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. Mr. E. Hatschek (Communicated) : The author points out that the formula fails to account for incomplete coagulation, where the number of particles remains constant after a certain time. He does not refer to the equally serious defect that it does not cover the case of cumfkfe coagulation either, since, if the equation applied strictly, the number of primary particles could not become zero except after an infinite time. It is difficult to prove that this is not the case, but it is certainly not the im- pression one derives from 1ooki.ng at a completely coagulated gold sol after, say, twenty-four hours. The author makes some suggestions to account for the discrepancies between the formula and the experimental results and in so doing appears to postulate electrostatic repulsions between the particles.Smoluchowski in his original paper is very emphatic on the point that, whatever may be the stabilising effect of the double layer on the particles, it cannot be electrostatic repulsion. There is also no justification for assuming, as the author does, that secondary particles are spheres with a radius v G that of the primary, an assumption which can hold only for liquid particles. As an alternative to his suggestions one may point out that the constant p contains the diffusion constant, which must change with the size of the particle, whatever its shape. I t is, however, obvious that the formula cannot be improved in principle by such corrections, and it seems as if no progress were to to be expected until the physically interesting part of the phenomenon, i.e. the effect of the double layer and of its removal, receives consideration. 634

ISSN:0014-7672    

DOI:10.1039/TF9241900634

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. THE EFFECT OF SUCROSE ON THE RATE OF COAGULATION OF A COLLOID BY AN ELECTROLYTE. BY LEONARD ANDERSON, B.Sc., PH.D. ( A Paper read befre THE FARADAY SOCIETY, Monday, November 12th, 1923. Sir ROBERT ROBERTSON, K.B.E., F.R.S., Presirtent, in the Choir,) Received August 2 8 f 4 1923. The concept that the activity of ions, rather than their concentration, was likely to play an important part in chemical kinetics was first suggested by Harned.l The interesting problem arises as to whether the activity of the added electrolyte ions does or does not play the significant roll in a typical coZZoidd phenomenon, namely the rate of coagulation of a colloid in presence of added electrolyte.The work of Moran2 and of Corran %* in this laboratory has shown that sucrose possesses the property of causing a large increase in the activ- ity of various ions, whilst not appreciably altering their concentration. I t was, therefore, considered of interest to investigate the influence of sucrose on the coagulation of a typical colloid, gold hydrosol. In any molecular (homogenous) system the kinetics of a reaction is characterised by a velocity constant the value of which can be determined in general with accuracy. In colloid systems the Smoluchowski equation is the only one, possessing a strictly theoretical basis, which deals with the kinetics of coagulation. From the work of Anderson and of other investigators,G however, this equation appears to be limited to rapid coagulation.In the present instance, a slow or moderately rapid rate of coagulation was considered best since, in this region, the rate of coagulation is very sensitive to slight variations in the concentration of the coagulant. In view of the inapplicability of the Smoluchowski equation to this region, a more arbitrary method of measur- ing the rate of coagulation was chosen, namely, the time required for the originally red gold sol to reach the tint of a partially coagulated gold sol, stabilised by addition of gelatin. From the literature it appears to be uncertain whether sucrose does or does not behave as a peptiser towards colloidal sols. Bancroft7 enumerates certain cases in which sucrose does peptise.Chatterji and 'Harried, Journ. Amer. Chem. SOC., 37,2482, 1915. 2 Moran and Lewis, Trans. Chem. Soc., 121, 1613, 1922. 3Corran and Lewis, yourn. Amer. Chem. SOL, 4, 1673, 1922. 4Corran, yourn. AMY. Chem. SOC., 45, 1627, 1923. 5 Smoluchowski, Zeitschr. physikal. Chem., 92, 129, XgI7. 6 Anderson (this issue, Trans. Faraa'uy SOC., p. 623). Kruyt and Arkel, Rec. Trav. Weztgren and Reitstotter, Zeitschr. physikal Chem. Chitn. Pgys-Bas., 3, 656, 1920. 92, 750, 1917. Mukherjee and Papaconstantinou, Phil. Mag., 4, 305, 1922. 7 Bancroft, Second Brit. Assoc. Report on Colloid Chemistry, p. 2, 1918. 635636 THE EFFECT OF SUCROSE ON THE RATE OF Dhar,l however, state that true peptisation by sucrose is doubtful. This point will be dealt with in the present paper.The addition of sucrose to the colloid system increases the viscosity and should thereby lower the speed of coagulation. The sucrose, however, besides affecting the mobility of the colloidal particles also affects the mobility of the coagulating ions. However, in the following experiments, the number of electrolyte ions per litre is always greater than one hundred times the number of colloid particles per litre. colloid particles per litre ; the former varied from 0.83 x 1042 ions per litre in the case of potassium chloride to approx. 0.83 x 1041 ions per litre in the case of hydrochloric acid. Adsorption of the electrolyte ions by the colloid particles may therefore be taken as rapid compared with the rate of collisions between the suspensoid particles themselves.The rate of collision.of the colloidal particles will vary with their diffusion coefficient which is given by The latter were calculated to be approximately 3 x 10 RT I * = r c where R = the gas constant. T = the absolute temperature. C = the frictional resistance. I n this case, since it is the colloidal particles which are diffusing, we can write C - 6 q r where and I = the radius of the particle. ‘I = the viscosity of the medium. Hence, if we coagulate two samples of the same colloidal solution and vary only the viscosity then, in the two cases, the rate of coagulation should be inversely proportional to the viscosity of the medium. I t was upon this basis that the following investigation was carried out. Aliquot portions of a colloidal solution were treated with electrolyte in presence of varying quantities of sucrose.In one series of determinations the cotrienfrafion of electrolyte was maintained constant in presence of vary- ing amounts of sucrose. In the other case the acfivity of the electrolyte cation was maintained constant in presence of varying amounts of sucrose. If fh activiiy of the conphfing ion as distinct from its concentration is a decidiag factor for coagdation, t h n under the above conditions the quantity representing fhc time of coaguzation divided by the viscosity of fh medium (assuming ih sucrose io be inert in respect of peptising power) shouZd behave as fo22ow.s :- I. At constant concentration of electrolyte the ratio should diminish as the sucrose content is increased; 2.At constant activity of electrolyte, the ratio should be constant. Preparative.-The gold sols used were prepared by the formaldehyde method as recommended by Elliott with the following modification. To the cold mixture of gold chloride and potassium carbonate solutions were added a few drops of acetone. On warming the solution a pinkish tinge appears. When the solution is boiling, the formaldehyde solution is slowly added drop by drop and the resultant sol is of a clear ruby red colour. Hatschek3 states that it is difficult to prepare larger batches of colloidal gold than I 5 0 c.c.~ at a time by Zsigmondy’s original formaldehyde method. 1 Chatterji and Dhar, Appendix to Trans. Faraday SOC., 16, 122, 1920-21. a Elliott, Journ. Indus. and Bng.Chem., 13, 699, 1921. 3 Hatschek, ‘‘ Laboratory Manual of Colloid Chemistry,” p. 31.COAGULATION OF A COLLOID BY AN ELECTROLYTE 637 If, however, the above slight modification is made, batches of any volume can be prepared and the resulting sol is beautifully clear. The sols used were dialysed against distilled water ; the specific conductivity of the dialysed sols ranged from 0.8 x I O - ~ to 1.8 x 10-5 mhos. All the solu- tions used were made up with conductivity water twice distilled from a quartz flask and collected in a hard glass vessel. All vessels used for the experi- ments were cleaned with hot chromic acid, washed out with distilled water and finally steamed out with steam from twice distilled water. This was done for each fresh sol. Furthermore the mixing vessels were steamed out after each experiment in each series.The sucrose used was a specially pure form supplied by Mews. Tate Sr Lyle, Liverpool, since this brand had been found experimentally to be most satisfactory. In fact, it was the only brand of sucrose used with which duplicate results could be obtained. A 10 per cent. solution of this brand of sucrose has a specific conductivity of 2.9 x I O - ~ mhos. The most probable impurity is a trace of an organic salt of calcium.' I t was found experimentally that the amount of calcium ion present did not measurably affect the speed of coagulation. Mctbd of Proccdure.-The investigation, as already stated, was carried out in two sections, the one at constant concentration of electrolyte, the other at constant activity of electrolyte cation whilst the sucrose content was varied from o per cent.to 50 per cent. The apparatus used was that of Hatschek.2 I t consists of two oblong cells about 75 millimetres long and about 30 millimetres broad and deep. The two sides are glass plates and the ends are ebonite. The two cells are placed one above the other and a screen of matt glass is placed behind them. Into the lower cell is poured the comparison sol stabilised by gelatin, the coagulating sol being placed in the upper. The field in the upper cell gradually changes in colour with time from port red through violet to blue. The time taken for the coagulating sol to reach the tint of the comparison sol, was taken to be a measure of the rate of coagulation. The varying amounts of sucrose were dissolved in 60 cc.s of gold sol and made up to 100 c.c.~ at 25' C.Into a quartz flask were placed 25 c.c.~ of this solution and the flask suspended in the thermostat at zs0 C. Into a beaker were placed x c.c.~ of electrolyte and ( 5 - 2) c.c.~ of twice distilled water and the beaker also suspended in the thermostat at 25' C. At constant concentra- tion of electrolyte the volume x is the same, in any one series, for all concentrations of sucrose. At constant activity of electrolyte cation the volume x was calculated, for any given concentration of sucrose, from the tables of relative activities of hydrogen ion (in the case of hydrochloric as electrolyte) as given by Moran and Lewis.3 For the case of barium chloride as electrolyte the volume x was calculated from the tables of Corran and Lewis,4 recent work by Corran5 having shown that the relative activity of the barium ion in presence of different amounts of sucrose is the same as that of the potassium ion.The sol and electrolyte were rapidly mixed (the time being noted) and allowed to remain in the thermostat until the tint was nearly equal to that of the comparison sol. The mixture was then poured into the upper cell, the colours matched, and the time of coagulation thus determined. This was done for varying amounts of sucrose and the time of coagulation was I Kieran, Trans. Faruduy SOC., 18, Part I., 1x9, 1922. a Hatschek, Trans. Faraday SOC., 17, 499, 1921. Moran and Lewis, Trarrs. Chctrr. SOL., 121, 1613, 1922. Corran and Lewis, Jown. Anam.Clrem. SOL, 44, 1673, 1922. SCorran, ~ o i ~ r n . Amer. Chem. SOC., 45, 1627, 1923.638 THE EFFECT OF SUCROSE ON THE RATE OF - divided by the relative viscosity of the medium. The viscosities employed were those of sucrose in water and the following values taken from measurements by Powell were used. Relative Viscosity. __--- 1'000 1.138 1.313 1'794 2.154 2'616 4'073 6'525 I Temperature 2 5 O C. Per Cent. Sucrose. 0 5 I0 20 25 30 40 50 In the following tables the results at constant concentration of electrolyte are given first. A fresh comparison sol was made for each gold 1, sol. I t is therefore to be noted that the values of comparable for one and the same sol, e.g. Sol 39. are only strictly rl EXPERIMENTAL RESULTS. SECTION I . CONSTANT CONCENTRATION OF ELECTROLYTE IN EACH SERIES.A. E r'ectroly fe- Nyu'roc~~oyic Acid. In the following tables, Column I gives the concentration of electrolyte in each series. Column 2 J , ,, percentage of sucrose in the coagulating mixture (volume 30 c.c.~). Column 3 ,, ), time required to reach the tint of the comparison sol. Column 4 ,, ,, ratio of this time to the relative viscosity of the medium. All the experiments recorded were duplicated and concordant results ob- tained to within 3 per cent. The times recorded are the average of the duplicate. Sol 35. Conc. HCI. 0-005 N Per Cent. Sucrose. 0 2 5 7'5 I0 I2 20 I5 25 30 Time (T). Secs. 10.5 14.0 16.8 43 53'5 44'2 33'0 24.0 23'4 I 7-6 T Relative Viscosity (7). 10.5 13.4 14.8 35.8 40'7 32-1 13'5 10.9 22'0 6.7 Powell, J O W I I .Chcm. Sot., 105, I, 1914.COAGULATION OF A COLLOID BY AN ELECTROLYTE 6-39 Cone. HC1. Per Cent. Sucrose. 0 0.0083' N 5 I5 30 I0 20 t Time (T). Secs. 28 25 I45 - 22 34'5 28.0 30-8 110*4 12.3 13.2 - 0 10 20 5 I5 30 40 Sol 39. Cooc. HCI. 0-0066 N I5 25 40 35 38 25 36 Per Cent. Sucrose. 0 5 15 30 I0 20 Time (T). Secs. Sol 41. In this case, two comparison sols were used, the second being more coagulated (i c, more violet) than the first No. I COMPARISON SOL. Conc. HCI. 0.0083 N ~ _ _ - No. 2 COMPARISON SOL. I5 21.8 30.0 22.8 21'2 9.6 8.9 Conc. HCl. 0.0083 N Per Cent. Sucrose. 