact difference is the average effect of these conditions. Where several kinds of atoms are present in the solution the average number of any one of them at the surface will depend on their concentbration, valency and mobility. The variation of contact Werence from negative to neutral and positive was observed with cotton and aluminium sulphate near the neutral point. These variations occurred during the same experiment, the readings being direct measurements of E.1I.F.s developed by filtration under pressure.This point would be covered by putting n2 = 1 and = 2 or 3 in Mukherjee’s equation No. 13.118 ELECTRICAL THEORY OF ADBORPTTON The writer considers the double layer as consisting of a swface of rigidly fixed atoms under continuous bombardment of positively and negatively charged ions, any particular point on the rigid surface becoming in turn negative, neutral and positive, these conditions arisdg in any order. The observed contact difference is the average effect of these conditions. Where several kinds of atoms are present in the solution the average number of any one of them at the surface will depend on their concentbration, valency and mobility. The variation of contact Werence from negative to neutral and positive was observed with cotton and aluminium sulphate near the neutral point.These variations occurred during the same experiment, the readings being direct measurements of E.1I.F.s developed by filtration under pressure. This point would be covered by putting n2 = 1 and = 2 or 3 in Mukherjee’s equation No. 13.118 ELECTRICAL THEORY OF ADBORPTTON The writer considers the double layer as consisting of a swface of rigidly fixed atoms under continuous bombardment of positively and negatively charged ions, any particular point on the rigid surface becoming in turn negative, neutral and positive, these conditions arisdg in any order. The observed contact difference is the average effect of these conditions. Where several kinds of atoms are present in the solution the average number of any one of them at the surface will depend on their concentbration, valency and mobility.The variation of contact Werence from negative to neutral and positive was observed with cotton and aluminium sulphate near the neutral point. These variations occurred during the same experiment, the readings being direct measurements of E.1I.F.s developed by filtration under pressure. This point would be covered by putting n2 = 1 and = 2 or 3 in Mukherjee’s equation No. 13. DISCUSSION 61 I Dr. J. N. Pring said he took it that the treatment of this charcoal with aqua regia was less drastic than with nitric acid alone, because as far as he could remember amorphous carbon was very rapidly decomposed by nitric acid.Was it the hct that aqua regia wasless drastic? Dr. J. B. Firth said that if it were too hot, the bulk of the carbon was lost by decomposition. If the charcoal were treated with strong nitric acid alone there was a tendency to lose a lot of the carbon, whereas by treating the charcoal with aqua regia they were treating it with an active form of chlorine. If it were too hot, however, the solution became dark brown and a lot of carbon was lost. Dr. Pring said that in purifying cocoa nut charcoal he always used chlorine gas at a high temperature followed by heating in hydrogen. In that way it was possible to eliminate the hydrocarbons and some ash with- out decomposing the carbon. Had the author attempted a treatment of that kind with aqua regia? Dr. Firth said he had tried to approximate to that, by heating up until a fairly heavy stream of chlorine was given off, but he did not allow the temperature to rise any higher, otherwise the whole of the carbon passed into solution, and a brown viscous residue was obtained.Dr. H. Borns asked of what the 6 per cent. residue consisted? Dr. Firth said it was almost entirely iron ; there was a slight trace of Dr. Borns: And you cannot get rid of that by means of bromine? Dr. Firth said not without very prolonged treatment was he able to reduce it to any extent greater than was indicated by the figures in the paper. I t was necessary not to make the treatment too drastic because the activity of the carbon would be altered by an alteration in the treatment and he wished to keep as far as possible to a uniform treatment and in- vestigate the effect of the impurity by other methods.Dr. R. Lessing said he was interested in conclusion No. 4 of the Summary to the paper dealing with the effect of the iron in the charcoal on the activation, which the author regarded as a kind of spacing action of the iron. In some work which he did some years ago-and in which he was again engaged-he found that the addition of inorganic substances to carbonaceous products of various kinds had what he believed to be a catalytic influence on the carbonisation. Therefore in the present case, when iron oxide or hydroxide was added to the sugar solution, one would expect an entirely different form of physical structure with carbon than one would get with iron. How iron or any other inorganic agent would act we did not know but he would like to throw out one suggestion for the authors’ consideration, and that was that although he did not find any action to speak of with the ashes or with iron reduced by coal gas, or reduced pure iron, it was not unlikely that the iron in the charcoal itself acted quite differently from the isolated iron.Particularly where one was dealing with ashes it was necessary to consider the question of sintering which depended very much on the temperature at which the ash was prepared and also on the gas with which the ash or iron were reduced; hydrogen, coal gas or whatever it may be. In the case of coal gas one has to guard against small traces of sulphur, although they did not come out in the figures in the paper as being of a poisonous nature.Certainly the iron in the charcoal might act quite differently and it might be of an entirely different physical silica but the bulk of it was iron.612 DECOMPOSITION OF HYDROGEN PEROXIDE condition; further, the nature of the reduction would be a different one. In the one case the reduction was presumably by solid carbon, with the formation of CO, whereas in the other case we had to deal with hydrogen reduction. Whether or not we had to deal with a uni-molecular layer of iron and carbon he did not know, but any way it would be quite on the cards that the iron would be in an entirely different form from that in other cases. The President said he took it that the authors assumed that the iron formed a kind of network throughout the carbonaceous mass, breaking it up and so gradually increasing the surface for action.If that be so, it would be interesting to try by some other manner entirely different from the one they had used, such as, perhaps, by taking a radiograph, to see if the iron were scattered through the mass in gross particles, or if there were a network. Dr. E. Fyleman pointed out that if charcoal is heated with iron oxide at gooo C., there will surely be considerable oxidation, and this would con- siderably increase the surface extension of the mass which would presum- ably increase the activity. I t seemed to him that a very large proportion of the results in the paper could be interpreted on these lines. Oxidation of part of the charcoal would account for the decrease in the weight of the the charcoal that the author obtained. There was another rather interest- ing analogy.If pure charcoal was treated with liquid oxygen, nothing happened, unless, of course, the mixture were detonated. If, however, they had charcoal containing more than a certain proportion of iron, they were very likely to obtain a violent explosion. Dr. Firth, replying to the discussion, said that with regard to breaking up and offering more surface, in the previous experiments of the authors with carbohydrate carbons it was shown that the actual bulk of the carbon is not as decisive a factor as was imagined. In the carbohydrate carbons the volume occupied by one gramme of finely divided carbon in the case of cellulose was 3.9 C.C. ; cellulose was exceedingly active relative to the other carbohydrates.In the case of carbon prepared from rice starch, one gramme of that charcoal occupied 6 c.c., and yet the one with the smaller volume was by far the more active. They had measured the bulk density of these carbons and there seemed to be no direct relation between the bulk density, which one could take approximately as a measure of the surface, and the activity. Their idea with regard to the action of iron was that the presence of iron in any carbonaceous substance distributes itself and prevents the formation of highly complex carbon molecules. I t was the simplest carbon molecules which possessed the high activity and they were more or less unstable, and that was one reason why the activity of the charcoal decays.It is shown in the paper that a certain amount of residue occupies 0.4 C.C. in the ash, whereas the same amount of ash is distributed through 4 C.C. in the actual carbon. We appreciate the remarks of Dr. Lessing and indicate in our paper that it is possible that the iron when distributed throughout the charcoal behaves differently from the iron in the ash or the reduced iron itself. On the other hand from our investigations with charcoals containing impurities other than iron, similar results have been obtained, that is, whilst the impurity, either as the substance itself or as the ash from the charcoal exhibits relatively small activity, it produces a considerable increase in the activity of the charcoal. Hence it would appear that the explanation, in the main, must be of a general character, and not necessarily a specific property of any particular impurity.DISCUSSION With regard to some later experiments which were not included in the paper, and which we do not wish published at present, we have investigated a commercial charcoal which it has been possible to activate to an activity which approximates to chemical decomposition.We took 0.1 25 gramme carbon in 12.5 C.C. of peroxide and got over go per cent. of the-available oxygen off in the first half minute, and the reaction was practically complete in three minutes. That carbon contained very little iron; the chief im- purity was phosphate. We have been investigating recently another carbon which possessed very great activity with regard to the absorption of colour- ing matter in solutions of iodine and sugar solutions, and although it was very highly active under these conditions, it showed, up to the present, very feeble activity with regard to decomposition of hydrogen peroxide. I t showed no alpha activity; it was only a slight activity, going on for about several hours. Therefore, this would indicate that there is no direct connection between sorption activity and catalytic activity, as we were inclined to believe in the first place.