0 I0 20 5 15 30 40 Time (T). Secs. 103.5 91.7 92.0 40' I 35'0640 THE EFFECT OF SUCROSE ON THE RATE OF so1 44, Conc. HCI. 0.0066 N 0'0033 N Per Cent. Sucrose. 0 I0 20 30 40 50 0 1'0 30 40 20 Time (7').Secs. 68 I I0 3 82 60 T . 68.0 37'3 26-0 9'0 '83.7 20'0 900 420 234 86 From the above data it will be observed that at first rises and then falls rapidly as the concentration of sucrose is further increased. I t would therefore appear that the sucrose has a two-fold effect. Firstly there is a peptising effect resulting in a slowing down of the rate of coagulation and then, secondly, the sucrose causes an increase in the rate of coagulation as T evinced by the rapid fall in the value of -. This second efect sugpsts tht rl the iweased acfivi@ of the hydrogen ion, due to tk presence of sucrose, i s coming infopday, which, in itseg is evidence that tk activio and not mere4 tk concentration of the coagulating ion is a factor in the coagulation. rl E Zectrodyte- Barium Ch doride.Sol 43. Conc. BaC12. 0.00166 N 0'00233 N 000266 N Per Cent. Sucrose. 0 5 I0 20 30 40 0 I0 20 30 40 0 5 I0 20 30 40 Time (T). T Sees. 390 1 390 900 786 Longer than 30 minutes 55 I35 43 63 73 31 55 90 35 45 57 20 I 90.6 60 '0 55 102% 23'9 24.1 17'8 31 48 68 19'5 I7 =4COAGULATION OF A COLLOID BY AN ELECTROLYTE 641 T 77 The values of - are similar to those obtained using hydrochloric acid, T t ks., a rise in - occurs followed by a rapid fall in the value of the ratio. EZectroZyte-Potassium Chloride. In the case of potassium chloride as coagulant, it was found that the time of coagulation (corrected for viscosity) increased continuously, as the concentration of the sucrose was increased. This is in contradistinction ta the results obtained for hydrogen and barium ions.In these two cases as we have seen the value of - passes through a maximum at about 10 per cent. sucrose. The values for the coagulation times using potassium chloride are given below. T t Sol 62. I--- 0.0666 N 0'0833 N 0 I0 20 5 I5 30 40 50 0 5 123 25 40 50 801 63. Conc. KCI. 0.0666 N Per Cent. Sucrose. 0 I0 20 15 30 40 Time (T). Secs. Less than 5 I2 22 38'5 '50 450 1320 60.0 Instantly 7 23 60 I95 I1 T - 8' - 1o.5 16.7 25.2 33'4 573 I 10.4 202.3 6.2 7'6 10.6 1.5 30 I Time (T). T Instantly 60 113 465 210 1200 - 45'6 74'1 117.0 1826 294'6 T t f t is obvious that the values of - rise continuously with increasing con- centration of sucrose. Since this is SO, when the potassium chloride con- centration is maintained constant, it is certain that - would behave similarly if the potassium chloride activity were maintained constar; t, because a T642 THE EFFECT OF SUCROSE ON THE RATE OF constant activity of electrolyte in presence of sucrose means a decreasing concentration of electrolyte.Experiments maintaining the activity constant in the case of potassium chloride were therefore not carried out in Section 2. We can conclude from the above results that sucrose possesses a definite peptising action, which increases with its concentration and, in the case of KCl, more than compensates for the increase in activity of the cation brought about by the increase in sucrose content of the system. The behaviour of the three electrolytes at constant concentration is shown in Fig. I. 1 0 2 0 so 40 Percentage Sucrose. FIG.r.-Showing the variation of - with percentage sucrose at constant concentra- No. I, Sol 41, Electrolyte HCI Conc. = 0*0083 N. No. 2, Sol 43, ,, BaCl, Conc. = 0-00266 N. No. 3, Sol 62, ,, KCI Conc. = 0.0666 N. T rl tion of electrolyte. SECTION 2. CONSTANT ACTIVITY OF ELECTROLYTE IN EACH SERIES. EZectrolyte- Hydroch Zoric Acid. The required volume of hydrochloric acid necessary to maintain the hydrogen ion activity constant, in any one series was calculated from the following table of relative activities, given by Moran and Lewis.' Per cent. sucrose (grams per IOO c . c . ~ ) o I0 20 30 Per cent. sucrose . . . . . 40 50 Relative activity Qf H-t ion . . . 1.0 1'2 1.4375 1.7375 Relative activity . . . . . 2'0875 2'5000 Moran and Lewis, Truns.Chenz. SOC., 121, 1613, 1922.COAGULATION OF A COLLOID BY AN ELECTROLYTE 643 0 I0 20 30 40 If the hydrogen ion activity is a factor governing the rate of coagulation, then the times of coagulation after being corrected for viscosity should be constant in any one series. In the following tables, Column I, gives the percentage sucrose in the coagulating mixture (whose total volume was always made up to 30 c.c.s with twice distilled water). Column 2, ,, ,, volume of decinormal hydrochloric acid added to make the activity of the H+ ion the same in all cases for any one series. The activity of H+ ion is that of a o.013gN solution (in water). Column 3, ,, ,, coagulation time observed and is the average of dupli- cate experiments. Column 4, ,, ,, coagulation corrected for viscosity.A fresh comparison sol was made for each sol prepared. 5 '0 4-17 3'48 2-88 2-38 Sol 45. Per Cent. C.C.S Sucrose. 0'1 N HCI. 0 4-80 I0 4'- 20 3'34 30 2'80 40 2.3 I 50 1-92 Per Cent. I c.cg;;; N Sucrose. T 1) - Time (T). Secs Lessthan5 - 20 15.2 25 13'9 36 13'6 60 14'7 80 I2'2 I- Time ("). Secs. 23 68 74 90 106 T - 1)' 23 57 42.0 34'4 25'7 Sol 46. Per Cent. Sucrose. 0 10 20 30 40 50 C.C.S 0'1 N Time (T). HCI. 1 Sets. 4-17 3-48 2'93 2-40 1.66 2'00 Less than 5 15 29 41 1 74 20 I T 1)' - - 11.4 11.4 11.3 11'1 10'0644 THE EFFECT OF SUCROSE ON THE RATE OF Per Cent. Sucrose. Sol 47. C.C.8 0'1 N HCl. wIg&l. I Time (Secs.). 1 - T I)' C.C.S 0.1 N HCI. Per Cent. Sucrose . Time (T). Secs. I 1 0 I0 20 30 40 3-60 3'00 2'50 2'08 I *So Less than 5 30 34 60 90 - 22'8 1g.g 22.9 22'0 Sol 52.T I)' Time (T). Secs. 0 10 20 30 40 50 4-00 3'33 2'77 2.30 I *g8 1-60 15.0 27'4 24'5 24.1 22.6 24.2 Sol 53. Per Cent. Snaose. T - I)' 1 I-- 0 I0 20 5 30 40 50 4'0 3'63 3 '33 2'77 230 1'98 1'60 I0 23 33 36 46 67 204 10'0 20'2 20'0 25'1 17.6 16.7 (3 1 '4) Sol 54. Per Cent. suaose. 0.1 N HCl. I C*c-s T 1)' Time (T). Sea. 1 -I- I- Less than 5 16 32 80 I0 20 58 5'0 ;:I; 3 '50 2-90 2-50 2'00 0 10 20 5 30 40 50 - 8.8 12'2 11'1 12'2 13.7 12.3COAGULATION OF A COLLOID BY AN ELECTROLYTE 645 Per Cent c.c.s Sucrose. 0'1 NHCl. 0 4.0 5 3'63 1 0 3'33 20 2'80 30 2-30 40 2'00 50 1-60 Time (T). T I)' - S e a . Less than 5 - 22.5 20'2 39'5 30.0 46.0 25'7 60.0 22-9 90.0 22'0 150.0 23 *o T In the above tables the values of - from 10 per cent.sucrose to 50 per rl cent. sucrose are reasonably constant in each series, in agreement with the view that coagulating power depends upon activity. In the light of results obtained with other electrolytes, however, the constancy of - in the case of HCl cannot be regarded as an unequivocal proof of the simple considera- tions outlined in the introduction. The value of - in presence of sucrose is, however, always greater than that in absence of sucrose (at the same hydrogen ion activity) indicating a definite peptising effect of the sucrose. T rl T 'I c.c.s 0'1 N BaClp Electrolyte-Barium Chloride. The required volume of barium chloride necessary to maintain the barium ion activity constant in any one series was calculated from the tables of relative activities of the potassium or barium ion given by Corran and Lewis1 and by Corran.2 Time (T).Secs. Per cent. sucrose (grams per 100 c.cs) . . o 10 20 Per cent. sucrose . . . . . 30 40 50 Relative activity of K+ or Ba++ ion . . I*OOO 1'0644 1.13S2 Relative activity . . . . . . 1'2118 1.3066 I * . + ~ Q SoE 57. Per Cent. Sucrose. I I i 0 I0 20 5 30 40 50 1'00 1'00 0'94 0-88 0'82 0.76 0.71 Less than 4 29 25 14.5 35 20 21 - 25'5 26.6 5'5 5'1 5'3 11'1 1 Corran and Lewis, Jour. Amer. Chern. SOC., 44,1673, 1922. a Corran, yourn. Amer. Chem. SOC., 45, 1627, 1923.646 THE EFFECT OF SUCROSE ON THE RATE OF I I i I I ! 158 81 '5 76 130 100 0 5 I0 20 30 40 50 9 25 20 20 25 40 I 0.70 0.70 0.65 0.61 0'57 0'53 0.50 9'0 19.0 7'6 6.1 6-1 11'1 123 45 I 80 61 34 35 69 12.5 39'9 144'7 34'0 13'3 10.5 8 '3 Sol 58.Per Cent. Sucrose. C.C.S 0.1 N BaCI2, 0 I0 20 30 40 50 0.5 0'47 0.44 0.4 I 0.38 0.36 T 7' 83 I 20.3 55 '7 3 1.1 18'5 19'9 - Per Cent. I c.c.s I Sucrose. 0.1 N BaClo. Time (Sets-). T - r)' 0 10 20 30 40 50 0 '8 0'75 0'70 0'66 0% 0'57 Sol 49. Per Cent. Sucrose. 0.1 N BaClP. Secs. I c . c . ~ 1 Time (T). I I 0 I0 20 30 40 50 1.1 1'0 0.90 0.85 0.76 0.70 ,Less than 5 60 30 29 32 20 - 45 '7 16.8 11.4 7'2 4-8 T rl I n the above tables it will be observed that the value of - rises up to I n the case of constant hydrogen EO per cent. sucrose and then falls rapidly. ion activity the ratio of concen&a&kn throughout any one series. was constant above 10 per cent. sucrose. rl The above results are similar to those obtained using constant barium The rate of coagulation is fasterCOAGULATION OF A COLLOID BY AN ELECTROLYTE 647 than was anticipated on the basis of the activity of the barium ion being alone a significant coagulating factor.The behaviour of the electrolyte at constant activity in presence of sucrose is shown in the figure 2. 1 0 a- SO *o SO u 70 se 1 0 0 Percentage Sucrose. m FIG. 2.-Showing the variation of with percentage sucrose at constant activity 11 of electrolyte. solution (in water). solution (in water). Curve No I : Sol 46. Curve No 2 : Sol 49. Electrolyte HCI, activity equivalent to that of a O'or3g N Electrolyte BaCI,, activity equivalent to that of a 0.00366 N DISCUSSION OF RESULTS. The experiments indicate that the sucrose has a two-fold action. The first is a definite peptising effect, and the second is an accelerating effect, upon the coagulation of colloidal gold by certain electrolytes.In the case of potassium chloride as electrolyte the peptising action is most marked and rapidly increases as the sucrose is increased. In the case of barium chloride, in presence of sucrose, the coagulation proceeds more rapidly (above 10 per cent. sucrose) than it should do on the basis that activity is the sole factor determining the rate of coagulation. From this it would appear that the sucrose in addition to its peptising effect also introduces a specific augmentation of coagulation in excess of what would be anticipated. The latter effect is apparently least in the case of potassium chloride and greatest in the case of barium chloride. In the case of hydrogen ion the value of - is constant above 10 per cent. sucrose which may indicate that the peptising effect of the sucrose (so marked in the case of potassium chloride) is just counterbalanced by the specific augmenting effect and con- sequently in the case of HC1 the true activity apparently manifests itself.T 17648 THE EFFECT OF SUCROSE BY AN ELECTROLYTE I t is important to observe that sucrose of itself has no apparent coagulating power on gold sol. As regards the augmentation of the coagulating efficiency of ions produced by the sucrose it might be suggested that possibly we are dealing with an effect which is due to the alteration in the dielectric capacity of the medium as a result of the increase in the sugar content. Measurements of the dielectric capacity of sugar solutions indicate a marked fall as compared with that of water.The electrical adsorption of the ions would therefore be intensified, involving an increase in neutralising efficiency on the electrical charge of the colloid particles, with a consequent increase in the rate of effective collisions, i.e., effective in respect of coagulation, on the part of the colloid particles. If this is. the case the horizontal line obtained in the case of hydrogen ion (fig. 2) is largely accidental and the results can only be regarded as qualitative evidence in favour of the activity of ions as a significant factor for coagulation. SUMMARY. I . The coagulation of gold sols by hydrochloric acid, barium chloride and potassium chloride in presence of varying amounts of sucrose has been investigated, at z 5' C. 2. In the case of hydrochloric acid two sets of conditions were used, (a) The concentration of the hydrogen ion was maintained constant. (b) The activity of the hydrogen ion was maintained constant. In the first case the time of coagulation, corrected for viscosity, was found to pass through a maximum at about 10 per cent. sucrose. In the second case the time of coagulation corrected for viscosity rose until 10 per cent. sucrose was reached and then became constant. 3. In the case of barium chloride the above two sets of conditions T were observed and in both cases the value of - passed through a maximum rl at about 10 per cent. sucrose. 4. In the case of potassium chloride the concentration of the potassium ion was maintained constant. It was found, contrary to anticipation that - increased continuously as the sucrose content increased. 5 . I t is concluded that sucrose exerts a definite peptising effect upon colloidal gold. 6. I t is also concluded that sucrose exerts a specific augmentation of coagulation in the case of hydrogen and barium ions over and above that of increasing the activity of these two ions. From ( 5 ) and (6) it is evident that sucrose is by no means inert towards ions and gold sols. I t exhibits. apparent antagonistic action. 7. The experiments indicate in general, however, that the coagulating power of an ion is dependent upon its activity rather than upon its con- centration, a conclusion which brings the typical colloid phenomenon of coagulation into line with the kinetics of chemical change in homogenous (molecular) systems. T rl The author wishes to express his thanks to Messrs. Tate and Lyle, Liverpool, for a generous supply of pure sucrose. Musprutt Laboratory of Physical und Electro- Chetnistry, Universify of Liverpool.

ISSN:0014-7672    

DOI:10.1039/TF9241900635

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. A METHOD OF MEASURING THE RATE OF COAGULATION OF COLLOIDAL SOLUTIONS OVER WIDE RANGES. BY H. H. PAINE, MA., B.Sc., AND G. T. R EVANS, B.k., UNIVERSITY OF THE WITWATERSRAND, JOHANNESBURG. ( A Paper readbefore THE FARADAY SOCIETY, Monday, November I 2th, 1923~ SIR ROBERT ROBERTSON, K.B.E., F.R.S., PRESIDENT, in the Chair.) XeceivedJuZy I 6 t 4 I g 2 3. $ I. InfroductoYy. Considerable light was thrown on the kinetics of coagulation some few years ago in two important papers, one by Smoluchowski,’ and the other by Freundlich.2 These writers considered a colloidal solution from the point of view of the kinetic theory. Coagulation is regarded as the “ coalescence ” of L‘ molecular ” particles resulting from their mutual collision.Though we know but little- of the nature of the forces which cause the coalescence, and though the behaviour of any single particle may be quite indeterminate, yet the law of probability enables us to draw certain conclusions regarding the collection of particles as a whole, just as it does for the molecules of a gas. Smoluchowski was concerned primarily with coagulation when it occurs at its maximum rate-that is, when all collisions between colloidal particles result in coalescence ( L 4 rasche koagulation ”). The existence of this maxi- mum is demonstrated by the experiments of Zsigmondy with gold and those of Kruyt and van Arkel with selenium When successive samples of the colloidal solution were treated with increasing amounts of an electrolyte, the rates of coagulation increased to a mcrximum or ZimitiRg, value.Freundlich has extended the theory to deal with slow coagulations, and has made a further inquiry into the forces between the particles. Briefly, his theory is as follows. In the case of a stable colloidal solution, the collisions between particles are (‘ elastic,” that is, they do not result in coalescence. A repulsion (ie. something which prevents coalescence) exists between two particles arising in some way from the existence of the electrical double layer. As soon as the potential difference across this double layer is reduced beyond a certain critical value, the repelling force vanishes, and a collision results in coale~cence.~ This critical point has, then, many of the properties originally assigned to the isoelectric point.Koll. Zcif., mi., 98 (1917). 3 Gotf. Ncrchr., 1917, discussed by Smoluchowski, loc. cit. 4 Koll. Zcit., xxxii., 29 (1923). 5 See Powig Zeit.f#rPhys. Chem., Ixxxix., 186 (19x5) ; Y. Chem. SOL, cix., 734 (1916). 2 Ibid., xxiii., 163 (1g18). Kruyt, Koll. Zeit, xxii., 81 (ISIS), and xxv., I (1919). 649650 METHOD OF MEASURING THE RATE OF COAGULATION If two particles have charges which are above the critical value, and if the particles are approaching one another with sufficient velocity, the repulsion between them may not be great enough to prevent an intimate collision which would result in coalescence.Consequently, even above the critical point some coalescence will take place. Freundlich called the relative velocity which two particles must have if they are to coalesce, the '' critical velocity " for the particular condition of the double layer. If the repelling forces are still high, the critical velocity will be high ; if the potential difference across the double layer is reduced, the critical velocity will be reduced correspondingly. If e be the charge on the colloid particle, (or, rather, the charge on the particle over and above that carried when at the critical point), then the critical velocity of a particle of mass m,, and radius r, would be given by an equation of the form (mv2)/2 = e2/r, or v = xl.e, where x1 is a constant. Now the particles of the solution may be regarded as a swarm of mole- cules having velocities distributed according to the probability law. Hence we can deduce the number of these particles which, at any instant, have a velocity greater than some particular velocity. According to Freundlich, the chance of any given particle coalescing with another is proportional to the number of particles which have velocities greater than the critical velocity. Obviously, the smaller the critical velocity (that is, the nearer the solution is to the critical point), the larger will be the number of particles for which the velocity exceeds the critical velocity, and the more rapid will be the coagulation. In order to deduce an expression showing how the rate of coagulation varies with the concentration of the electrolyte, it is necessary to assume a relation connecting the charge e on the particle with this concentration.From the curves given by Kruyt, Freundlich concluded that we could write e = x2 log y, where x2 and y are constants, and t is the concentration of the electrolyte. y is then the concentration of the electrolyte at the critical point. The final formula given by Freundlich is Here X is a constant, and is equal to the product of the two constants x1 and x2 already mentioned. R is the rate of coagulation measured in such a way that its maximum value is equal to +JG. Freundlich showed that this formula fitted in well with the results of experiments on the rate of coagulation as far as such experiments had been carried out.2 However, these experiments were of rather a limited range, and therefore could not serve as a good test of the formula.In particular, the formula requires that as the concentration of the electrolyte increases, the rate of coagulation should approach a certain limiting value, equal to 3 JG. In the above experiments there was no certain indication of the approach of such a limit. I n fact, the rate of coagulation was represented with fair accuracy by the expression k . cp, where? and k are constants. We should now conclude, however, that the maximum rate was too great tu come within the range of these experiments. 1 L O C . cit. 2 Gann, Koll. Beihefte, viii., 63 (1916). Freundlich and Ishizaka, Zeit. fur Phys. Chem., lxxxv, 398 (19x3); Paine, Proc.Camb. Phil. SOC., xvi., 430 (rgrz), and KolZ. Beihefe, iv., 24 (1912).OF COLLOIDAL SOLUTIONS OVER WIDE RANGES 651 I t occurred to us that these experiments could be extended by the use of a “protective colloid.” The addition of gelatin or starch results in making the colloid less sensitive to electrolytes; for example, the experi- ments of Luersl show that the time taken for some definite change to be observed in a Congo ruby sol on the addition of an electrolyte is increased if gelatine is present. Further, the retardation produced is greater, the greater the amount of gelatin present. Hence, coagulations which normally proceed very rapidly, can be retarded until the methods for studying relatively slow coagulations can be applied to them.We have thus been able to observe coagulation proceeding at its limiting rate ” (rasche koagulation) for a colloidal copper solution to which a suitable quantity of starch had been added. $ 2. Experimental. The method of measuring the relative rates of coagulation was the sa,me as that described by one of us in the paper already referred to.2 The colloidal solutions of copper (or, more correctly, copper hydroxide) were prepared by Bredig’s method in conductivity water. To about 150 C.C. of this solution a certain amount of electrolyte was added. After varying intervals of time, between 25 and 30 C.C. of the coagulating solution was drawn off, heated quickly, and left to settle. The precipitate quickly separated, and 20 C.C. of the clear liquid was then drawn off and titrated with very dilute nitric acid, the acid dissolving the copper in equivalent proportions, leaving the solution colourless. From the results of these titrations a curve was drawn showing the quantity of colloid still remaining in solution after varying intervals of time.The “ relative rates of coagulation ” for various concentrations of the electrolyte were obtained from curves of this type by comparing the “times ’’ taken by the different solutions to reach some definite stage in the coagula- tion process, this ratio being the same whatever stage we take for the com- parison. In this way the variation of the rate of coagulation with the concentration of the electrolyte was measured. When the concentration of the electrolyte had been increased until the coagulation was so rapid that it could only just be measured satisfactorily, a certain quantity of starch solution was added to the copper before the addition of the electrolyte I t was then found that another series of experi- ments could be carried out with greater electrolyte concentrations. When again the rate of coagulation became rapid, a greater amount of starch solution was added initially, and so on, until an increasing concentration of electrolyte no longer produced any increase in the observed rate of coagulation. On each occasion on which the starch concentration was increased, two solutions having the two different amounts of starch were treated with equal quantities of electrolyte.The coagulation was slower where more starch was present. The “ relative rates of coagulation ’’ of these two solutions gave a factor for transforming the results obtained with the one starch concentration into what they would have been with the other starch con- centration and thus, ultimately, into what they would have been for the pure colloid.In some cases these transformed rates were many thousand times greater than any that could have been measured directly. When a comparison was made of the coagulation curves for solutions to which starch had been added, it was found that the relative rates of Koll. Zeit., xxvii., 123 (1920). 2 Paine, loc. c i t .6 5 2 METHOD OF MEASURING THE RATE OF COAGULATION coagulation, as deduced from the relative “ times ” that elapsed before some particular stage in the coagulation process was reached, were not as con- stant for different parts of the curves as in the experiments with pure colloid.The divergence increased as the quantity of starch used increased. A much greater consistency was obtained when the relative rates of coagulation were deduced not from the relative times, but from the relative sZojes ( = d . C/dt, where C is the concentration of the colloid at any instant) of the coagulation curves at equal concentrations of the colloid. This makes it seem probable that some, at any rate, of the above divergence is due to initial irregularities on the addition of the electrolyte. Where but small concentrations of the electrolyte were used, the effects were relatively small, but they became more prominent in those experiments in which larger amounts of the electrolyte were added.In the experiments with large quantities of electrolyte quite an appreciable amount of the copper was precipitated Log c + FIG. I. almost immediately, the rest of the coagulation proceeding, as far as one could see, quite normally. It is possible that the individual colloid particles are not “ protected ’’ equally by the starch, so that some of them are made to coalesce much more rapidly than the remainder. The starch solution was prepared from pure “soluble starch” (Kahlbaum’s) by adding about 5 grams of the starch powder to a litre of conductivity water, heating the mixture on a (boiling) water bath for 2 0 minutes, and then putting it aside to settle. After two or three days the almost clear upper portion of the liquid was drawn off.The addition of small quantities of this solution to the colloidal solution did not bring about any visible change, and did not affect the quantity of nitric acid required tc dissolve the copper when the colloidal solution was titrated. Potassium sulphate was used as the coagulating agent. A solution con- taining 3-5 grams K2S0, per litre was found to be suitable for these experiments.OF COLLOIDAL SOLUTIONS OVER WIDE RANGES 653 relative to Solution I. 1-00 The following tables contain the results of three series of experiments. All the solutions of any one series were made up from the same original stock of colloidal copper, but the stocks used for the different series were quite distinct and were prepared on different occasions.TABLE I. -SERIES I. I LITRE OF THE STOCK COLLOIDAL SOLUTION CONTAINED 0.176 GR. COPPER. Factor for the Series. 1'0 N a o f Solution kansformed Rate. (= R). 1'0 3'5 9'94 '9'4 17'4 Starch Solutior (C.C.) ;&SO4 Solutior (C.C.) (= c). I 2 3 4 - 0.30 0'35 0.40 0.42 0'43 0'43 0'43 0'51 0'0 1'0 17-41 17'8 17% 0.87 4'9 0.25 4.8 5 1-24 17.410 '87 9810'25 I 88010.39 63 IOO 0.65 IIIOOOO 0'55 5400000 I *6 17'4 98 2 '0 3'0 9s 1880 0.5 I 0.71 - I '85 so8 0'00 4'0 0.71 1-00 1'20 0'39 3'2 13.1 0'65 4'2 11.4 I0 I1 12 I3 I4 15 16 17 18 19 20 21 22 23 0'08 26 *38 1-20 1-83 2-40 5'0 6'0 0.3 8 '47 '5 4 2.40 3 '0 3'5 6.3 (6'73) 6.99 6-99 6'95 6'93 3'5 4'0 5'0 7'0 10'0 0'54 '60 '70 '85 1-00 In the case of Solution No. I of this Series, half the copper had separated out after Each solution (Column I ) contained 150 C.C. of the stock colloidal solution, and sufficient water was added so that, with the starch and salt solutions to be added, the total volume should be I 70 C.C.in every case, These results are plotted-log R against log c-in Fig. I. The indivi- dual points marked are the experimental results contained in the preceding tables, but the continuous lines are theoretical curves calculated from Freundlich's equation as described later. The three curves correspond to a period of 97 minutes.654 METHOD OF MEASURING THE RATE OF COAGULATION TABLE 11.-SERIES 11. I LITRE OF THB STOCK COLLOIDAL SOLUTION CONTAINED 0.179 GR. COPPER. I I ---I - No. of Solution I I I -_ Actual Rate relative to Solution I . I Transfor ma tion Factor for the Series.I I Starch Solution (C.C.) 0'0 0.158 1-09 <zS04 Solutior (C.C.) (= c). 0.38 1.66/0.158 Transformec Rate. (= R). 1'00 I '00 1'00 11'0 21.5 18.0 0-100 0.203 1-09 11~5/0~1oo Log10 R. 1-58 I 1'00 1'0 0'00 2 1'0 0'77 1*0/0*77 0.38 0.38 0'50 0.60 0.60 0.60 I '0 1'2 3 4 5 6 0.043 0'47 0'92 0'78 0.0123 0'34 0.65 0.0123 0.27 0.25 0'20 -- 2'0 3'0 4'0 1'0 0.043 18.0 494 950 18.0 0.0123 7 8 9 10 I1 12 I3 0.08 '40 '47 '70 (2'98) 4'19 4'32 4'54 950 I5 95- 20,800 35,000 950 0.0123 In the case of Solution No. I of this Series, half the copper had separated out afteH a period of 44 minutes. TABLE 111.-SERIES 111. I LITRE OF THE STOCK COLLOIDAL SOLUTION CONTAINED 0'160 GR. COPPER. Starch Solution (C.C.) No. of Solution. 1-00 I 1'0 (extrapolated) 0.40 0'43 1 '00 1-66 0'00 (0'22) 0'0 2'0 I - 2 3 - 1-63 -80 0.43 0.63 1-66 11.5 0 '22 1-06 -.1-80 '90 '99 -- - 1-99 0'10 '20 '30 '70 -48 0.63 0.80 0'97 (1'06) 1-37 2'10 (2'10) 2.36 2.60 2-87 2-89 3'17 4 5 6 3'0 4'0 0.9 7 1-26 I -6 2'0 3 '0 5'0 0'0845 0'160 0'267 0'500 0.522 I '00 12 5 10 '084 5 125 237 395 740 773 1480 7 8 9 I0 I1 I2 In the case of Solution No. I of this Series, half the copper had separated out after a period of 3 minutes.OF COLLOIDAL SOLUTIONS OVER WIDE RANGES 655 the three series of experiments. They are drawn on the same diagram so that they may be compared more easily. The electrolyte concentrations in the three series are directly comparable, being expressed in the same units. The rates of coagulation, however, are quite independent, and each series has its own ‘‘ unit ” of measurement, the rate of coagulation of Solution No.I in each series being taken as unit rate for that series. Several conclusions can be drawn from the experimental results. (I) In each series a maximum rate of coagulation is evident. (2) This maximum rate appears to have been reached with about the same electrolyte concentration in each series. (3) The variation in the relative rate of coagulation with the concen- tration of the electrolyte is different in the different series. (4) For small concentrations of the electrolyte, the slope of each curve varies but slowly with a change in that concentration, so that over small ranges the curve differs but little from a straight line. 3. Theoretical. A closer examination of these results was made in order to find’ what support they gave to Freundlich’s theory. I n order to plot the relation between R and c in his equation, it is necessary to determine the value of the constant A.Freundlich gave X = 1-735 for all the cases which he ex- amined, but the results of the experiments detailed above do not fit in with that value. Further, each series seems to require a separate value for X I f we calculate the rate of coagulation R in this equation, giving X any likely value (such as 1*735), it will be seen that when the concentration of the electrolyte c becomes small (below the value y / ~ o for example), the integral term in the equation becomes so nearly equal to JG/z, that R is practically determined by the magnitude of the first, or exponential, term. Hence, in calculating (d.R/dc) for small concentrations, we can take account ( f this first term only, and obtain a resu t quite accurate enough for our purposes. The result can be put in the form The quantity in the square brackets then represents the tangent to the curve log R : log c at any point.’ The tangents were determined from the experimental curves in the region immediately below the point corresponding to c = 7/10. The maxima of the cclrves (c = y) can be located approximately, and hence an approximate position for c = y/ro deduced. The approximate value of A can then be calculated from the above equation. To obtain a more accu- rate result we can proceed as follows. Using this approximate value of A, we can calculate the rate of coagulation for c = 7/10 from Freundlich’s equation.The difference between the logarithms of this quantity and of In this equation, as in the original one of Freundlich, the logarithms are natural logarithms, and A is calculated on that assumption. It is only in plotting one logarithm against another (log R : log c) that we have, for simplicity, used common logarithms (to base IO), in whlch case, of course, the result is the same whichever logarithms are employed.656 MIETHOD OF MEASURING THE RATE OF COAGULATION 111. 2 3 4 &/2, the maximum rate, will be the difference between the corresponding ordinates of the experimental curve. The maximum ordinate is observed directly. Hence the ordinate for the point c = Y/IO is deduced and the position of this point determined. For Series I., 11. and HI., the tangents were found to be 8.3, 6.4, and 5 -2, respectively, whence X equals I '3 2 , I S T 7, and I -06, in the three cases.With these values for A, three theoretical curves were drawn ; they are the smooth curves shown in Fig. I. They were "placed" in reference to the experimental points so as to pass as nearly as possible through these points. The positions of the " maxima " of these theoretical curves (c = 7) then came out as shown in Fig. I . The abscissz of these points (= log c) are 0.89, 0.86, and 0.78 in the three cases, corresponding to c = 7-8, 7.2 and 6.0 c.c.~ of K2S04 solution, respectively. The mean value is 7.0 C.C. This reduces to a concentration in the experimental solution of 0.83 millimols per litre. I t must be remarked that the theoretical equation of Freundlich de- pends to some extent on the accuracy of his assumption taken from Kruyt's experiments, that e = x2 Iog 2.A discrepancy here would have its effect on the form of the curve. C 3 4. Retarding Efect of the Starch. ,4 simple relation was found to hold for the retarding effect on coagula- tion produced by the addition of starch to the colloid. This retarding effect is expressed by the " transformation factors " of Tables I., II., and 111. It will be seen that the first additions of starch produce no appreciable effect on the rate of coagulation,' it is the later additions which are effective. Table IV. contains the transformation factors of Tables I., II., and 111, with the respective starch concentrations. TABLE IV. 10'5 1'02 11.5 2 *06 I430 3-17 No.of c . c . ~ of Starch Added (8). I I- 2 3 4 5 6 6.3 I 2 3 4 Log10F. Transformation Factor (F). 0'98 20'0 392 4820 97,000 2,000,000 3,400,000 - 1-99 1-30 2'59 3-68 4'99 6-30 6.5 3 I------ 1-30 23.2 1460 77,000 0'1 I "37 3-16 4'89 These data are plotted in Figure 2, which shows the relation between the logarithm of the transformation factor and the concentration of the starch. The curves that pass through these points are straight lines. 1 See Luers, loc. cit.OF COLLOIDAL SOLUTlONS OVER WIDE RANGES 657 Now it seems certain that the starch is in some way absorbed by the colloid. An experiment was carried out with two colloidal solutions of copper, one about twice as concentrated as the other. In order that the same concentration of electrolyte (sufficient to cause thepure colloid to be precipitated almost instantaneously) should bring about coagulation at about the same rate in the two solutions, something like twice as much starch had to be added to the stronger copper as was necessary in the case of the weaker.The curves of Figure 2 make it evident that the first additions of starch do not affect the rate of coagulation. Hence it appears as though the retarding effect of the starch did not commence until the copper particles had absorbed a definite amount of starch-perhaps until the surfaces of the particles are completely covered or coated with starch molecules. Concentration of Starch 3 FIG. 2. I t also follows from Figure 2 that the effect of the starch can be ex- pressed with close approximation by the equation Log F = K(s - SO), or where (s - so) is the concentration of the starch over and above that necessary to start influencing the rate of coagulation. F = &s - sn) $ 5 . Summary. ( I ) The rate of coagulation of colloidal copper solutions has been studied for a wide range of electrolyte concentrations by making use of the retarding effect of starch. Very rapid coagulations can thus be brought into the region of observation by ordinary methods. A ‘‘ transformation factor” can be obtained which enables us to calculate what the rate of coagulation would have been for the pure colloid. VOL. XIX-T25658 MEASURING RATE OF COLLOIDAL SOLUTIONS (2) The results of these experiments agree closely with the equation deduced by Freundlich for the variation of the rate of coagulation with the concentration of the electrolyte. In particular, they confirm the ex- istence of a maximum rate of coagulation. (3) Concentrations of starch below a certain minimum do not influence the rate of coagulation. I f we express this influence by the “transformation factors,” a linear relation holds between the logarithm of the transformation factor and the concentration of the starch over and above some minimum value. The water still and other apparatus used in this investigation were purchased by means of a South African Government Research Grant.