ISSN:0014-7672    

DOI:10.1039/TF9241900611

出版商:RSC    

年代:1924

数据来源: RSC

摘要:

118 ELECTRICAL THEORY OF ADBORPTTON The writer considers the double layer as consisting of a swface of rigidly fixed atoms under continuous bombardment of positively and negatively charged ions, any particular point on the rigid surface becoming in turn negative, neutral and positive, these conditions arisdg in any order. The observed contact difference is the average effect of these conditions. Where several kinds of atoms are present in the solution the average number of any one of them at the surface will depend on their concentbration, valency and mobility. The variation of contact Werence from negative to neutral and positive was observed with cotton and aluminium sulphate near the neutral point. These variations occurred during the same experiment, the readings being direct measurements of E.1I.F.s developed by filtration under pressure.This point would be covered by putting n2 = 1 and = 2 or 3 in Mukherjee’s equation No. 13.118 ELECTRICAL THEORY OF ADBORPTTON The writer considers the double layer as consisting of a swface of rigidly fixed atoms under continuous bombardment of positively and negatively charged ions, any particular point on the rigid surface becoming in turn negative, neutral and positive, these conditions arisdg in any order. The observed contact difference is the average effect of these conditions. Where several kinds of atoms are present in the solution the average number of any one of them at the surface will depend on their concentbration, valency and mobility. The variation of contact Werence from negative to neutral and positive was observed with cotton and aluminium sulphate near the neutral point.These variations occurred during the same experiment, the readings being direct measurements of E.1I.F.s developed by filtration under pressure. This point would be covered by putting n2 = 1 and = 2 or 3 in Mukherjee’s equation No. 13.118 ELECTRICAL THEORY OF ADBORPTTON The writer considers the double layer as consisting of a swface of rigidly fixed atoms under continuous bombardment of positively and negatively charged ions, any particular point on the rigid surface becoming in turn negative, neutral and positive, these conditions arisdg in any order. The observed contact difference is the average effect of these conditions. Where several kinds of atoms are present in the solution the average number of any one of them at the surface will depend on their concentbration, valency and mobility.The variation of contact Werence from negative to neutral and positive was observed with cotton and aluminium sulphate near the neutral point. These variations occurred during the same experiment, the readings being direct measurements of E.1I.F.s developed by filtration under pressure. This point would be covered by putting n2 = 1 and = 2 or 3 in Mukherjee’s equation No. 13.118 ELECTRICAL THEORY OF ADBORPTTON The writer considers the double layer as consisting of a swface of rigidly fixed atoms under continuous bombardment of positively and negatively charged ions, any particular point on the rigid surface becoming in turn negative, neutral and positive, these conditions arisdg in any order.The observed contact difference is the average effect of these conditions. Where several kinds of atoms are present in the solution the average number of any one of them at the surface will depend on their concentbration, valency and mobility. The variation of contact Werence from negative to neutral and positive was observed with cotton and aluminium sulphate near the neutral point. These variations occurred during the same experiment, the readings being direct measurements of E.1I.F.s developed by filtration under pressure. This point would be covered by putting n2 = 1 and = 2 or 3 in Mukherjee’s equation No. 13.118 ELECTRICAL THEORY OF ADBORPTTON The writer considers the double layer as consisting of a swface of rigidly fixed atoms under continuous bombardment of positively and negatively charged ions, any particular point on the rigid surface becoming in turn negative, neutral and positive, these conditions arisdg in any order.The observed contact difference is the average effect of these conditions. Where several kinds of atoms are present in the solution the average number of any one of them at the surface will depend on their concentbration, valency and mobility. The variation of contact Werence from negative to neutral and positive was observed with cotton and aluminium sulphate near the neutral point. These variations occurred during the same experiment, the readings being direct measurements of E.1I.F.s developed by filtration under pressure.This point would be covered by putting n2 = 1 and = 2 or 3 in Mukherjee’s equation No. 13.118 ELECTRICAL THEORY OF ADBORPTTON The writer considers the double layer as consisting of a swface of rigidly fixed atoms under continuous bombardment of positively and negatively charged ions, any particular point on the rigid surface becoming in turn negative, neutral and positive, these conditions arisdg in any order. The observed contact difference is the average effect of these conditions. Where several kinds of atoms are present in the solution the average number of any one of them at the surface will depend on their concentbration, valency and mobility. The variation of contact Werence from negative to neutral and positive was observed with cotton and aluminium sulphate near the neutral point.These variations occurred during the same experiment, the readings being direct measurements of E.1I.F.s developed by filtration under pressure. This point would be covered by putting n2 = 1 and = 2 or 3 in Mukherjee’s equation No. 13.118 ELECTRICAL THEORY OF ADBORPTTON The writer considers the double layer as consisting of a swface of rigidly fixed atoms under continuous bombardment of positively and negatively charged ions, any particular point on the rigid surface becoming in turn negative, neutral and positive, these conditions arisdg in any order. The observed contact difference is the average effect of these conditions. Where several kinds of atoms are present in the solution the average number of any one of them at the surface will depend on their concentbration, valency and mobility.The variation of contact Werence from negative to neutral and positive was observed with cotton and aluminium sulphate near the neutral point. These variations occurred during the same experiment, the readings being direct measurements of E.1I.F.s developed by filtration under pressure. This point would be covered by putting n2 = 1 and = 2 or 3 in Mukherjee’s equation No. 13. THE PROPERTIES OF POWDERS. PART VIII. THE INFLUENCE OF THE VELOCITY OF COMPRESSION ON THE APPARENT COMPRESSIBILITY OF POWDERS. BY E. E. WALKER, D.Sc. (A Paper read before THE FARADAY SOCIETY, Monday, November 1 2 t / t , 1923, SIR ROBERT ROBERTSON, K.B.E., F.R.S., PRESIDENT, i f 2 t& Chair.) Received October 3 4 I 9 2 3.Th Creeping of Powders Under Pressure. When a powder is compressed in a cylinder by a constant load, the velocity of compression is very high to begin with and then rapidly falls off, but no case has been observed up to the present in which it falls off to zero before the powder is completely compressed. TABLE I. Time in Seconds. Volume Ratio. K'. 5 10 20 40 80 160 320 640 1280 2560 5120 10,240 20,480 81,920 40,960 1.329 1.316) 1.295 1.257 1.236) 1-214 1-18,) 1.158 1.030 1-007 1'000 J -019 '0185 '021 '0195 .orgo '015 '012 '0035 This phenomenon has been investigated, using the apparatus described in Part VI. of these studies. The general method of procedure, including the preparation of the powders, was the same as described in that com- munication, but, instead of varying the load, observations were made on the height of the plunger after various intervals of time, the load being kept constant.As a rule the load was applied by means of a small Tangye press, and kept constant by manipulation of the hand-operated oil pump- but in certain cases where it was necessary to keep the load constant for several hours a dead load was employed. The load was applied as rapidly and gently as possible, and zero time taken from the moment at which the 614THE PROPERTIES OF POWDERS-PART VIIL 615 ,789 '759 ,736 '653 full load was applied. On account of errors arising from the estimation of zero time, observations made in less than 2 0 seconds were liable to consider- able error. One series of observations made on ammonium nitrate is recorded in Table I.The relationship between the volume ratio' (V) and the load (t) is given by the empirical equation :- V, - V2 = IOK' (t2h - t,is) . * ( I ) -032 '034 '03 I -03 I TABLE 11. - Potassium Nitrate. Load ~ o r j kiloelsq. cm. Ammonium Nitrate. Load 101*5 kilos/sq. cm. K' K - K' K K' K V t V 1.548 1.532 1'516 1'497 1.477 1.455 1'409 K' '040 '03 7 '037 '03 7 -032 *028 -026 - K '518 '501 '478 '469 '449 '421 ,386 - *076 '074 '077 '079 -071 '47 '067 - - 10 20 40 80 160 320 640 1280 2560 "339 1.303 1-267 1,229 1.188 1.150 1.114 =.q9 - -0165 '0155 *or7 -017 *018 ,016 '506 '5 15 322 '531 '536 '033 -030 '03 3 '03 2 '03 4 '547 ,029 Mean = '0318 Potassium Chloride. Load 203 kiloslsq. cm. Ammonium Chloride. Load 183 kilos/sq. cm. V K I K' K' '0021 'OOIg -0017 t 40 320 2560 54820 t I0 40 160 640 2560 1.478 1.431 1.374 1.318 1'260 '425 -418 ,411 '025 '026 ,023 '020 - Mean = -0044 - Values of the velocity coefficient K' are recorded in Table I., and it is seen that they vary within very narrow limits until compression is practically complete, when its value falls rapidly to zero.In the diagram experimental 1 The volume ratio is apparent Of powder actual volume of solid particles.616 INFLUENCE OF VELOCITY OF COMPRESSION volume-ratios are plotted as ordinates and logarithms of the time as abscissz. V = 1.5665 - -1985tih A curve calculated from the equation :- is drawn for comparison and is seen to be in good agreement with the results of the experiment. It is found that equation I is capable of representing the relationship between V and t for all the powders hitherto investigated over that range of volume ratios for which the logarithmic law V = C - K l o g R TABLE 111.Class I . *Ammonium nitrate. . . . . . Sample A2 {Trinitrotoluene 20 per cent. } Sample B2 Trinitrotoluene . . . . . . "Sodium chloride . . . . . . *Potassium chloride . . . . . . Potassium nitrate . . . . . . Ammonium nitrate 80 per cent. class I I . Class III. *Ammonium chloride (pure) . . . . 