ISSN:0014-7672    

DOI:10.1039/TF9241900649

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. STUDIES IN HETEROGENEOUS EQUILIBRIA. PART 1. CONDITIONS AT THE BOUNDARY SURFACE OF CRYSTALLINE SOLIDS AND LIQUIDS AND THE APPLI- CATION OF STATISTlCAL MECHANICS. BY J. A. V. BUTLER, MSc. ( A Paper read before THE FARADAY SOCIETY, Monday, Nmcmbcr 12th~ 1923, SIR ROBERT ROBERTSON, K.B.E., F.R.S., PRESIDENT, Sit the Chair.) Received SGptember I oth, I 9 2 3.Introduction. Equilibrium between two phases occurs kinetically when equal numbers of molecules of every species concerned pass the boundary surface in both directions in the same time. The methods of statistical mechanics hare been applied to the development of the kinetics of such processes only in the simplest cases. Langmuir, whose pioneer work has inspired most of the recent progress in this direction, was chiefly concerned with vaporisation and the kinetics of gas reactions at solid surfaces. Rideal and Rodebush have employed special forms of the statistical equations with some success in the calcula- tion of the constants of vaporisation. Langmuir’s treatment has been extended to give a kinetic theory of the adsorption of gases by Henry.* A tentative attempt to apply statistical equations to the calculation of solubility is mentioned by Dushman.s The subject is here approached from a somewhat different point of view, which may perhaps be made clearer by means of an analogy.In the study of homogeneous equilibrium the law of mass action proved a reliable guide and inspiration for many years, although it gave merely the form of the equilibrium equation and did not lead to the calculation of the equilibrium constant in any case. The earlier workers on heterogene- ous equilibrium approached the subject from the point of view of the law of mass action, but the results were comparatively meagre. Certain analogies with mass action obviously exist, but the backward state of our knowledge of the kinetics of phase equilibria may be traced to the fact that no general kinetic law of surface action has been available to replace the law of mass action.The author hopes to show that although it is not yet possible by the ’3’. Amm. Chem. SOC., 1916,38,2231; 1917, 3, 1848; Trans. Faraduy SOC., 1922, 2 Proc. Camb. Phil. SOC., 1921,#), 291. 3y. Amer. Chem. SOC., 1g23,45,606. 4PhiZ. Mag. [VI.], 11922, 44, 68g. 17, 607, 621. ‘‘ A Theory of Chemical Reactivity,” y. Amcr. Chcm. SOC., 1921, 43, 397. 659660 CONDITIONS AT THE BOUNDARY SURFACE OF application of statistical methods to calculate the values of the equilibrium constants, nevertheless these methods may be used to co-ordinate and throw much light on a number of diverse cases of heterogeneous equi- librium, in particular solubility, the solubility product, the electromotive equilibria of metals and oxidation potentials.Although this treatment does not aim primarily at the calculation of equilibrium constants and may have a very distinct usefulness if it is at present unable to achieve that objective, it is evident that every oppor- tunity should be taken of making a comparison with numerical data. In these complex cases the exact calculation of equilibrium constants is only likely to be achieved when the special conditions of each case have been qualitatively explored. The present paper is devoted to a preliminary discussion of the con- ditions at the boundary surfaces between crystalline solids and liquids from this standpoint and the deduction of appropriate statistical equations.TJIG Surface Conditions. The attractive forces that bind the molecules of a crystal together must extend somewhat beyond the surface, constituting a field of force which surface molecules must overcome before they can escape, and which attracts molecules of the same kind, and so causes the growth of the The total effective range of surface forces is a disputed question at the present time.’ But the fact that molecules reaching the surface are fixed in definite positions so as to continue the crystal lattice, indicates the existence near the surface of specific and highly localised forces. There may exist, of course, general attractive forces, having a considerable range, responsible for capillarity and adsorption.But the forces holding mole- cules of the crystal at the surface are essentially localised, and consequently fall off rapidly with the distance. The liquid may also exert an attractive force on the molecules at the surface of the crystal. Consider an ordinary non-volatile solid ; the surface molecules show little tendency to escape into space-that is, a surface molecule acquires sufficient kinetic energy to carry it out of the range of the attractive forces of the surface only at very rare intervals. The greater tendency of the surface molecules of a solid to pass into solution indicates that the work which must be done in getting clear of the surface is less than in the former case. This can only be the result of an attraction exerted by the solvent on molecules at the surface. Again, the minuteness of the vapour pressures of the metals indicates that the atoms of the surface only rarely acquire sufficient kinetic energy to get free, yet the electromotive behaviour of the less noble metals at any rate indicates a considerable tendency to pass into solution.According to the view advanced here the solubility of a solid is de- termined by these two factors, the surface attractive forces of the solid and the attractive forces exerted by the liquid on the molecules of the solid. Now the attraction of a crystal surface on a molecule is at a maximum when the molecule is only very slightly displaced from its equilibrium position at the surface, and, as we have seen, there is good reason to believe that it falls off rapidly with increase of distance from the surface.The attraction exerted by a liquid is also greatest at or near the surface, crystal. Evans and George, Proc. Roy. Soc. [A], 1923, 103, 190.CRYS’I’ALLINE SOLIDS AND LIQUIDS 66r since the attractive forces exerted by the liquid are here all in one direction, that is, outwards from the surface. I t falls off on entering the liquid, and ultimately becomes zero when the forces are exerted equally in all directions. In saturated solution at any rate, the attractive force holding a surface molecule in its place in the crystal lattice is greater than the attraction of the liquid at this point. Otherwise the existence of the solid in contact with the liquid would be impossible. We picture, then, the general state of affairs at the surface owing to the interaction of these two opposing attractive forces as in Fig.I . “ A ” represents the attractive force exerted by the surface; “B” that of the liquid, and ‘‘ C ” the resultant of the two opposing attractions. In general a molecule passing outwards from the surface experiences first an attraction towards the surface; after a certain point an attraction into the liquid. There exists a balance point at which the two opposing attractions are equal. I Distance from surface. FIG. I. 0 We shall suppose with Langmuirl that collisions at the surface are practically inelastic. A moiecule from the surface will escape if it has acquired by thermal agitation sufficient kinetic energy to carry it past the balance point. Molecules from the interior of the liquid which reach the balance point come within the field of attraction towards the surface and pass on. Equilibrium is attained when equal numbers of molecules pass the balance point from both sides in any interval of time.I t may happen, of course, when the attraction of the liquid is so small (i.e. small solubility) that it is at all points less than the attraction of the su face. The net attractive force towards the surface will extend in- definitely into the liquid, a state of affairs analogous to that existing at the surface of a solid in contact with its own vapour. This may be regarded as a special case of that illustrated in Fig. I. The latter is regarded as the general case, but the expressions obtained can be applied with slight modification to the former.1 J. Ameu. Chem. SOC., 1916,*, 2221 ; 1 9 1 7 , ~ ~ 1848.662 CONDITIONS AT THE BOUNDARY SURFACE OF Th AppZication of Statistical Mechmcs. Consider a crystalline surface of a substance of molecular weight M, containing N molecules in the surface layer per square centimetre, in contact with a solution containing N, molecules of the same substance per cubic centi- metre, or a concentration of C gram molecules per litre. For the purpose of this discussion the " molecule )) is taken as the unit which independently and as a whole takes part on the surface equilibrium. An expression for the number of molecules reaching the boundary surface of a gas with kinetic energy greater than a ctrtain amount h has been given by Langmukl We shall give a deduction of this equation which is also applicable to the case of a liquid on the assumption that the kinetic energy of the molecules has the equipartition value in the liquid state.Consider a slice of solution at the surface of a solid, of unit area and thickness equal to the mean free path of solute molecules in the solution, and containing N' solute molecules. According to Maxwell's distribution law the number of molecules in the slice moving towards the surface with kinetic energy E gram molecule at any instant is :- E=a The number of molecules escaping per second is equal to this quantity multiplied by the mean collision-frequency, that is, the mean velocity v' divided by the mean free path S. But N' = N, S therefore the number of solute molecules reaching the surface per second is which on integration gives Langmuir's equation A 8 = N,A I/Te-E where A = ,/R/27rM .In reaching the balance point Q (Fig. I) from the interior a. solute molecule does work represented by the area QRS, say Wl per gram mole- cule. The number of solute molecules reaching the balance point from the solution per second, is then This expression is not entirely applicable to the molecules at the surface of a crystal whose only movement is one of vibration about an equilibrium position. I n such a case only molecules in the surface layer can get free, and the number of molecules escaping per second is given by the product of three factors : (I) the number of molecules in the surface layer, (2) the fraction of these molccules having at any instant kinetic energy greater than the quantity X per gram molecule, and (3) the number of times on an average the outward movement is repeated per second (assuming that the normal distribution of energy among the surface molecules is attained "jf.Amer. Cketn. SOC., 1913, 35, 122.663 CRYSTALLINE SOLIDS AND LIQUIDS afresh for each new outward movement). collision frequency by a vibration frequency v and obtain :- That is, we replace the mean E=x This expression can only be integrated in terms of a series giving :- RT which, if A is considerably greater than - reduces to I t may be observed that Rodebush, has deduced a similar expression on somewhat different grounds for the case of gaseous dissociation and has also applied it to the vapourisation of mercury. Now, in reaching the balance point, a molecule from the surface layer does work represented by the area POQ, say W, per gram molecule.The number of molecules reaching the balance point from the surface per second is therefore :- Wa - - __ e' = NA' JTe RT where A' = 11 ,/R/Wp . (7) I t is recognised that equations (3) and (7) can only be regarded as first approximations. They have been deduced, moreover, on the assumption that the equipirtition of energy holds good whereas the motions of the molecules of a solid body are almost certainly determined by quantum conditions. However, the kinetic equations will still be of the same form and except at low temperatures the terms will not be greatly different. We are therefore justified in taking these equations as a basis in discussing different cases of equilibrium between crystalline solids and liquids. Solubility.We can wite at once an expression for solubility when no electrical factors are concerned, i.e. for neutral molecules. The condition for equilibrium is that 8, = 02, that is :- N,4' JTe - WdRT = N,A ,/% - W'IIRT . - (8) Writing c = N,/N, x 1000 where No = Avogadro number = 6-06 x 1 0 ~ ~ we obtain the following expression for solubility :- Since IyZ - W,, is the total work done by a molecule in passing from the surface into the interior of the liquid, it is equal to the heat absorbed in the process, that is Qv the heat of saturated solution. Therefore on 1 For this expansion I am indebted to Mr. E. S. Keeping. ". A m . Chem. Soc., 1923,45, 606.664 CONDITIONS AT THE BOUNDL4RY SUKFACE OF Substance.differentiation (9) yields the well known le Chatelier-van't Hoff equation for change of solubility with temperature log,c. dlog c/dT = QV/RT2 . 5'9 7200 5 4'5 6400 3'3 10,100 3'6 15,900 The importance of heat of solution as a factor determining solubility is emphasised by equation (9). This is illustrated by the data given in Table I., in which it is seen that the values of log,c are closely parallel with those of -', The existence of a close relationship between these quantities RT was, of course, evident from the well known equation (IO), but solubility has received very little systematic study from this standpoint, largely owing to the confusion in the literature between the different heats of solution, and also to the obscure nature of the integration constant.This integration constant is given in (9) by the term log,AN,/IoooAN. The direct calculation of this constant would permit the calculation of solubility if the heat of saturated solution were known, but the term A' in- cludes two factors v and W, for which no data exist. In order to make a rough comparison, we may assign plausible values to these quantities, for v the characteristic vibration frequency of the solid as determined by " rest strahlen," and for W, the total heat absorbed in solution, unless that is smaller than the latent heat of fusion of the substance, when the latter is used. The justification of this is that the latent heat of fusion is the difference between the energy of the orderly arrangement of the crystal lattice and the haphazard arrangement of the liquid, and this amount of work at least must be done when a molecule leaves its place in the crystal lattice.The only series of compounds for which all the neces- sary data is available is to be found in the alkali chlorides. The equations that have been deduced are strictly applicable only to the case of non- electrolytes. I t can be shown, however, that the same equations apply to the case of a binary electrolyte,l provided that each ion is regarded as a b'molecule'' in our treatment, 5:e. Qv is now half the molecular heat of the saturated solution. The essential data is given in Table I. It is satisfactory to note that the values of the integration constant obtained in this way are of the right order of magnitude. Q 2-09 3'72 4.66 1'55 TABLE I. NaCl K C1 AgCl TlCl 1.81 I -40 -11.46 -4.13 I- '28 2-32 13-01 8'79 ; The extension of these methods to the special case of meti AN l o g , O IOOOA'N. 5'3 5'0 5-r 4'9 Is dipping into their salt solutions, taking into account the electric factors concerned, leads to a physical interpretation of Nernst's electrolytic solution tension. This case will be discussed in detail in connection with the solubility product. r. Amer. Chem. Soc , $3. 347, 1920. Calculated from solubility. Neat of saturated solution, Q,. 3 Landolt Bornstein. 5 Latent heats of fusion.CRYSTALLINE SOLIDS AND LIQUIDS 665 A further extension provides a reasonable explanation of the mechanism whereby reversible oxidation potentials are set up at inert electrodes. These applications will be given in further papers. In conclusion I must express my best thanks to Professor J. E. Coates, DSc., for his kind interest in the progress of this work, and to Mr. E. S.. Keeping, B.Sc., A.R.C.S., for some helpful discussion of it. Chemistry Department, Universio CoZZege of Swansea. FOY Parts 11. and 111, see pp. 729 and 734.

ISSN:0014-7672    

DOI:10.1039/TF9241900659

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. ELECTRODE REACTION AND EQUILIBRIA. A GENERAL DISCUSSION. At the meeting of the Faraday Society held on Monday, November 26th, 1923, in the Hall of the Institution of Electrical Engineers, London, a GENERAL DISCUSSION took place on (( ELECTRODE REACTIONS AND EQUILIBRIA.” The meeting con- sisted of two Sessions, from 3 to 5 and from 5.30 to 7.30 p.m. The President, Sir Robert Robertson, opened the proceedings with the following remarks : I t surely must be important to know exactly what processes take place among the atoms at the electrode surface. When we have a metal which some believe to consist essentially of a meshwork of metallic ions inter- mingled with another meshwork of free electrons, and when that metal is dipped into a liquid, by some means the meshwork of electrons and the meshwork of metallic ions part company with one another.I t is a matter of pressing importance nowadays to ascertain exactly how this occurs and why it occurs. That is to say, what are the factors which accelerate or retard the separation of the ions from the free electrons ; what factors are there in the solvent or elsewhere which allow the electron to part from the metal and become attached to other atoms or to other molecules. Such matters as these, all of which have to be investigated first in the light of the simple original theory of Nernst, but to which must be applied the modified conception of activity, will receive attention as to the mechanism involved, in the papers of this programme. Another aspect is that which deals with the practical chemical applica- tion of the electrode condition, namely, that of the separation of compounds both organic and inorganic, which are otherwise difficult to obtain. Still another aspect is to confirm the measurements of the energy of chemical reaction between elements and between compounds, and this forms a very important part of modern progress. A great deal, however, of the elucidation of the detailed mechanism must depend on irreversible electrode effects and their investigation, which belong to the second part of the programme. 666