9 , .. (commercial) . . . Tetranitromethylaniline . . . . . "Barium nitrate . . . . . . Calcium carbonate (precipitated) . . . K' K - . '0731 '0552 . '0299 . '0272 . -0128 . s0320 . '0318 . -0032 . '0044 . -0176 . '0039 . '0094 holds as a first approximation ; but where K in this expression is variable K' is found to vary also.In fact it is the ratio - which is constant for a K' K See Part. VI. of this series, Vol. XIX., p. 79. 2 Sample A was prepared by mixing the two powders under heavy rollers in edge runner mills. Sample B was prepared by mixing the hot ammonium nitrate powder with molten trinitrotoluene so that every particle was coated. It is interesting to note that K' whereas sample A has the value of -, which might be expected from its composition, K the value for sample B is almost the same as that found for pure trinitrotoluene.ON COMPRESSIBILITY OF POWDERS 61 7 Load, *pproxirnate Idean Kilos per Sq. Cm. Volume Ratio. 81 1.25 I 62 1-14 324 1-06 given powder rather than K' itself; as the following observations on a mixture of ammonium nitrate and trinitrotoluene show :- K' ir K Mean Value.K' Mean Value. '339 '0209 '0615 '296 '0177 '060 '227 '0135 '0595 K' K been found that - is very nearly constant in the case of all the substances, the compressibility of which was measured in Part VI. of these studies ; but in some cases the value of K' is so small that the experimental error in determining it is very high. Examples of the application of equation I to observations on various substances are given in Table 11. Collected values of 5' are given for a variety of substances in Table 111. Those marked with a star refer to samples of powder, the compressibilities of which are recorded in Part VI. K Inzuzme 01 t h VeZocity of Compression. The relationship between the length of time for which the load has been applied and the velocity with which compression takes place can be calculated simply from equation I as follows :- dv Velocity of compression = - - dt = K't - 0'9 .* ( 2 ) When a powder is compressed by blows in the manner described in Part VI. of this series the impact velocity is known and, if equation I is applicable to very small time intervals, as it is for time intervals ranging from 5 to 20,000 seconds, it should be possible to calculate the relationship between the resistance which a powder offers to compression by impact of known velocity (RJ and its resistance to a static load of known duration (Rp). Taking the mean velocity of compression as half the velocity of impact, we get dv v x A - = velocity of compression = ~ dt 2 4 v 3 impact velocity of falling weight in cm./sec.A = cross sectional area of cylinder Q = quantity of powder (in cubic centimetres) hence for a container having a cross sectional area of 10 sq. cm. where (3) ' (4) K't- 0'9 = K5 Q and since :- &2 V, - Vz = K log - R. a we get by equations I, 2, 3 and 4 log,o iy- Ri = I - from which 5 may be calculated. The results are recorded in Table IV., RP618 INFLUENCE OF VELOCITY OF COMPRESSION '0399 -0xg8 '0122 '00487 and the observed values are given for comparison. I n classes I. and 111. the average observed value of -* is given for all volume ratios (excluding those below I '2, since these values are rather abnormal, see Part VI., p. 80). The value of -- is not even approximately constant for substances in Class 11. and the limits are therefore given instead of the mean value.I n the case of substances in Class I. the calculated and observed values are in fair agreement, but there is no agreement in the other two classes. This lack of agreement is quite consistent with the hypothesis put forward in Part VI. to explain the behaviour of these substances towards slow and rapid compression. Substances in Class I. are regarded as normal substances, which are com- pressed by deformation of the particles both when static loads are employed and when compression is brought about by the impact of falling weights. Accordingly the calculated and observed values of - are in fair agreement. Substances in Class 11. are regarded as behaving normally when they are compressed slowly by static loads, but when the compression is brought about suddenly by blows the particles are broken down into fragments which fall together into closer order, so that the resistance to compression is much less than the calculated value to begin with.As compression proceeds further disintegration takes place causing an increase of resistance analogous to the hardening of metals by cold working. Accordingly the ratio -' rises and finally reaches or exceeds the calculated value. In Class 111. conditions are exceedingly complex since disintegration is caused both by slow and by rapid compression (impact), and accordingly there is no connection between the calculated and observed values of R. RP Ri RP Ri R, R. RP 14'53 Ammonium nitrate 5'6 4-16 14'68 Mixture sample B 1.92 I -86 15-23 Trinitrotoluene I'&9 1-62 14-68 Mixture sample A 3-14 3'32 - Ri.Calcium carbonate is quoted in Table IV. as an example of this total Rb '0079 '02 I5 '0175 lack of agreement. TABLE IV. _ _ ~ ~- 1 Ri I 11-48 Sodium chloride 1-30 I 1:13 - 2.78 1 5 12-53 Potassium chloride 2-01 I 04 - 2-06 6 Potassium nitrate 1'97 I '875 - 2-18 7 1°'00 I I i I l- ___- ___ I '0040 1 4-97 1 Calcium carbonate 1'26 I 2'91 , 8 1 : 1 3 For 2, 3 and 4 v = 173 and t = 20. The remaining data required tor these calcu- lations are in Table 111. of this paper, and in Table 11. Part VI. of these studies.ON COMPRESSIBILITY OF POWDERS 619 Th Shrinkage of Powdered Ammomkn Nifra fe. If ammonium nitrate is finely ground in a mortar or some form of mill, and is then pressed into pellets, the pellets will often shrink consider- ably.The rate at which this shrinkage takes place depends largely on the fineness of the grinding and on the quantity of moisture present ; 0.2 per cent. of moisture is quite sufficient to promote shrinkage. The rate of shrinkage also depends on the extent to which the pellets are compressed, and decreases with increasing compression. The following experiments were carried out on 50 gram blocks of powder which had been milled in edge runner mills. The volumes of the blocks were measured in mercury. Original volume ratios 1.270 1.181 1.103 Percentage reduction in volume after 12 hours 2.05 1-80 1-22 It is seen that the rate of shrinkage falls off with increasing compression. Professor Lowry and the present author described a case of expansion and shrinkage of potassium carbonate in Part V.of these studies, but this referred to the uncompressed salt, which is much more liable to shrink. The author believes that ammonium nitrate is quite exceptional in the readiness with which highly compressed pellets will shrink under the influ- ence of minute traces of water. I t is hoped that further light may be thrown on this question during the discussion. Although the capillary forces which tend to produce shrinkage are present in all powders containing moisture, it can be shown by means of the following calculations that in the case of ammonium nitrate a small force of this kind is particularly likely to cause a large contraction. From equations I, 2, and 4, we get for the velocity of compression under any load P when the volume ratio = Vl where P, is the load required to compress the powder to a volume ratio of V, in t, seconds.dv df From the data given in Part VI. it is possible to calculate - for any load at any given volume ratio from this expression. For example :- if P = 30 kilos./sq. cm. and V, = 1.3 then the following values of fl are obtained :- df dV dt I_ ammonium nitrate potassium nitrate sodium chloride ammonium chloride 8.5 x I O - ~ = 1.8 per cent. per hour. 3.0 x I O - ~ = 7.3 per cent. per year. 4.6 x 1 0 - l ~ = 1.1 x I O - ~ per cent. per year. 4-9 x 1 0 - l ~ = 1-2 x I O - ~ per cent. per year. These figures demonstrate that ammonium nitrate is quite specially liable to shrink under the influence of prolonged external pressure, and this fact explains why powdered ammonium nitrate is particularly liable to shrink when acted upon by capillary forces.620 COMPRESSIBILITY OF POWDERS Summary. compressed powder has been investigated. (a) The influence of the duration of the load on the volume ratio of (b) The isobaric curve has been correlated with the value of the ratio resistance to impact resistance to static load’ and further evidence for the validity of the classifi- cation of powders suggested in Part VI. of these studies has been obtained. (t) The exceptional readiness with which powdered ammonium nitrate shrinks has been shown to be dependent chiefly on the high value of its velocity coefficient. The author desires to thank Professor Lowry for suggesting that this work on powders, which was originally undertaken to settle certain practi- cal problems, should be extended to a variety of substances with the object of publishing it in its present form. (Parts VI., VII., and VIII. of these studies.) The author is also indebted to the Department of Scientific and Industrial Research for a grant which made this work possible, and to the Chief Superintendent, Research Department, Woolwich, for permission to use the Fairbanks compression testing machine and the Tangye press mentioned in these communications.