ISSN:0014-7672    

DOI:10.1039/TF9241900666

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. PART I.-" CONDITIONS OF EQUILIBRIUM AT' REVERSIBLE ELECTRODES." Chairman .- SIR ROBERT ROBERTSON, K.B.E., F.R.S., PRESIDENT. INTRODUCTORY ADDRESS ON THE MECHANISM OF THE REVERSIBLE ELECTRODE. BY ERIC K. RIDEAL. Received Nwember 9th, 1923. One of the oldest unsolved problems in physical chemistry 1s the source of E.M.F. in the simple galvanic cell and the mechanism of its production. Ever since the investigations of Volta opinion has swung to extremes oscillating between the contact and the chemical theories. At the present time there are supporters of each view, whilst a centre party of opinion wishes to ascribe the observed potential differences to both effects. For our purpose we must exclude all irreversible effects caused by film formation at the electrodes or alteration in concentration of the electrolytes at the electrode surfaces due to a difference in the rate of removal of the ions from the skin in contact with the electrode and the rate of supply by diffusion.Furthermore, in the simple cell of the most general type /-7 M 1 MX,1 NY I N 7r1 we will assume that the liquid-liquid potential junction TI, is either non-existent or can be made more reproducible by some suitable artifice such as the sand method of Bjerrum or the flowing method of Lamb and Larson, and can be calculated with the aid of the equations of Planck, Henderson, Cumming or G. N. Lewis. Volta and his school maintained that the E.M.F. of such a cell had its seat in the contact potential between the metals MN possibly supplemented by smaller contact potentials between the metals and their respective electrolytes.The experimental work of Ritter, Faraday and Favre on the other hand lent conviction to the view that since the electrical energy of the cell was derived from chemical processes all observed contact potential differences had their origin in chemical changes occurring between metals and electrolytes. This hypothesis was greatly strengthened by Le Roux confusing the contact potential with the Peltier thermoelectric effect. Lord Kelvin corrected this error in showing that the Peltier effect was not related to the contact potential but only to its temperature coefficient. W 3V Volta 3T ' V Peltier = T668 THE MECHANISM OF THE REVERSIBLE ELECTRODE The investigations of Nernst on the electromotive forces of concentration cells showed that the observed E.M.F.of such a cell agreed remarkably closely with the value calculated from the osmotic pressure of the solutions : RT II IZF II, v = -- log 2. For dilute solutions where the activities of the ions are equal to their concentrations we may re-write the equation I n a perfectly general manner, however, the equation becomes wherehf, are the activity coefficients, a conclusion amply confirmed by the work of Harned, Bronsted, Richards, Debye, Bjerrum, McLewis and others. The success of this method when applied to concentration cells led to its extension to general cells and we find the introduction of the term electro- lytic solution pressure as a fundamental characteristic for a homogeneous metal.According to Nernst we must ascribe to each metal a tendency to throw off positive ions into its environment ; thus the metal can be replaced for purposes of calculation by a fictitious solution of positive ions of an equivalent concentration. The very fact that in order to remove the atoms from a metal surface energy has to be expended to do work against the cohesive forces, and that to remove electrons from a metal energy has like- wise to be expended as in the photoelectric effect or thermionic emission, makes it extremely unlikely that positive ions in the metal are in reality only retained there presumably by the slowness with which equilibrium is arrived at. The enormous pressures for the alkali metals and the extra- ordinarily small pressures for the noble elements which are, on the Nernst hypothesis, existent in the electrodes likewise led rapidly to a criticism of the idea.Thus we find Van Laar suggesting that the hypothetical pressures were non-existent, but Nernst's electrolytic solution pressures were in reality equivalent to " concentrations " or " solubilities." Thus, for example, the metal potassium which according to the Nemst conception has an electro- lytic solution pressure of 1 0 5 0 atmospheres would according to Van Laar have a positive ion solubility of 1050 normal. I t might be pointed out that since the atomic volume of potassium is w] = 34 the solid metal contains only some 2.1022 atoms per C.C. or 2 . 1 0 ~ ~ atoms per litre, thus a concentration of 1050 normal is a physical impossibility. This idea of equivalent concentration as opposed to equivalent pressure has been amplified and extended by Smits in his theory of allotropy.On this view there exists in the interior of a solid metal a series of equilibria of which the simplest may be formulated as follows :- M 2 M - t . C . Not only do the atoms but also metallic ions and electrons pass into the solution, and a dynamic equilibrium is set up between these at the metal surface and the electrolyte. As in the cases of adsorption investigated by Rona and Michaelis the liquid will not be saturated with both ions and electrons unless K = C M solid - t i solid CM liquid CC liquid)THE MECHANLSM OF THE REVERSIBLE ELECTRODE 669 since if the solubility ratio in the two phases of the metallic ion differs from that of the electron the liquid will not become saturated with each, but will be either supersaturated in respect to electrons or not saturated with respect to the positive ions thus giving rise to the metal liquid potential difference.The point of view advanced by Smits appears to offer itself to criticism in various directions, thus the exceedingly complicated space lattice systems suggested for even the simple metals which would be necessary to account for the various equilibria presupposed in the theory is at variance with the X-ray examination. The apparent independence of the electrolytic solution pressure of a metal with its temperature, and the presence of free electrons in solution in the light both of the enormous potential of such an entity and the accuracy of Faraday's laws of electrolysis are factors militating very strongly against the hypothesis.Richardson, likewise, from a study of electrical conductivity data and thermionic emission has come to the conclusion that the number of free electrons per unit volume is practically the same for all metals. We are therefore forced to the conclusion that the potential differences between metals when immersed in electrolytes are not to be attributed to differences in either ionic solubilities or to solution or corpuscular pressures of the metals. The failure of the hypothesis of Nernst even when subjected to some- what drastic modifications naturally reopened the question of the existence of a true Volta or contact effect.The more recent experiments of Richard- son on thermionic emission, of Millikan, Hughes, and Langmuir on the photoelectric effect in high vacuo, and of Dushmann and Millikan 1 on con- tact E.M.F. in vacuo have shown that the electron affinity + is a character- istic of a metal, that it varies from element to element, but that the magni- tude as determined by any of the above methods is dependent on the cleanliness of the metallic surface. The electron affinity in volts characteristic of each metal may be calcu- lated with the aid of the Richardson thermionic equation where w = +F or from the Einstein photoelectric emission relationship + = '" - F ' There will in general be a difference in the + values or the electron affinities of two metals. Thus if two metal strips be placed in vacuo, the difference in electron affinity between the two metals will give rise to a potential differ- ence VA - VB = +A - 4~3, the electrons being drawn from the metal having the smallest electron affinity, whilst the potential difference between the plates will balance exactly the contact potential where the potential difference is developed by the Volta effect.In the table on p. 4 are given the + and EP values for a number of the elements.2 I t will be noticed that there is a very fair agreement between the electron affinity series of the elements and their electromotive order. Ac- cording to Langmuir the value of t$ - EP increases as we advance towards the electropositive end of the series, but the data are somewhat inconclusive on this point.If The mean value of + - EP for eight elements is 4-42. 1 Phys. Rev., 18, 241 (1921). 2 The values of 9 for tin, lead and mercury are obtained with the aid of the Linde- mann equation for the ultra violet frequency.670 THE MECHANISM OF THE REVERSIBLE ELECTRODE we assign to hydrogen the arbitrary value of + = 4-42 we can obtain from the relationship VA - VB = +A - +B the value of Ep calc. These values are found in the fifth column. Element. 6 Volts. 1-45 1-87 2.65 3'27 3'70 4'35 0'76 4'93 4-00 4'1 5-48 3.20 2'73 1'49 0 '77 0'34 0.15 - 0.15 - 0'33 - 0'75 - 0'77 I 4-Ep. 4'65 4-60 4'14 4-04 4-04 4-50 0.76 4-78 3'67 4'63 + 3'33 E, Calc. 2'95 2'55 1'77 1-15 0.72 0.07 - 031 - 0.42 - 1.06 -t 0.32 - There is thus little doubt that there is a relationship between the electron affinity of a metal and its position in the electropotential series, but the method of ascribing the potential differences in a cell entirely to the contact potential between the electrodes suffers from the fundamental disadvantage that the E.M.F.of such a cell should be independent of the concentration of the electrolyte which we know is not the case. The de- pendence of the E.M.F. of a cell on the ionic concentration of the elec- trolyte does, however, reach a limiting value for small concentrations, a fact not to be anticipated from the Nernst equation but one readily under- standable from the conception of the Volta effect as the prime cause of the E.M.F. A second factor militating against the hypothesis of the metal- metal junction as the seat of the E.M.F.is that experience in the direct determination of the + values of the elements has indicated that the value of + is extremely sensitive to small traces of impurities. One would there- fore expect that unless the seat of the E.M.F. was really at the metal-liquid interfaces which can always be prepared in a ,perfectly reproducible manner, wide fluctuation in the observed E.M.Fs. of cells would result. The cells such as those of Weston and Clark show no significant variation even to the extent of one hundredth of a millivolt when maintained under uniform conditions. Whilst accepting the concept of the difference in electron affinity of the elements as the cause of the potential difference we may re- gard the mechanism of its function in a somewhat different light.A clean metal surface in a vacuum consists of the cross-section of the lattice work of the metal. If now a gas for which the metal has some chemical affinity be admitted a surface reaction will occur. The surface must now be represented as in a state of chemical equilibrium given by the expression or M + X Z M + X M zM+E the electron existing as the anion adsorbed on the metal surface.l The adsorption of various cations and anions when a metal surface is immersed in an elect] olyte rives rise to the formation of an electric double la) er the magnitude of which can be measured electrical y in terms of the elec iokinetic potential 01 Freundlich. Whatever be the magnitudes of the electrokinetic potentials, however, the E.M. F. of the cell as a whole must be independent of their values.THE MECHANISM OF THE REVERSIBLE ELECTRODE 671 For estimating the position of this surface equilibrium let us imagine the case of a metal, e.g.zinc, in which the metal/vacuum potential difference is 3-27 volts. The metal is now exposed to air, oxygen is adsorbed and partly reacts with the metallic surface, so that the double layer created is equal and opposite to the intrinsic potential of 3-27 volts. Assuming the thick- ness of the double layer to be 2-5 10-* crns. that of an oxygen molecule, we obtain 47rrTs v - V'= - K ' where V - V = 3-27 volts = -010g E.S.U. 6 = 2-5 10-* crns. and K = r hence Q = 3.4 I O - ~ E.S.U. per sq. cm., equivalent to 4.0 I O - ~ gms. zinc per sq. cm. whilst there are actually some 140 10- 9 gms.of zinc per sq. cm Thus a 3 per cent. oxidation of the surface would be sufficient to create an electrical double layer of the requisite charge. Let us now consider the effect of passing a gram equivalent of electrons isothermally and reversibly through a cell consisting of the system. M MX' NX N c, c, On entering M an equivalent of work equal to 9 ,F will be liberated, M now will attempt to acquire a negative charge ; as a result a gram equivalent of M ions will be attracted and condense on the surface in the form of rne,al due to an attempted shift of the surface equilibrium in the direction M + c + M . If C, be the reciprocal of the atomic volume of the element the work required to effect the compression will be When the gram equivalent of electrons leave the metal N by the expendi- ture of energy &F the surface equilibrium of this metal will be shifted in the direction due to the removal of the electrons.The surface will thus tend to acquire a positive charge, and as a result a gram equivalent of fi ions will be driven into the solution of concentration C , with an energy expenditure of N + N + C C RT log 2. c2 The total energy in threading the circuit on the assumption that the passase of the electrons from the outside of N to the outside of M can take place without any change in energy is evidently or RT C RT C, v = 4u - +N - - log 4 - - log -. l! c2 F c, This expression thus involves both the Volta effects, the electrolyte concentrations and the atomic volumes Gf the elements forming the elec-672 THE MECHANISM OF THE REVERSIBLE ELECTRODE trodes.In the case of gas evolution it is evident that C , can be equated to C, as representing gases in equilibrium with the atmospheric pressure at each electrode. For amalgam cells the equation reduces to the form Kr c1 v = ----log- F C,’ which is the desired form, and for concentration cells without transfer I t will be noted that the value of E, is obtained when +N = o and c, = I RT I E, = a#BM - -- log -. F CPd or E, = +H - RTlog V, or + - E, = RT log V , . We should thus expect that the values of 4 - E, should increase with the atomic volume of the element. Such an increase is noted in the more electropositive elements : a fact noted and commented on by Langmuir. I t must, however, be admitted that the + values of any elements are not known as yet with sufficient accuracy to justify entirely the above assump- tions.A solution to the problem of reversible equilibrium at the boundary between an electrode and an electrolyte has likewise been sought in terms of anodic discharge. Sackur was the first to advance the hypothesis that the anodic solution of a metal took place by means of anionic discharge, 6.e. the anodic process was to be represented not as generally suggested by the following scheme but the result of a more complicated change. M - s M + Z x’d + ‘ } or M + X’+MX + Z. M + X + M + X ’ These views have been formulated more precisely by Reichinstein who suggested that the anodic process of solution could be represented by the expression M + 0” 3 MO + 2c, whilst according to Kistiakowsky the following anodic reaction takes place :- M + OH’+MOH + Z, Electrodes are on this view in reality oxygen or oxide electrodes, and the existence of a potential difference must be ascribed to a difference in affinities of the metals for oxygen, a process ultimately involving ionisation of the metallic electrode.These views have been amplified and extended by Heyrovskf who re- gards the electrodes as being continuously bombarded by OH‘ ions, some of these react with the metallic atoms on the surface to form MOH which passes into solution. The electrode is thus left with a negative charge ’Zeit. Phys. Chem., 95, 457. Ibid., 70, 200, 1910.THE MECHANISM OF THE REVERSIBLE ELECTRODE 673 which increases in amount until it attracts as many positive metallic ions per second as are removed by the hydroxyl ions.By a process of virtual work Heyrovsky' obtains the following expression in lieu of the Nernst - - log P term, viz. :- ?ZF K + i - !?log P} - M - log K.}. -L Where K, is the basic dissociation constant of the base MOH, i - log P being approximately identical with the Richardson, Einstein work func- tion +, K and M potentials corresponding to the ionisation of water and the base MOH. The objection to this method of formulation is that no other ion except the OH' is considered as active, whilst the actual E.M.F. observed is dependent only on the activity of the metallic ion in the solvent which may contain either no hydroxyl ions or a number of other anions. Furthermore it is tacitly assumed that all metals acquire a negative charge when immersed in aqueous solution.Numerous investigations on the dropping electrode by Palmaer, Smith, Billitzer and others have confirmed the fact that a metal is not always electronegatively charged with respect to the surrounding solution. Although no exact value for the null electrode has as yet been established on account of disturbances created by the adsorp- tion effects of both cation and anions giving rise to electrokinetic potentials, yet metal/liquid potential differences ranging from negative to positive values far outside electrokinetic values have been observed, thus disproving the basic postulate. I t consequently appears much more plausible to assume some type of mechanism as outlined above. A full interpretation of reversible electrode processes will, however, not be possible until we have obtained more information on two points; the absolute potential values of electrodes immersed in electrolytes of known activity and both the degree and extent of solvation of the ions.N Adopting the value of E = -3380 volts for the - calomel electrode at 18" C. and 0.2864 for the N calomel electrode at the same temperature, we find values for the N calomel electrode on the absolute scale ranging from - 0.60 volts to + 0.60 volts. Palmaer's value of + 0.56 volts being commonly accepted which would give a value of + 0.28 to the N hydrogen electrode. Quite recently Garrison has re-opened the question by obtaining a value of - 0.13 volt for the - calomel electrode, equiva- lent to - 0.18 volt for the N electrode, in agreement with Billitzer's value which received much adverse criticism.A close examination of the literature on the capillary electrometer may readily lead to the conclusion that the problem of the factors in- fluencing the surface tension of a curved mercury surface in contact with an electrolyte have not yet been solved. I t is possible that the Quincke double layer has no objective existence and that the interfacial electrification is the result of ionic adsorption. The data of Smith and others clearly point to both cationic and anionic adsorption, whilst the Lippmann curve may be as readily deduced from the Gibbs surface tension equation as from the hypothesis of a Quincke electric double layer. 1 0 N 1 0674 THE MECHANISM OF THE REVERSIBLE ELECTRODE The data of Jahn and Ostwaldl on the heats of ionisation in solution of metals, e.g., M 3 Maq, which were determined by applying the Gibbs-Helmholtz equation to a cell in which one electrode consisted of a known or null electrode are thus open to criticism.From these data together with the ionising potentials and the lattice energies calculated by Fajans,2 Born has determined the heat of hydration of various ions with the following results :- Hydration Heat in Large Calories. Ion. I 57 K 82 Br 68 Na 103 c1 77 Li IIO cs 74 H 262 Pb 73 Adopting the hypothesis that the envelope of the ions consists of the water dipoles orientated to the ion by the electrostatic field round the ion we would expect that the loss in potential energy of the ion due to immersion in a polar medium would be recovered in the form of heat liberated as the energy of hydration, or that the heat of hydration would vary as the potential, i.e., be approximately inversely proportional to the ionic diameter. In the following table the product of Hr for the positive ions of the above series is given.H. Li . N.. K. Pb. cs. Average. 295 259 295 290 279 305 290 It will be noted that the data agree we!l with the hypothesis outlined above as to the nature of the solvate atmosphere round each ion. I t might be noted in passing that the degree of solvation of the hydrogen ion appears to be somewhat high, a fact somewhat difficult to reconcile with its apparent extreme mobility in aqueous solvents. The above relationship Hr = constant does not hold for the electro- negative ions on which the water dipoles are presumably orientated in opposite sense. I t may be pointed out, however, that in this case the relationship H v r where N is the atomic number of the element yields an approximately constant value of 2 10. The enormous values of the heats of hydration of the ions determined in this way lead to some interesting conclusions. Thus, Born has shon-n that the energy required to vaporke a salt e.g., LiCl into its component ions is given by the expression : NX t t - - I w = - - 6 tt 1Zkt.f. Physikal Chem., 18, 421, 1895. 'Bey., 20, 512, 1918 ; Ber., 21, 542, 1919; ZGit. f. Physikal Chem., 2, 328, 1920. SBer., 21, 679, 1919.THE MECHANISM OF THE REVERSIBLE ELECTRODE 675 where x = 13.g9c~ n = 9 for the alkalis with the exception of Li where a = 5 8 is the lattice constant = 5.4 I O - ~ for sodium chloride. For lithium chloride W = 179 large calories. Whilst the heat of In the following table are appended a solvation H of the ions is 185. few comparative values of W and H :- W. H. W. H. KI . . 144 139 NaI . . 158 160 KCI . . 163 159 NaCl . . 182 180 KBr . . 155 150 NaBr . . 171 171 These figures have been somewhat modified by Fajans and Henfeldl in that the W values have all been somewhat reduced. I t is evident that the energy of hydration alone is quite sufficient to dissociate all the ionic duplets which may exist at the moment of solution of a salt in water. The point is even more clearly shown when it is remembered that the W term represents the electrical work of ionic separation in vacuo which would be W reduced in water to the extent of - K - ZGit. Phys., 2, 309, rgm.

ISSN:0014-7672    

DOI:10.1039/TF9241900667

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. 13.118 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. OXIDATION AND REDUCTION POTENTIALS OF ORGANIC COMPOUNDS. BY EINAR BIILMANN. ReceZ'vtd November 8th, I 9 2 3. I. THE QUINHYDRONES. (a) Reduction Potenrial of Benmequi9zhydrme. The affinity of organic chemical processes has only been measured in very few cases.In organic chemistry there has been much talk about affinity, but very little exact investigation, which could give numerical values for the affinity of distinct transformations. However, since I 9 2 0 several investigations of this kind have been carried out, concerning the reduction-oxidation poten- tials of couples of organic compounds, especially of the type quinone-hydro- quinone, which was examined by Haber and Russ as far back as 1904. A series of American investigators have determined the reduction potentials of several quinones, and at the same time, but independently of them, I have been working on the same problem. Still there has been a difference in the scope of our work, for while the American investigators have extended their experiments over a vast number of quinones, I have tried to study the reduction potentials of some few quinones in electrolytes with different hydrion concentration.In consequence of this difference in plan the American scientists have given us a very valuable impression of the influence of the chemical constitution on the reduction affinity of quinones, and, on the other hand, my investigations have shown that by means of quinhydrone, which is a combination of one molecule hydroquinone and one molecule quinone, an electrode may easily be made up, which in many cases may be used as a very good hydrogen electrode corresponding to a hydrogen pres- sure which is exceedingly feeble, but in the case of dilute electrolytes con- stant and well defined at a given temperature.This was defined by measurement of a series of cells of the type Pt I quinhydrone, a-nH+ I a-nH+, H21 atm. 1 Pt where a-nH+ represents an a-normal, dilute acid electrolyte, whose molar concentration does not exceed 0-1. The measurements, which were camed out at 18" C. and at 25' C. are shown in Table I. and Table 11. 676ORGANIC COMPOUNDS 67 7 TABLE I. QUINHYDRONE-HYDROGEN AT 18' C. Concentration Quinhydrone. Electrolyte. of Potential. 0'7043 0.7043 0'7046 HCI 0.1 IL 0.005 0'7038 0'7038 0'7038 Sodium citrate {:::; PH = 4'96 0'00 j 0'7045 Phosphate mixture pH = 6.81 Average 0-7044 TABLE 11. QU~NHYDRONE-HYDROGEN AT Concentration Quinhydrone. Electrolyte. of HCI 0.1 za solid solid H,SO, 0.1 $1 H,SO, 0.02 t~ Sodium citrate pH = 4-96 Phosphate mixture 1:::; pH = 6.8~ 25' C.Potential. 0.6987 0.6988 0-€& 0.699 I 0.6ggO 0.6992 0'6988 0.6993 0.6987 0.6985 0.6991 0-6ggo 06992 0.6989 Average 0.6g& In the cells the platinum in the quinhydrone solution is positive to the If we send a current from right to left inside the chain, the process at platinum of the hydrogen-electrode. the hydrogen-electrode will be and at the quinhydrone-electrode H, + 2F + 2H+ (F = 95,540 coulombs) C,H,Og + 2H+ + 2F + CGH,O?Ht, quinone hydroquinone so that the total change in the celI consists in the transformation of quinone in hydroquinone. A current in the inverse direction will produce the in- verse transformation, or the chemical process in the cell will be C6H402 + H2 fs C,H,02H2. Accordicg to the law of mass action we have which says that at a particular temperature the hydrogen concentration in the quinone-hydroquinone electrode is exclusively dependent on the ratio betwden the concentrations of hydroquinone and quinone.And to this hydrogen concentration in the solution must correspond a hydrogen pres- sure in the solution, which is678 OXIDATION AND REDUCTION POTENTIALS OF Therefore, if we consider the electrode as a hydrogen-electrode with the hydrogen pressure P,, the potential of the cell will be = o-oooo992~. log P,, + 0*0000gg2 . T . log P,, where P,, is the hydrogen pressure in the hydrogen-electrode. From the figures in Table I. and 11. we can calculaie the hydrogen pressure in solu- tions of quinhydrone, that is in solutions containing quinhydrone and quinone in equal molarity, and the calculation gives us the values P, I O - ~ ~ * ~ ~ at 18" C.P, 1 0 - 2 ~ ~ ~ ~ at 25" C. These hydrogen pressures correspond to less than one single molecule of hydrogen in I litre, and as for free hydrogen in the solutions of quin- hydrone there will not be one single molecule present in the quantity of this solution contained in the electrode vessel. As a matter of course we cannot attribute to these figures any physical significance. However, the calculated values of hydrogen F ressures corresponding to the normal potentials may be very useful characteristics of the reducing powers of the substances examined. As for the organic chemist who desires to work with these problems, and who usually may not be very familiar with thermo- dynamics, I think he will eliminate many errors by referring oxidation- reduction electrodes to the type of hydrogen electrodes.In the quinhydrone-electrode we do not employ a fortuitous mixture of hydroquinone and quinone but the compound quinhydrone, which in aqueous solution dissociates widely in to equal molecules of its components. I n this special case we have [CsH402Hd = [Cd&Od and consequently [H,] = K and P, = K'. The constancy of P,, independent of the quinhydrone concentration, is of very great importance for the practical applicability of the quinhydrone electrode, and the figures in Tables I. and 11. show that this independence is realised in the electrode, for solutions which are 0.005 molar in regard to quinhydrone produce the same potential as solutions which are saturated with quinhydrone (" solid ") corresponding to 0.018 molar solutions at 2 5 O c. (6) Influences on the PoteittiaZ.We shall now consider the influence which chemical compounds present in the quinhydrone electrode may be able to exercise on the potential. First we have to consider that one of the components of the quin- hydrone, the hydroquinone, is itself a feeble acid. v. Euler and Bohlin have found that its first dissociation constant is 1-1 x I O - ~ O . In markedly acid solutions, therefore, the hydroquinone will not be able to alter the hydrion concentration. But as the hydrion concentration of a saturated solution is calculated to be c . 10 -6, the addition of quinhydrone to a feebly acid solution without buffer effect may affect the hydrion concentration.On the other hand, in the case of a buffer mixture, the acidity of which does not differ much from pH6, the influence may be negligible. So I found in a phosphate buffer mixture the value 6-80 for pH with quinhydrone electrode, and 6-79 with the hydrogen electrode. Another influence of the dissociation, which may be important in alkaline solutions, is that it alters the ratio between hydroquinone andORGANIC COMPOUNDS 67 9 quinone, which in the markedly acid solutions of quinhydrone is unity. I f t is the total concentration of hydroquinone and G' is the concentration of undissociated! hydroquinone, and I O - ~ O the approximate value of the dis- sociation constant of hydroquinone, we have C 10-** - - - I f - Ct P + l ' and the potential between a quinhydrone electrode with only undissociated hydroquinone and one in which the ratio between total and undissociated hydroquinone is c : t' is AT = 0~0000g92 .T . log -;. t C By means of this equation we find W+l. c : c'. AT. 10-9 1-01 0'00012 volts. IO-~ 1'10 o'ooIIg ,, We see that this influence should, in the first place, be important in markedly alkaline solutions. Next we shall regard an influence of quite another kind, namely the influence of acids and salts on the solubilities of hydroquinone and quinone, that is to say, on their activity. After my first publication on the quin- hydrone electrode at the Oersted-meeting in Copenhagen in September, 1920, S. P. L. Soerensen, Margrethe Soerensen and Linderstroem Lang very carefully studied this problem.By determinations of the solubilities of hydroquinone, quinone and mixtures of these components of the quin- hydrone in acid solutions of sodium chloride and by measurement of the potentials of electrodes made by means of such solutions, the authors have studied the activity theory and at the same time have made a very interest- ing contribution to the chemistry of the quinhydrone electrode in salt-rich solutions. They find that the potential decreases when the salt concentra- tion increases and that this is due to the influence of the salts on the solubilities of hydroquinone and quinone, which both diminish but not in the same degree. The hydroquinone is affected more than the quinone, and in consequence the ratio between the activities of hydroquinone and quinone has a value higher than unity.Therefore the quinhydrone electrode in a solution rich in salt is more negative than that of a quin- hydrone electrode in a solution with a lower salt concentration. The authors have only examined the influence of sodium chloride in 0-01-n HCl. We therefore know very little about the nature of the salt effect on the quinhydrone electrode. But at a meeting at Gothenburg last summer, Mr. Linderstroem Lang communicated some researches he had done on the influence of the kations Li+, Na+, K+, Rb+ and Cs+ and the anions Cl', Brj, and I+ on the solubilities of hydroquinone, quinone and some other substances. These experiments seem to indicate a relation between the reducing and oxidising effect of hydroquinone and quinone on one side and the electrical charges and the size of the ions on the other side.As a matter of course, these effects are of great importance for the practical use of the quinhydrone electrode, I n Table 111. is shown the influence of salt- and acid-concentration on the potentials of chains of the type- Pt I Quinhydrone, Electrolyte, H, I Pt at 18" C. and I atm. hydrogen.680 OXIDATION AND REDUCTION POTENTIALS OF TABLE 111. Electrolyte. 0.01 HCI . . . . . . . 0'7044 Volts. 0'01 HCI + o'og NaCl . . . . . 0.7042 ,, 0.01 HCl + 1.99 NaCI . . . . . . 0,6978 ,, 0-01 HCl -t 3-99 NaC . . . . . 0.6921 ,, 0.50 HCl . . . . . . . 0.7029 ,, 0.50 HCI f 0.50 NaCl . . . . 0 - 7 q ,, 0.50 H,SO, . . . . . . 0'7039 ,, The figures, are taken from the paper of Biilmann and Lund on the quinhydrone electrode.The potentials of the first two electrolytes are quite consistent with the figures in Table I., and the values for the a-molar and the 4-molar solution of hydrochloric acid + sodium chloride agree well with the observations of Soerensen and his collaborators. (6) 2% Quinhydrone Elecfrode, th Quino-guiahydrone Electrode and the H j d ~ ~ - p t h h ~ d ~ o ~ Electrode. The chemical process in the quinhydrone electrode is a transformation of dissolved hydroquinone in dissolved quinone and hydrogen, and vice R FIG. I. versa. We must not suppose that the affinity of this transformation and in consequence the potential of the electrode should not be affected by the nature and the composition of the solvent. Only experiment can tell us the magnitude of this influence, but as a matter of course, if we make up an electrode in which the process is a transformation of a solid substance into another solid substance, we may suppose that the affinity and the potential will be found to be independent of the composition of the solvent, and this may easily be tried by means of electrodes saturated with quinhydrone and with the one or other of its components.Together with Mr. Lund I have examined electrodes of these types, and I think that it may be useful to describe the practical arrangement of these electrodes as well as of the quinhydrone electrode itself. As for the quinhydrone electrode we employ electrode vessels of theORGANIC COMPOUNDS 68 F form usual for calomel electrodes (see Fig. IA). The platinum electrode is a blank platinum foil or a platinum wire.For making the electrode ready for use c. 15 C.C. of the electrolyte are shaken for t minute or so with some c. gm. of quinhydrone, the mixture is poured in the vessel and the platinum electrode fitted in place. As soon as the solution has the desired temperature, the electrode is ready for the measurement of potentials. I n the electrodes containing saturated solutions of quinhydrone and quinone or hydroquinone-we have called them the quino-quinhydrone electrode and the hydro-quinhydrone electrode-it is only that part of the solution in contact with the platinum which needs to be saturated with quinhydrone and with quinone or hydroquinone. By means of the electrode vessel shown in Fig. I B we have succeeded in preparing these saturated solutions in the electrode vessel itself.For this purpose we shake c. 15 cc. of the electrolyte for + minute with a mixture of 0.1 gm. quin- hydrone and 0.5 gm. quinone (for the quino-quinhydrone electrode) or with a mixture of 0-1 gm. quinhydrone and I gm. hydroquinone (for the hydro- quinhydrone electrode). The mixtures of solutiop and solid substance are poured into the electrode vessel, and for platinum electrode is used a platinum wire as shown in Fig. IB. The figures in Tables 1V.-VII. show that by means of this arrangement the solution moistening the platinum very rapidly attains a complete and well-defined saturation. All the measurements were carried out at 18" C. and with 2 electrodes. TABLE IV. Pt. I Quinone., Quid drone Electrolyte, Hz I Pt.&ids): Electrolyte. t. lr. Hrr. Mins. 0.1 ti HCl 0 45 0'7563 0.7563 z:;;?} O'75% 1 15 - 0.7561 0.756 I 0.5 ti H,SO, 1 25 0'7558 0.7560 - 1 55 0.7558 0.7560 - 4 15 0.7562 0.7558 - 5 I5 0.7565 0.7561 0.7562 0.7561 0.7562 0.7561 0.5 n HCI 0 15 0.5 ti NaCl 0 30 I 00 Average 0.7562 TABLE V. (Solids). pt. I Hydroquinone, Quinhydrone, HCl, Ho I Pt. Electrolyte. t . m. 0.1 10. HCI I 00 0'6175 0.6174 - 1 45 0.6174 0.6174 0.1 rz HCl 0 30 0.6178 0.6181 - I 0 0 0.6177 o617g 0.5 ts HCI 2 25 0.6178 0.6173 - 3 15 06178 0'6175 - 6 0 0 0.6177 06175 - 18 00 0.6176 0.6174 1.0 n HCl I 00 0.6172 0.6171 - 2 0 0 0.6174 06174 - 3 30 0.6174 0.6174 Hrs. Mins. 0.6177 06176 1 1 0.6174 } Average 0.6176 A capillary quinhydrone electrode, working with 2-3 drops of the electrolyte, has See also the April been described by Biilmann and Lund (Annales dc Chimie, 1921). number, 1924, of the 3!'owrnal of Agricultural Science.682 OXIDATION AND REDUCTION POTENTIALS OF TABLE VI.Pt. I Hydroquinone, Quinhydrone, HCl + NaCl, H., I Pt. (Solids). Electrolyte. t. ?r. Hrs. Mins. 0.6181 0 30 0.6183 1 30 0.6 182 0'6180 0'6 I 80 0.6180 ~6 00 0.6180 0.1 11 HC1 + 0.9 n NaCl 0.6176 1 '5 0.6174 0.6176 1 30 0.6176 0.6177 2 I5 06176 0.6178 0-5 IL HC1 i- 0.5 12 NaCl Average 0.6179 TABLE VII. I Hydroquinone, Quinhydrone, HzSOp, HZ I pt. P t . (Solids). Electrolyte. t . 