ISSN:0014-7672    

DOI:10.1039/TF9241900614

出版商:RSC    

年代:1924

数据来源: RSC

摘要:

118 ELECTRICAL THEORY OF ADBORPTTON The writer considers the double layer as consisting of a swface of rigidly fixed atoms under continuous bombardment of positively and negatively charged ions, any particular point on the rigid surface becoming in turn negative, neutral and positive, these conditions arisdg in any order. The observed contact difference is the average effect of these conditions. Where several kinds of atoms are present in the solution the average number of any one of them at the surface will depend on their concentbration, valency and mobility. The variation of contact Werence from negative to neutral and positive was observed with cotton and aluminium sulphate near the neutral point. These variations occurred during the same experiment, the readings being direct measurements of E.1I.F.s developed by filtration under pressure.This point would be covered by putting n2 = 1 and = 2 or 3 in Mukherjee’s equation No. 13.118 ELECTRICAL THEORY OF ADBORPTTON The writer considers the double layer as consisting of a swface of rigidly fixed atoms under continuous bombardment of positively and negatively charged ions, any particular point on the rigid surface becoming in turn negative, neutral and positive, these conditions arisdg in any order. The observed contact difference is the average effect of these conditions. Where several kinds of atoms are present in the solution the average number of any one of them at the surface will depend on their concentbration, valency and mobility. The variation of contact Werence from negative to neutral and positive was observed with cotton and aluminium sulphate near the neutral point.These variations occurred during the same experiment, the readings being direct measurements of E.1I.F.s developed by filtration under pressure. This point would be covered by putting n2 = 1 and = 2 or 3 in Mukherjee’s equation No. 13.118 ELECTRICAL THEORY OF ADBORPTTON The writer considers the double layer as consisting of a swface of rigidly fixed atoms under continuous bombardment of positively and negatively charged ions, any particular point on the rigid surface becoming in turn negative, neutral and positive, these conditions arisdg in any order. The observed contact difference is the average effect of these conditions. Where several kinds of atoms are present in the solution the average number of any one of them at the surface will depend on their concentbration, valency and mobility.The variation of contact Werence from negative to neutral and positive was observed with cotton and aluminium sulphate near the neutral point. These variations occurred during the same experiment, the readings being direct measurements of E.1I.F.s developed by filtration under pressure. This point would be covered by putting n2 = 1 and = 2 or 3 in Mukherjee’s equation No. 13. 620 COMPRESSIBILITY OF POWDERS DISCUSSION. Dr. E. P. Perman (communicated): During the war I examined the shrinkage of a large number of cylindrical blocks of ammonium nitrate and other substances made by compression with blows. The proportion of moisture was varied in the different blocks, which were kept in a closed vessel to prevent loss or gain of moisture.The volume of the blocks was measured at definite time intervals by their mercury displacement. A beaker containing mercury was placed on the pan of a balance and counter- poised; the block was then forced under the mercury by a support from above and the balance brought again into equilibrium. The added weights represented the displaced mercury which gave at once the volume of the block. The shrinkage was most rapid at first, the greater part of the action taking place in three or four days, but it continued measureable for several weeks. This problem is different from Dr. Walker's main problem, as the shrinkage took place without external pressure (beyond that of the atmos- phere) but the forces acting must have played a part in his experiments also.Factory managers always asserted that the shrinkage was due to the work done in grinding the material. I t is possible that an amorphous and less dense form is produced by grinding or hammering,l and that this tends to revert to its original state after the grinding is over, thus causing con- traction, but my own opinion has always been that the shrinkage is due mainly to solution and crystallisation in the surface film. 'Cp. Roy, PYOC. Roy. SOL., A. 101, 509 1922.DISCUSSION 62 I My experiments gave the following results :- (I) No moisture, no shrinkage. (2) Rapid increase of shrinkage with increase of moisture. (3) The greater the solubility, the greater the shrinkage.Ammonium nitrate is very soluble and therefore shows a high rate of shrinkage. Barium sulphate and calcium carbonate showed no measure- able shrinkage. Shrinkage and caking of salts are intimately connected and, I believe, are usually brought about somewhat as follows. There is always a film of saturated solution over the surface of the powdered salt. At the points of contact the pressure will increase the solubility (with most salts); at these points, more salt dissolves and the pressure is relieved ; recrystallisation then takes place between the surfaces in contact, and caking is produced Another factor acting in a similar way is the increased solubility on rounded surfaces; this would tend to flatten the points of contact, and cause shrinkage and consolidation of the whole mass.I believe also that moisture may travel from one part to another of the crystalline mass, which is full of air spaces. This would be similar to what takes place in chemical reaction between imperfectly dried sa1ts.l The President said that Dr. Walker was to be congratulated upon determining the ratio of the velocity coefficient, and on obtaining the calculated ratio between the impact and resistance under pressure to agree so well with the observed values. A most interesting feature was the application to ammonium nitrate, which was so abnormal in several ways. I t had been shown by Dr. Lowry that moisture was necessary for the caking effect of ammonium nitrate, and that if water was not present there was no caking.He had found recently in the case of another salt- ammonium sulphate-nitrate which, it might be- remembered, was found to have caked very badly at Oppau-that if it be dried thoroughly, con- siderable pressure could be applied to it without any caking whatever; but if there was only a slight amount of moisture it caked in the same manner as ammonium nitrate did. There remained, of course, a lot of work to be done in this connection. The three different classes which these chemical compounds fell into were by no means clear from the point of view of the chemical composition of the bodies. I t was not clear, for example, why, in the case of ammonium nitrate and T.N.T., the ratio of resistance should agree so well between the calculated and observed results, whereas in the case of other bodies, such as barium nitrate and tetranitromethylaniline, or trinitrophenylmethylnitroamine as it should more properly be called, it should come outside of the calculations.I t was necessary to have other physical constants for these bodies before it would be possible to give a complete account of them, but the results Dr. Walker had got were most important and were the first, to his knowledge, to throw light on this very difficult subject, which also had a very practical bearing. Dr. E. E. Walker: With reference to the President's remarks, the classification referred to is based entirely on the behaviour of the powders towards compression. Probably a substance which belonged to Class 11. or 1x1. at ordinary temperature would be found to belong to Class I.if the temperature were raised sufficiently. Lead which anneals itself almost in- stantaneously, would certainly behave quite differently at low temperatures at which this process takes place slowly. I t is not surprising, therefore, 'Proc. Roy. Soc., A. 79, 1907, 310.622 COMPRESSIBILITY OF POWDERS that the lines of demarcation should not coincide with any recognised chemical classification. The fact that the calculated and observed values resistance to impact resistance to static load of the ratio are in fair agreement in the case of sub- stances belonging to Class I., but not in the case of substances in the other two classes, is explained adequately by the theory on which this classification is based, and is, in fact, a necessary consequence if this theory is correct. I agree on the whole with Dr. Perman’s remarks regarding the mechanism by which shrinkage is brought about, but I am of the opinion that, what- ever the mechanism, the motive force is capillary in nature. It is a well- known physical fact that, the smaller the particles are, the greater is the capillary force with which a film of moisture will draw them together. Probably in this fact is to be found the reason for the connection which undoubtedly exists between the degree of fine grinding (or “ work ”) and the amount of shrinkage observed. In my experiments the capillary force was replaced or greatly augmented by an external load, and the processes suggested by Dr. Perman are probably supplemented by simple plastic deformation, and in the earlier stages when compresson is rapid this process probably overshadows all the others.

ISSN:0014-7672    

DOI:10.1039/TF9241900620

出版商:RSC    

年代:1924

数据来源: RSC

摘要:

118 ELECTRICAL THEORY OF ADBORPTTON The writer considers the double layer as consisting of a swface of rigidly fixed atoms under continuous bombardment of positively and negatively charged ions, any particular point on the rigid surface becoming in turn negative, neutral and positive, these conditions arisdg in any order. The observed contact difference is the average effect of these conditions. Where several kinds of atoms are present in the solution the average number of any one of them at the surface will depend on their concentbration, valency and mobility. The variation of contact Werence from negative to neutral and positive was observed with cotton and aluminium sulphate near the neutral point. These variations occurred during the same experiment, the readings being direct measurements of E.1I.F.s developed by filtration under pressure.This point would be covered by putting n2 = 1 and = 2 or 3 in Mukherjee’s equation No. 13.118 ELECTRICAL THEORY OF ADBORPTTON The writer considers the double layer as consisting of a swface of rigidly fixed atoms under continuous bombardment of positively and negatively charged ions, any particular point on the rigid surface becoming in turn negative, neutral and positive, these conditions arisdg in any order. The observed contact difference is the average effect of these conditions. Where several kinds of atoms are present in the solution the average number of any one of them at the surface will depend on their concentbration, valency and mobility. The variation of contact Werence from negative to neutral and positive was observed with cotton and aluminium sulphate near the neutral point.These variations occurred during the same experiment, the readings being direct measurements of E.1I.F.s developed by filtration under pressure. This point would be covered by putting n2 = 1 and = 2 or 3 in Mukherjee’s equation No. 13.118 ELECTRICAL THEORY OF ADBORPTTON The writer considers the double layer as consisting of a swface of rigidly fixed atoms under continuous bombardment of positively and negatively charged ions, any particular point on the rigid surface becoming in turn negative, neutral and positive, these conditions arisdg in any order. The observed contact difference is the average effect of these conditions. Where several kinds of atoms are present in the solution the average number of any one of them at the surface will depend on their concentbration, valency and mobility.The variation of contact Werence from negative to neutral and positive was observed with cotton and aluminium sulphate near the neutral point. These variations occurred during the same experiment, the readings being direct measurements of E.1I.F.s developed by filtration under pressure. This point would be covered by putting n2 = 1 and = 2 or 3 in Mukherjee’s equation No. 13.118 ELECTRICAL THEORY OF ADBORPTTON The writer considers the double layer as consisting of a swface of rigidly fixed atoms under continuous bombardment of positively and negatively charged ions, any particular point on the rigid surface becoming in turn negative, neutral and positive, these conditions arisdg in any order.The observed contact difference is the average effect of these conditions. Where several kinds of atoms are present in the solution the average number of any one of them at the surface will depend on their concentbration, valency and mobility. The variation of contact Werence from negative to neutral and positive was observed with cotton and aluminium sulphate near the neutral point. These variations occurred during the same experiment, the readings being direct measurements of E.1I.F.s developed by filtration under pressure. This point would be covered by putting n2 = 1 and = 2 or 3 in Mukherjee’s equation No. 13.118 ELECTRICAL THEORY OF ADBORPTTON The writer considers the double layer as consisting of a swface of rigidly fixed atoms under continuous bombardment of positively and negatively charged ions, any particular point on the rigid surface becoming in turn negative, neutral and positive, these conditions arisdg in any order.The observed contact difference is the average effect of these conditions. Where several kinds of atoms are present in the solution the average number of any one of them at the surface will depend on their concentbration, valency and mobility. The variation of contact Werence from negative to neutral and positive was observed with cotton and aluminium sulphate near the neutral point. These variations occurred during the same experiment, the readings being direct measurements of E.1I.F.s developed by filtration under pressure.This point would be covered by putting n2 = 1 and = 2 or 3 in Mukherjee’s equation No. 13.118 ELECTRICAL THEORY OF ADBORPTTON The writer considers the double layer as consisting of a swface of rigidly fixed atoms under continuous bombardment of positively and negatively charged ions, any particular point on the rigid surface becoming in turn negative, neutral and positive, these conditions arisdg in any order. The observed contact difference is the average effect of these conditions. Where several kinds of atoms are present in the solution the average number of any one of them at the surface will depend on their concentbration, valency and mobility. The variation of contact Werence from negative to neutral and positive was observed with cotton and aluminium sulphate near the neutral point.These variations occurred during the same experiment, the readings being direct measurements of E.1I.F.s developed by filtration under pressure. This point would be covered by putting n2 = 1 and = 2 or 3 in Mukherjee’s equation No. 13.118 ELECTRICAL THEORY OF ADBORPTTON The writer considers the double layer as consisting of a swface of rigidly fixed atoms under continuous bombardment of positively and negatively charged ions, any particular point on the rigid surface becoming in turn negative, neutral and positive, these conditions arisdg in any order. The observed contact difference is the average effect of these conditions. Where several kinds of atoms are present in the solution the average number of any one of them at the surface will depend on their concentbration, valency and mobility.The variation of contact Werence from negative to neutral and positive was observed with cotton and aluminium sulphate near the neutral point. These variations occurred during the same experiment, the readings being direct measurements of E.1I.F.s developed by filtration under pressure. This point would be covered by putting n2 = 1 and = 2 or 3 in Mukherjee’s equation No. 13.118 ELECTRICAL THEORY OF ADBORPTTON The writer considers the double layer as consisting of a swface of rigidly fixed atoms under continuous bombardment of positively and negatively charged ions, any particular point on the rigid surface becoming in turn negative, neutral and positive, these conditions arisdg in any order.The observed contact difference is the average effect of these conditions. Where several kinds of atoms are present in the solution the average number of any one of them at the surface will depend on their concentbration, valency and mobility. The variation of contact Werence from negative to neutral and positive was observed with cotton and aluminium sulphate near the neutral point. These variations occurred during the same experiment, the readings being direct measurements of E.1I.F.s developed by filtration under pressure. This point would be covered by putting n2 = 1 and = 2 or 3 in Mukherjee’s equation No. 13.118 ELECTRICAL THEORY OF ADBORPTTON The writer considers the double layer as consisting of a swface of rigidly fixed atoms under continuous bombardment of positively and negatively charged ions, any particular point on the rigid surface becoming in turn negative, neutral and positive, these conditions arisdg in any order.The observed contact difference is the average effect of these conditions. Where several kinds of atoms are present in the solution the average number of any one of them at the surface will depend on their concentbration, valency and mobility. The variation of contact Werence from negative to neutral and positive was observed with cotton and aluminium sulphate near the neutral point. These variations occurred during the same experiment, the readings being direct measurements of E.1I.F.s developed by filtration under pressure. This point would be covered by putting n2 = 1 and = 2 or 3 in Mukherjee’s equation No.13.118 ELECTRICAL THEORY OF ADBORPTTON The writer considers the double layer as consisting of a swface of rigidly fixed atoms under continuous bombardment of positively and negatively charged ions, any particular point on the rigid surface becoming in turn negative, neutral and positive, these conditions arisdg in any order. The observed contact difference is the average effect of these conditions. Where several kinds of atoms are present in the solution the average number of any one of them at the surface will depend on their concentbration, valency and mobility. The variation of contact Werence from negative to neutral and positive was observed with cotton and aluminium sulphate near the neutral point. These variations occurred during the same experiment, the readings being direct measurements of E.1I.F.s developed by filtration under pressure.This point would be covered by putting n2 = 1 and = 2 or 3 in Mukherjee’s equation No. 13.118 ELECTRICAL THEORY OF ADBORPTTON The writer considers the double layer as consisting of a swface of rigidly fixed atoms under continuous bombardment of positively and negatively charged ions, any particular point on the rigid surface becoming in turn negative, neutral and positive, these conditions arisdg in any order. The observed contact difference is the average effect of these conditions. Where several kinds of atoms are present in the solution the average number of any one of them at the surface will depend on their concentbration, valency and mobility.The variation of contact Werence from negative to neutral and positive was observed with cotton and aluminium sulphate near the neutral point. These variations occurred during the same experiment, the readings being direct measurements of E.1I.F.s developed by filtration under pressure. This point would be covered by putting n2 = 1 and = 2 or 3 in Mukherjee’s equation No. 13. AN INVESTIGATION OF SMOLUCHOWSKI’S EQUATION AS APPLIED TO THE COAGULATION OF GOLD HYDROSOL. BY LEONARD ANDERSON, B.Sc., PH.D. (A Papey read before T H E FARADAY SOCIETY, Monday, November 1 2 t 4 1923, SIR ROBERT ROBERTSON, K.B.E., F.R.S., in fh chi^.) Received June 2 7 fh, I g 2 3. INTRODUCTION. As regards the kinetics of coagulation of colloidal solutions the Smoluchowski equation is the only one which has a theoretical basis, Freundlich,l working with aluminium hydroxide sols, deduced an equation, based on measurements of the variation of the viscosity of a sol during co- agulation, which gives the relationship between the time and the amount of precipitated colloid. I t has the drawback, however, that no definite relationship is known to exist between the size of particles and the viscosity of a colloidal solution.Smoluchowski 2 was led to the theoretical treatment of the problem of coagulation from a consideration of certain experiments of Zsigmondy on colloidal gold. Zsigmondy found that, on coagulating a gold sol by means of electrolyte, the speed of coagulation increased with increasing electrolyte concentration until a maximum speed was obtained.This maximum speed is independent of all further increase in electrolyte concentration. This fact formed the basis of Smoluchowski’s theory of “ rapid coagulation.” I n the absence of electrolyte, the electrical double layer on the particles prevents coalescence taking place on impact of one or more particles. Addition of electrolyte to the colloid system diminishes the electrical double layer on the particles, and a force of attraction comes into play. The region of ‘‘ rapid coagulation,” as formulated by Smoluchowski, corre- sponds to a state of complete electrical discharge of the particles and con- sequently of maximum attractive forces between them. Smoluchskis Theory.3-According to this theory each particle in a homogenous sol is supposed to possess a sphere of attraction R, within which the attraction is so strong that any other particle, whose centre enters this sphere, is firmly held.In an originally uniform sol, whose particles have been completely discharged, the initial number of particles whose centres are less than R apart is vanishingly small. In course of time, Brownian movement brings the particles into all possible configurations. In consequence of Brownian movement and of Freundlich, Trans. Fuuaday SOL, 9, 66, 1913. Smoluchowski, Zecfschr. physikai Chcm., 9, 129, 1917. 3 Ibid. 623624 AN INVESTIGATION OF SMOLUCHOWSKI’S EQUATION AS the existence of ‘‘ spheres of attraction ” an irreversible state of coagulation is finally set up. It would be futile to merely give the deduction of Smoluchowski’s equation; that will be found in the original paper.Smoluchowski, combining probability considerations and the laws of diffusion, derives a series of equations which give the rate of disappearance of the particles in a colloid solution during “ rapid ” coagulation. The following equation which gives the rate of disappearance of primary particles is the one used for the following investigation : VO (zqp where yo is the number of primaries at zero time, v1 is the number of primaries at a time t, and /3 is a constant equivalent to ~TDRv,, where D is the ditrusion coefficient and R is the radius of the sphere of attrac- tion. Smoluchowski also attempts to extend his theory to slow coagulation. In this case owing to incomplete discharge of the electric layers, the attractive forces between the particles are not at their maximum and hence only a fraction of the collisions result in union.A probability factor c is therefore introduced to allow for this. The resulting equations obtained by Smoluchowski are identical in form with those obtained for rapid coagulation except that the term /3 is now replaced by cP. Thus the vo The probability factor equation v1 = is assumed by Smoluchowski to be constant throughout the course of ( I + €/3t)2’ vo becomes v1 = ( I + pry - . coagulation. A fundamental assumption of Smoluchowski’s theory is that the rate of disappearance of primaries is greater than that of a simple K g bimolecular ” reaction. As a corrollary to the above it would follow that if coagulation were treated as a bimolecular process, the bimolecular velocity ‘( constant ” kbi should always increase with time for both slow and rapid coagulation.In the case of slow coagulation, however, a rapid fall in the Smoluchowski constant and also in kbi is obtained experimentally, although theoretically the former should remain constant and the latter should rise. PREVIOUS INVESTIGATIONS OF SMOLUCHOWSKI’S EQUATION. Smoluchowski’s equations have been tested by Zsigrnondy, by Westgren and Reitstiitter, and more recently by Kruyt and Arkel, and by Mukherjee and Papaconstantinou. These investigators, with the exception of the last named, used an ultramicroscopic method and followed the coagulation by making a count of the number of particles present at various intervals of time.The experiments of Zsigmondyl gave reasonable constants for /3 (variation of about 5 0 per cent.) when rapid coagulation was studied. Westgren and Reitstiitter worked with coarse gold sols, and found the ratio of i.e. the radius of the sphere of attraction, divided by the radius R of the primary particles. The values of -, which of course should be constant, varied in some cases by IOO per cent. Y ’ 1 Zsigmondy, Zeitschr. physikal Chcm., 92, 600. 1917. a Westgren and Reitstotter, Zeitsthr. physikal Chem., 92, 750, 1917.APPLIED TO THE COAGULATION OF GOLD HYDROSOL 625 Using the data of Westgren and Reitstotter,' the values of /3 have been calculated by the present writer using the equation DATA OF WESTGRBN AND RBITST~TTBR. Serirs I.swim 2. t Sea. 0 60 I20 240 420 600 900 1320 (Re%ve). 10'0 8-70 8-36 7-51 6*29 5 46 5-06 9-46 B (Calcuia ted). - 0'149 0-098 0'083 0.083 0.083 0.065 0.057 t Secr. 0 30 60 I80 300 420 600 120 L V (Relative). 10'0 6 *63 5'45 3 '92 3.12 2'3 9 I%+ I '42 B (Cdculatcd). - 1'0 0.834 0'775 0'73s 0'645 0'635 0.604 It will be observed that the tendency of /3 to fall is quite marked in spite of the fact that in series 2 the speed of coagulation is such that the total number of particles is halved in the first minute. Kruyt and Arkel,2 working with selenium sol, find that Smoluchowski's equation holds in the rapid region but that fl falls rapidly for coagulation at inter- mediate speed. Mukherjee and Papaconstantinou s measured the varia- tion in the optical absorption coefficient, which accompanied the change of colour of colloidal gold in presence of electrolyte.If t,, t,, and ts are the times required by the coagulating sol to reach the same absorption coefficient, using a different concentration of electrolyte in each case, it can be deduced from Smoluchowski's equation that where Mukherjee and Papaconstantinou obtained data which gave reasonable constants for the ratio of T1 : T, : T,. The act of coagulation of colloidal gold involves a change of colour from red to blue. The red is supposed to be due to primary particles. On the assumption that the percentage of red remaining is proportional to the change in the absorption coefficient, the values of /3 were calculated by the writer from Mukherjee and Papaconstantinou's data, using the YO ( I + / 3 t ) Z * equation v1 = In the case of potassium chloride and potassium nitrate as electrolyte, the value of p calculated on the above basis showed good constancy €or p..However, in the case of barium chloride (the speed of coagulation being slower than in the two previous cases) the value1 of /3 calculated 1 Ibid. 2 Kruyt and Arkel, Rec. Trav. Ckim. Pays Bas., s, 656,1920. Mukherjee and Papaconstantinou, Phil. Mag., 44, 305,1922. VOL. XIX-T2&626 .4N INVESTIGATION OF SMOLUCHOWSKI'S EQUATION AS Time in Minutes. 0 I 2 4 5 7 9 I3 16 - from the data of Mukherjee and Papaconstantinou was found to fall as coagulation proceeded. This is shown in the following table :- DATA OF MUKHERJEE AND PAPACONSTANTINOU. Absorption Per Cent. Red Coefficient. (Calc.).0'0453 JOO'O 0*1603 71.56 0'2007 61.5 7 0.2687 44'76 35'73 0'3237 31-15 0'3527 23'93 0'3732 18-91 0'4497 0'3051 Conc. BaC12. 100'0 40 25 IS 5 0.00075 N - 0.380 0'333 0.287 0.280 B (Calc.). - 0.182 0.137 0'123 0.134 0-1 13 0.116 0'096 0.081 - I n the last column the value of p falls by more than one-half. I t is certain that the alteration in the value of p is real, as will be shown later. In addition to the above investigations, attention must be drawn to those of Hatschek,l who devised a colorimetric method of testing the applicability of Smoluchowski's equation. The details of the method will be found in the original paper. The principle of the method is as follows : A rectangular cell divided by an oblique partition contains, in one half red gold sol and in the other half blue sol in suspension stabilised by gelatin.This cell when viewed from the front shows a colour range varying from IOO per cent. red to 100 per cent. blue. A second cell, similar to the first, is placed on top and into it is poured the coagulating sol which is being examined. The colour of the sol in the upper cell varies gradually, with time, from IOO per cent. red to blue. By direct aomparison of the tints in the upper and lower cells, the percentage of red remaining at any instant can be estimated directly. Using this method, Hatschek tested the applicability of the equation VO 'l = ( I + Pt)Z where yo = IOO per cent. red. Y1 = the percentage of red at time f . The following values are taken from Hatschek's paper :- DATA OF HATSCHEK.0.0075 N 0 2'0 3'0 4'5 8'0 14.0 28 '0 Per Cent. Red. 100'0 60 50 40 30 20 I0 8. 0.145 0.138 0.130 0.103 0.088 0.077 Conc. HCI. 0.00826 N Time in Minutes. 0 1'5 3'0 5 '5 16.0 ' Per Cent. Red. 1 B* I- -___ I I Hatschek, I'raus. Faraday Soc., 17, 499, 1921.APPLIED TO THE COAGULATION OF GOLD HYDROSOL 627 The constancy of /3 is not satisfactory although in the faster reaction the fall in p is much less marked. Hatschek also gives instances in which coagulation does not go to completion, Le. no further colour change takes place even after many hours. I t follows from this that p ultimately be- comes zero and consequently a fall of p from the initial value is inevitable. The work to be recorded in the present paper consists of a further and more detailed examination of the Smoluchowski equation using the method of Hatschek.It is convenient to divide the various cases encountered into three divisions, namely :- (i) Rapid coagulation. (ii) Intermediate speed of coagulation. (iii) Slow coagulation. PREPARATIVE. Preparation of CoZoidaZ GoZd.-The water used for this purpose was conductivity water which had been redistilled from a silver-lined copper vessel. All standard solutions used (electrolytes, etc.) were made up with this redistilled water. The gold sols themselves were made by three methods. ( a ) Sodium Citrate Mt.thod.-To 240 C.C. of redistilled water are added 2.5 C.C. of a 0.6 per cent. solution of gold chloride and the whole heated to boiling. Three cubic centimetres of a I per cent. solution of sodium citrate are then added.The solution turns a clear port red and then a further 2 - 5 C.C. of gold chloride are added and the solution again boiled. (6) Method of Hatschek.-3oo C.C. of redistilled water are placed in a flask together with 0.06 grams of white dextrin and 2 C.C. of a normal caustic soda solution. Five C.C. of a 0.6 per cent. solution of gold chloride are added and the mixture slowly heated to boiling. Between 95' C. and 100' C. the whole turns ruby red. (c) T h Formaldehyde Method.-480 C.C. of redistilled water are brought to boiling-point and 10 C.C. of a 0.6 per cent. gold chloride solution to- gether with 14 C.C. of o.18N potassium carbonate solution are added. When the solution is boiling vigorously a 0.3 per cent. solution of formalde- hyde is slowly added as recommended by El1iott.I The addition of formal- dehyde is continued until no further change in colour occurs. The sols prepared were dialysed in collodion dialysers against distilled water.The specific conductivity of the dialysed sols ranged from 0-8 x 10 - to 2 x 10 - mhos. The dialysed gold sols contained 56 milligrams of gold per litre. Method of Procedure.-All glass and quartz vessels were cleansed, for each fresh sol, with chromic acid and aqua regia, washed out with distilled water and finally with steam from redistilled conductivity water. The con- centration of colloid in the comparison sol was in all cases identical with that of the coagulating sol. In all cases the sol was poured into the electrolyte and rapidly mixed. Hydrochloric acid, potassium chloride, barium chloride and aluminium chloride were used as coagulating agents and the comparison sol was made by using the electrolyte, which was being examined .The whole is heated. Elliott, '3!wrn. Iitdiis. and En:. CIum., 13, 699, 192 1. Note.-The present investigation was already partly completed before the writer became aware of the results obtained by Mukherjee and Papaconstantinou by the absorp- tion coefficient method, It is entirely accidental, therefore, that the same precipitating electrolytes were used in both cases.628 AN INVESTIGATION OF SMOLUCHOWSKI'S EQUATION AS 100'0 25.6 19.2 12.8 6.4 EXPERIMENTAL RESULTS. In the following tables, selected from numerous results, /3 was calculated, using the equation - 1-54 1-25 1.06 1-17 VO "l = ( I + /3t)2' - 2'34 2.48 2-15 1-98 where vo = IOO per cent.red, and v1 = the per cent. red at time f. In several cases the value of kbi has been calculated assuming that the primaries (i.e. the percentage red) disappear simply by union with each other. In the results recorded below, only those experiments which gave concordant results, after two or more repetitions have been utilised. - 7'0 12.4 7-10 11*8 A. CONDITIONS APPROXIMATING TO RAPID COAGULATION. Time. (!ha.). I . EZech.olyte-Uyd~oc~Zoric Acid. Per Cent. 8. Red. 1 Sol 12A (Formaldehyde). HC1 = 0*00833 N. Sol rzA. HCl =O'OI N. Time (Secs.). 0 35 57 95 150 Per Cent. I Red. 1 B. t I- Time (Stcr.). 0 25 35 50 90 Per Cent. Red. --- 100'0 25.0 16'6 12.8 6.4 2. Blectro&fe- Potassium ChZoride. Sol 45 (Formaldehyyde).-KCl = 0.033 N.Time. Per Cent. (Secs.). 1 Red. 1 0 17 45 60 84 160 240 I 100'0 ~ 60.0 40.0 29'3 12.9 6.4 20'0 - ' 0.928 0'748 0'848 0'884 0.930 0'737 3. ElectroZyte-Barium Chlode. Sol 22 (Fomaldchyde). Sol 19 (Formaldehyde). BaC1, = 0.00332 N. BaCI, = o*ooxM N. I I 0 I5 25 45 80 130 100.0 46'6 33'3 20 '0 9'3 4'3 - 1.860 1-760 1.650 1.710 1-850 Time (Secs.). Per Cent Red. 100'0 53'3 38'6 29'3 24.0 I 6'0 10'6 6.6 B. - 1'11 0'915 0.925 0'777 0'782 0.828 (3.913APPLIED TO THE COAGULATION OF GOLD HYDROSOL 629 Time. Per Cent. The constancy of p in the above tables is good, especially in the case of barium chloride as electrolyte. Analogous behaviour was observed with aluminium chloride. The results may be taken as indicating that the Smoluchowski equation is holding in the region observed.In fact, the values of p in the case of barium ion are apparently more concordant than have been previously obtained. 1 B. B. CONDITIONS OF INTERMEDIATE SPEED OF COAGULATION. EZeciroZyte- Hydroch Zovic Acid. Sol 7 (Formaldehyde). HCl = 0.00847 N. Sol 10 (Fmaldchyde). HCI = 0*0053 N. I--1 100'0 30 67.9 60 59.0 5x-3 I80 44'9 25'6 360 540 I I0 20.5 1 Time. (secs.). - I 0.428 0.302 0'212 0'164 0'162 , 0.134 > Per Cent. Red. 100'0 56 '4 38'4 25'5 19'4 12.7 6.4 (Secs.). 1 Red. - 0.662 0.526 0'490 0.282 0.258 0'211 B. -____ 4 1'5 1-26 0.87 0'50 I *08 1'11 EZectro@#e- Potassiu m Chloride. Sol 45 (Formaldehyde). KCI = 0.03166 N. Time. (Secs.). 0 30 so 130 I80 360 Per Cent. Red. 100'0 60 '0 33'3 26'0 22 06 '3'3 I kbi X 102. - 1'33 1-60 1-07 0'70 I '50 EZectroZyte-Barium Ch Zoride.Sol 19 (Formaldehyde). 3aC1, = 0.00134 N. Time. (Secr.). Per Cent. 1 Red. 0 30 55 560 I00 220 1080 100'0 73'3 60.0 46'6 33 '3 13'3 3 '3 k - 0.336 0.317 0.279 0'200 0'187 - kbi x Id. - 0.72 0.72 0'63 0'43 0'60 - In the above tables it will be observed that the value of /3 falls con- tinuously as coagulation proceeds, indicating that the Srnoluchowski equation is not depicting the true rate of coagulation.630 AN INVESTIGATION OF SMOLUCHOM'SKI'S EQUATION AS c. SLOW COAGULATION. Electroote- HydrochZo?-ic Acid. Sol 7 (Citrute Method).-HC1 = 0*00508 N. Time (Minutes). 0 I 2 4 6 8-25 11'5 24 35 Per Cent. 1 Red. .I 100'0 76.0 66'7 53'3 44'0 40'0 35'9 26-7 22.5 B* - 0.146 0-117 0.08 j 0.070 0.060 0.039 0.031 0'1 I2 - 3'1 1.8 1.8 2'0 1'0 0.88 0'77 0'63 The change to 100'0 per cent.blue was incomplete after 2 hours. Electrody te-Potassium Chloride. Sol 45 (Formaldehyde Method).-KCl = 0,0266 N. Time I Per Cent. ~ kbi x 102. (Secs.). Red. I I I 100'0 73'3 60.0 50.8 44'0 37'3 30'7 - 0-214 0.1 66 0.147 0.113 0.067 0'100 - 0.4 8 0'34 0'26 0.17 0'22 0'10 EZectro(yte-Barium ChZoride. Sol 19 (Formaldehyde Method).-BaCl! = O ~ O I N. Time (Minutes). 0 I 2-33 3 '75 5 *66 24-50 3 6.0 67'5 107.0 Per Cent. Red. I 100'0 86.7 73'3 66.6 60'0 46.6 33'3 221.0 17'3 - 0*07+ 0.071 0'060 0.05 I 0.018 0.013 om08 0'020 - 1-53 1-58 0.96 0.86 0'74 0'18 0.056 0'20 I t will be seen in the above tables that p and even kbi fall rapidly-the more so the slower the speed of coagulation. Although in nearly all the results quoted the sols used were made by the formaldehyde method, experiments were conducted using sols made by the other methods mentioned previously.for a given concentration of electrolyte varies with the mode of preparation of the sol. A lesser degree of variation occurs when sols are prepared in an identical manner. Probably this is a question of difference in size of the particles and possibly also of their structure. The results obtained show that the value ofAPPLIED TO THE COAGULATION OF GOLD HYDROSOL 631 In addition to a fall in p, it is possible to arrange the electrolyte con- centration so that the sol never reaches IOO per cent. blue after standing many hours. I t would perhaps be of interest to compare the values of /3 for slow coagulation with hydrochloric acid as electrolyte, with those obtained by other workers cited in the Introduction.I n series I of Westgren’s data the value of p fell from 0.149 at the end of the first minute to 0-057 after 2 2 minutes. I n the recalculation of Mukherjee and Papaconstantinods data, using barium chloride as electrolyte, the value of p fell from 0.182 after the first minute to 0.081 after 16 minutes. In the first table for slow coagulation by the author, given above, the value of p fell from 0.146 after the first minute to 0.060 after I 1.5 minutes. Although the data are obtained by three totally different methods, the rate of fall of p is about the same in each case. This is evidence in favour of the general applicability of the various experimental methods employed.DISCUSSION OF RESULTS. The experiments indicate that for the coagulation of gold sols, by means of the electrolytes chosen, there is a “rapid” region in which Smoluchowski’s equation holds reasonably well. The constancy of /3 is quite good, especially in the case of barium chloride as electrolyte. In a region of smaller electrolyte concentration than the above, an excessive slowing down in the speed of coagulation with time is observed. This is in agreement with the data of Kruyt and Arkel and also with the one case (incidentally the slowest speed) of Mukherjee and Papaconstantinou in which the value of p, calculated by the writer, was shown to fall. The general conclusion arrived at by the writer is that the Smoluchow- ski equation is strictly limited in its application.Smoluchowski asserts that the curves depicting slow and rapid coagu- lation should have a similar form, the only varying factor being the proba- bility that an impact will give union. For so-called “ rapid ” coagulation this probability factor is unity and for non-rapid coagulation it is 6, where r < I. This factor z is assumed by Smoluchowski to be constant through- out the course of any one coagulation, but this is not found to be the case. The factor depends upon, and must be some function of, the residual nett charge on the particle. When two charged particles unite, the surface density of the charge on the complex is different from that on the original particle and therefore different repulsive forces come into play. I t is therefore very probable that the factor for union between a charged primary and a charged complex, is less than the factor z for union between two primaries.Let us assume that each primary particle has a radius r and possesses a nett charge E. When two primaries approach one another, a force of repulsion comes into play, reaching a maximum value of E? F = - @K’ where K is the dielectric constant of the medium. that this force of repulsion is overcome. Before two such particles can unite, their relative velocity must be such632 AN INVESTIGATION OF SMOLUCHOWSKI’S EQUATION AS If W,, is the work done in overcoming this force of repulsion, the probability that any two particles will possess the necessary critical velocity is given by This probability factor P,, is the factor Z.Let us now consider a union between a primary and a secondary par- If the density is The ticle. constant, the radius of the secondary will be Y viand its charge 2E. force of repulsion between a primary and a secondary will now be For simplicity consider the latter to be a sphere. The work Wl, to be done before union can take place is now greater and the probability of union becomes - Wl!. P,, = z kT L- W1,. 8 Where W1, i3: __--- (I + 72)2 It is thus evident that the probability factor is varying continuously as the complexes become larger. The falling value of c would partly account for the filling value of /3 observed in this region, since the /3 calculated in the previous tables implicitly contains Z. However even if this modification were introduced into the Smoluchowski equation, the rate of disappearance of primaries would, theoretically, always be greater than that obtained by a bimolecular process, assuming primaries simply united with each other. The data obtained however show that in some cases the value of kh falls, i.e. coagulation is proceeding even more slowly than would be expected on the basis of a bimolecular process.Furthermore, if primaries did disappear simply by union with each other it would follow that once coagulation has commenced it should proceed until no more primaries are left. On this basis incomplete colour change from red to blue should not be possible, since any red colour remaining would indicate unchanged primaries (attributing the red colour to the latter). However, as we have seen, incomplete colour change does occur and simply depends upon the con- centration of the electrolyte present.This phenomenon admits of two ex- planations : I . I t may be due to the possibility that the rate of disappearance of primaries is counterbalanced by an opposing effect : that is, primaries are being reformed either by spontaneous disruption of complexes or by collision of complexes with each other. Such reversibility however would seem to entail a behaviour, on dialysis, of incompletely coagulated sol which has not yet been observed. 2. A more probable explanation would seem to be that the initial primary particles (giving the red colour) are unequaZZy charged. In the case of slow and eventually incomplete coagulation very small amounts of electrolyte are used and it is conceivable that the amount adsorbed is not sufficient to reduce the charge of some of the particles (which initially carry an excessive charge) below the critical limit which will permit coagulation to take place.If this conception of unequal charge is correct, the Smoluchowski equation could not be expected to be applicable in general.APPLIED TO THE COAGULATION OF GOLD HYDROSOL 633 In reviewing the whole problem of coagulation, it would appear that the Smoluchowski equation in its present form is limited in its application. Before it can be applied to all types of coagulation it apparently requires modification to allow for the two factors : (a) The decrease of the probability factor as coagulation proceeds. (b) The existence of incomplete coagulation as a consequence of unequal, and in some cases therefore, of excessive initial electrical charge on the primary particles. SUMMARY. I. A survey of investigations bearing on the Smoluchowski equation has been given. 2. Colorimetric determinations of the rate of coagulation of gold sols by hydrochloric acid, potassium chloride, barium chloride and aluminium chloride have been carried out, using the method of Hatschek. 3. I n agreement with previous investigators, a region of rapid coagula- tion is found in which Smoluchowski's equation holds fairly well. The equation holds most satisfactorily in the case of a certain concentration range of barium chloride as coagulant. 4. A slower region of coagulation is found in which the equation is inapplicable. 5. Possible explanations of (4) have been suggested. 6. I t is concluded that, on the whole, the Smoluchowski equation in its present form is strictly limited to rapid coagulation. 7. Certain of the results obtained can be interpreted as indicating con- siderable variations in magnitude of the charge on individual primary par- ticles of the sol. Addendum-Since the above paper was written the author has found that Kruyt and Arkel working with selenium sol have investigated the Smoluchowski equation, by the method of counting the particles. These observers find that the theory of Smoluchowski holds in the region in which the velocity of coagulation does not differ greatly from the velocity for completely discharged particles. At lower electrolyte concentration the coagulation takes place more slowly than the theory demands. This is in agreement with the results obtained in the above work. The investigation described in the Paper was carried out under the direction of Professor W. C. M. Lewis. 1 Kruyt and Arkel, Koll. Zeitsch., p, 29, 1923. Muspratt Laboratory of Physical and El'ectroehemisity, University of Liverpool. VOL. XIX-T24

ISSN:0014-7672    

DOI:10.1039/TF9241900623

出版商:RSC    

年代:1924

数据来源: RSC