7r. Hrs. Mins. 0.6180 0.6181 0 45 Od180 0.6181) 0'6181 1 I5 I 00 0.6183 0.6185 0.6182 0'6184 1 45 0.6181 0.6184 2 45 0.6176 1 15 06177 - 1 30 0'6179 - 2 00 0.6180 0.1 n H,SO, 0.5 F,SO, - 0.6183 - - 0.6 I 78 r o n H,SO, 0 45 - '9 15 0.6 I 76 20 0 0.6176 Average 0.6181 The constancy of the potentials is striking.As for the chemical re- actions in these electrodes, in the quino-quinhydrone electrode we measure the affinity of the transformation 2 mol quinone (solid) + H, (gas) + I mol quinhydrone (solid) and in the hydro-quinhydrone electrode the affinity of the reaction I mol quinhydrone, (solid) + H, (gas) 4 z mol hydroquinone (solid). I t remains to say that the platinum electrodes must be blank. They are cleaned by treatment with a hot mixture of chromic acid and strong sulphuric acid, then washed with distilled water and heated to a red heat in an alcohol steam lamp or in a benzine blow lamp, but not in a gas flame. As for the quinhydrone it has been found suitable to prepare it as follows : roo gms.ferric ammonium alum are dissolved in 300 C.C. water at c. 65' C., and this solution is turned into a warm solution of 25 gms. hydroquinone in 300 C.C. water. The quinhydrone precipitates in fine needles. The mixture is cooled in ice and filtered by suction and the pre- cipitate then washed four or five times with cold water. The yield is 15-16 gm:. The preparation may contain a feeble trace of iron, which is without serious effect. Quinone may easily be prepared by using a double quantity of ferric ammonium alum and distilling with water-steam.ORGANIC COMPOTJNDS 683 (d) ApgZications of the Quhhyd~ow Elecirodts. The quinhydrone electrode may mainly be applied for two purposes : as an electrode of comparison and for the determination of hydrion con- centrations.As for the first of these applications there are many cases where we have to measure the potential of an acid electrode, and by using a calomel electrode for the comparison we introduce a liquid junction potential. In such cases it may often be advantageous to apply a quinhydrone electrode with an electrolyte corresponding to that of the examined electrode. Of course, we could also use a hydrogen electrode, but the quinhydrone electrode is so much easier to prepare than a hydrogen electrode. Here in my laboratory we have now no calomel electrodes at all. We always use quinhydrone electrodes as comparison electrodes, and Mr. Stig Veibel has examined in my laboratory a quinhydrone electrode with 0.01-n HC1+ 0-09-t: KC1, which is reproduced so quickly and so constant that it may be a very good standard electrode.For the determination of hydrion concentrations the quinhydrone electrode has the advantage that it is very quickly prepared and its hydrogen pressure is so feeble that it gives well defined potentials in many cases where the usual hydrogen electrode does not work, because it reduces one of the chemical compounds in the electrolyte of the electrode. In Table VIII. are shown values for pH determined by means of the quinhydrone electrode and, for comparison, the values determined by means of the hydrogen electrode or calculated from the dissociation constants. As examples, unsaturated organic acids are mainly taken or else halogen-substituted organic acids, that is to say, chemical compounds which are reduced in the hydrogen electrode and which do not give well-defined values for the dissociation constant by means of the conductivity.I only give the values of pH, not the measurements or other particulars. But I may say that all the electrodes presented well defined and very stable potentials. TABLE VIII. pH-VALU ES. Substance. ________ ~ Phosphate mixture . . Citrate mixture . . . Acrylic acid . . , . Crotonic acid . . . Fumaric acid . . . Pheny propiolic acid . . Ch:oraceticacid . . . Iodpropionic acid . . . B bromosuccinic acid . . 180-bibromo6uccmic acid. . Quinhydrone Electrode. 6-80 2'93 3-10 2'28 2'65 1.71 1-57 4'96 2-12 2'12 6 79 4-96 2-89 3-10 2-29 2'28 2-18 3-18 1-82 1-77 (Hydrogen electrode) ( Yoerensen) (0 st wald) (White and Jones) (O&dd[' " (Chz&3ler) (Holmberg) 9, 1 Stig Verbel, I.c., p.2004. Very striking results were obtained by measuring o - ~ n HCl, mixtures of these in chains of the type Quinhydrme, HClor HNO, KCl KCI HgCl Pt I o*oog-n or HCl+HNO, 1 3-5-72 13-5-72, solid HNOa and H g684 OXIDATION AND REDUCl'ION POTENTIALS OF TABLE IX. Acid. Normality. o Hrs. o Hrs. 30 Mins. H C1 0'1 0.3880 0.3880 ,? 0.3879 0.3880 9 , 9 9 0'3881 0.3877 HCI Y ? 0'3882 0.3880 3 , 9 9 0.388~ 0.3880 9 , 9 1 0.3882 03881 H NOs 9 , 0.3880 0'3877 o Hrs. o Hrs. 40 Mins. 0.3882 - HNO,'; HCl :; 0.3882 0.3881 28 Hts. 30 Mins 0'3878 volts. 0'3875 1, 0'3874 ? I - $ 9 21 Hn. 0.3880 volts. 0.3880 ,, - l ? $ 9 - 0.3879 9 , These figures do not call for comment.Further, we have found that the quinhydrone electrode may be applied for the determination of hydrion concentrations in mixtures of soils and water. Together with Mr. Hakon Lund I have studied the application in some few characteristic cases and then the method has been examined on a large scale by Mr. Tovborg Jensen of the Danish State Laboratory for Plant Culture (Statens Planteavls laboratorium, Lyngby, Denmark, Director Mr. Harald R. Christensen), and it was stated that electrodes prepared simply by addition of a feeble quantity of quinhydrone to the mixture of soil and water give sufficiently exact pH-values up to pH 8.4. This means that the electrode may be applied to the entire interval of pH-values which are of practical interest in soil-investigations. For these applications it is important that the electrode be not affected by nitrates as is the hydrogen e1ectrode.l But, of course, the quinhydrone electrode is by no means a universal electrode, and there may be soils containing constituents which affect the electrode potential.For determining pH-values by means of the quinhydrone electrodes we have to measure the potential difference between these electrodes and a standard electrode. Taking as basis the figures used by Soerensen in his work on standard buffer solutions we have at 18' C :- if the standard electrode is the 0.1-n calomel electrode for the quinhydrone electrode 0'4183 - T for the quino-quinhydrone electrode pH = 0'0577 ' 0.2800 - z €or the hydro-quinhydrone eIectrode pH = and if the standard electrode is the quinhydrone electrode described by Veibel (electrolyte 0.01 -n HC1- o-og-n KC1) 0 .0 5 7 7 ' 7T for the quinhydrone electrode pH = 2.04 + ___ "'"577' In all the formulae T is the potential of the electrode examined relatively to the standard electrode. I n the last combination we measure the potential difference between two quinhydrone electrodes with different hydrion concentrations. As the " hydrogen pressure " is supposed to be the same in the two quinhydrone electrodes, the constant 2-04 in the formula is equal to the value of pH in the standard electrode.2 1 See the April number, 1924, of the Journal of Agricultural Science. a For applications of the quinhydrone electrode not described here see the papers of La Mer and Parsons, Kolthoff, Bodforss, Schreiner, Harris, and of Larsson recorded in the Bibliography.ORGANIC COMPOUNDS 685 do.0.7124 0.7120 0-7230 0.7228 0.7088 (e) Reduchbn Potentials of Dz;fJerent Quinones. In the preceding we have dealt exclusively with the simplest of all quinones and hydroquinones, the benzoequinone and the benzoehydro- quinone. I t has already been mentioned that these compounds were examined by Haber and Russ (1904). These investigators used as electrolyte a mixture of water and dilute sulphuric acid, and they could not obtainan exact reproduction of the potentials. All the same, their work must always be considered a very important contribution to the chemistry of the reduction potentials. Until my first paper in xg20 and a communi- cation made by Granger and Nelson (I 92 I) no further work on the problem seems to have been carried out.Other quinones were examined by Victor K. Lamer and Lillian Baker (I 92 2), who successively oxidised solutions of hydroquinones with chromic acid and reduced solutions of quinones with titanous chloride and measured the potentials of the solutions at different stages of the oxidation or reduction. The values obtained by this method seem to be very exact. Conant, Kahn, Fieser and Kurt (I 92 2) examined in the same way anthraquinone derivatives and Conant and Fieser made a comparative study of the potentials in aqueous solutions and in mixtures of water and alcohol. Together with Mr. Langseth Jensen I am trying to determine the reduction potentials of quinones in a very simple way, namely by measuring electrodes made by means of equimolar quantities of a hydroquinone AH2 and a quinone B, against a quinhydrone electrode A,AH, or B,BH9 with known reduction potential.In the mixture of AH, and I3 there will be formed the quinone A as well as the hydroquinone BH, :- a AH2 + aB -a (a - x)AH2 + xA + (a - x)B + xBH2. At the equilibrium the potential a produced by the mixture (a - x)AH2 -+ xA may be equal to the potential produced by the mixture (a - x)B + xBH2. If a'o is the normal potential of the quinhydrone AH2,A and rO is the normal potential of the quinhydrone BH2,B, we have ~~ ~ Other Determinations. 0'7125 (La Mer) 0.7152 ,, 0.7210 ,, As we do not measure the potential of the electrode against a hydrogen electrode but against a quinhydrone electrode BH2,B with the normal potential x0, the measured potential 4 is equal to I + TO or a = + x,,.Then we have In the Table X. are shown the values of w'o found in this way for a series of quinhydrones at 25' C. = a. + 24. TABLE X. Hydroquinone. Monochlorhydroquinone . . Monobromhydroquinone . . 2, 5-Dichlorhydroquinone . . 2, g-Dibromhydroquinone . . Tribromhydroquinone . . . Toluhydroquinone . . . Quinone. Benzoquinone ,, ** 1 9 9 9 9 9 I Xylohydroquinone . . . I Toluquinone686 OXIDATION AND REDUCTION POTENTIALS OF 11. REDUCTION POTENTIALS OF ALLOXANTHINES. In the dialuric acid and the alloxane we have two compounds which in many accounts resemble the hydroquinone and the quinone. One is easily converted into the other by oxidation or reduction, and one molecule of the dialuric acid combines with one molecule of the alloxane to alloxanthine, which is sparingly soluble in water.Of course, the quinone is yellow and the alloxane is colourless, and in the constitution there are also great differences. For instance, the quinone contains the group : C : 0 and the alloxane contains the group : C\OH),. But, as Biltz has shown, the alloxane forms an anhydride, which is yellow, and, together with Mrs. Bentzon, I have found that a concentrated, aqueous solution of alloxane, if heated, has a yellow colour. We may therefore suppose that at ordinary temperature also a part of the alloxane in an aqueous solution is present in the anhydride form. logether with Mr. Hakon Lund, I have determined the potentials of solutions containing alloxanthine, which in aqueous solution dissociates in H .N - C : O 0 : C - N . H dialuric acid and alloxane, and of tetramethyl alloxanthine, which dissociates in CH3: N-C : 0 0 : C - N . CH3 HO 0 : c CH3. N - C : 0 dimethyldialuric acid and dimethylalloxane. 0 C - N . CH3 For the experiments we used the vessel shown in Fig. 2 , which permits B FIG. 2. the experiments to be carried out in an atmosphere of carbon dioxide. The electrolyte was o-zn and 0.05" H,S04.ORGANIC COMPOUNDS 687 0'3944 0.4055 0.4169 0.4274 0.4386 0.4582 For chains of the type Pt 1 alloxanthine (solid), dilute sulphuric acid, H2 (I atm.) 1 Pt we found the following potentials :- TABLE XI. - 0.098 0'094 0.094 0.093 0'102 ~ 'I Hg-Pressure." I I' I I Alloxanthine. . . 0.10 0.3696 volt 0,3664 volt 10-l~~ Tetrakethylalloxanthin; I z::: 1 0.3657 :: 1 1 1 - .. 0.3699 111. REDUCTION POTENTIAL OF Azo COMPOUNDS. Together with Mr. Jakob Blom, I am studying the reduction potentials of azo compounds. The reaction R . N : N . R + H, +R. NH. NH. R is analogous to the transformation of a quinone to a hydroquinone, and so the treatment of the problem is analogous to that of the quinhydrones. But the properties of the hydrazo compounds occasion some special difficulties. First, the azo- and hydrazo-benzene are not soluble in water, acids, or alkalies. We are therefore compelled to apply derivatives con- taining amino groups or acid groups. Further, the oxygen of the atmos- phere oxidises many hydrazo compounds, and the experiments have therefore to be carried out in absence of free oxygen.And last, but not least, many hydrazo compounds are converted into benzidines or semidines by the action of acids and other reagents, so that the concentration of the hydrazo compound may decrease during the experiment. Here I only shall mention one example, namely, the reduction of the azo-toluidine H,N . . NH, H,C . o N = N - a . CH, to the corresponding hydrazo compound. We have measured the chain azotoluidine hydrazotoluidine HCl quinhydrone 1 Pt, pt I o.oo4o8 m o.oo4o8 m 0.1 n Particulars will be communicated in a special paper. In Table XII. we give the potentials 1 of the azo-hydrazo electrode against a hydrogen electrode, that is to say, the normal potential of the quinhydrone electrode (at 18') minus the measured potentials ; f is the time in minutes.TABLE XII. t. 21 31 41 50 60 72 81 - 1'0 ' - 1.4 - 1.8 - 2'2 - 2.6 - 3'3 - 3.6688 OXIDATION AND REDUCTION POTENTIALS OF We see that T increases, and this means that the hydrogen pressure in the azo-hydrazo electrode decreases in consequence of the transformation of the hydrazo compound into a semidine. The curve in Fig. 3 shows the relation between time and potential, and we see that from f = 22 to near t = 72 the relation is almoit linear.. An extrapolation to t = o gives value 0.3763. To this value we have to add 0.0044 volt, because the the .Mih!&?J FIG. 3. amino groups neutralise a part of the acid in the azo-hydrazoelectrode. We have then the normal reduction potential of the examined azotoluidine is r0 = 0.3807 volt, corresponding to a hydrogen pressure of 10 + 1 3 2 atm.If At, At’, Ht and Hp are the concentrations of the azo- and hydrazo compound at the times fland t’, we haveORGANIC COMPOUNDS 689 as the concentration of the azo compound is not altered. As for the transformation of the hydrazo compound in a semidine, this reaction may be supposed to be of the first order, catalysed by the hydrions. We find, therefore, that the velocity constant K at the hydrion concentration existing in the electrode is where Ht = a and Hp = (a - x). have By combination of (I) and (2) we K ( i - t). RT vF 7rp - 7rt = - As Y =I 2 and T = 291, the velocity constant is 7rtf - .rrt K =I 80 x -. t' - f The values of K are shown in Table XII. The velocity is exceedingly I think it should be dficult to determine the constant by quantita- high.tive analysis. From the equations (I) and (2) we derive a - x log - - Z ? L 2 7 t e a 0'028 The values of log (a - 'x) : a show, that after a short time we have to do with exceedingly feeble concentrations. IV. FINAL REMARKS. We have here dealt with some special cases of oxidation and reduction reactions of organic compounds. Some few others (indophenol, indigo- sulphonates and methylenblue) have been examined by W. M. Clark in 1920 and 1921. I think it useful to say that measurements of this kind should not be done exclusively by physical chemists, who may in most cases prefer to examine substances which are suitable for their special purposes and at the same time easily procured. But the examination of many other compounds has to be done by organic chemists, in the first place, because the prepara- tion and purification of organic compounds may take weeks or months and demand in many cases a high degree of exercise in practical organic chemical work.Further, it may demand the full skill of an expert in organic chemistry to estimate all possibilities of side reactions interfering with the reversibility, which is the condifzr'o sine qua non for the measure- ment of the potential of a particular reaction. For instance, it may seem very easy to examine electrometrically the reaction CH, : CH . COOH + Br, CH2Br. CHBr . COOH, but the organic chemist knows that this reaction need not be reversible in aqueous solution, as the main reaction of bromine in water on a double linking is an addition of HOBr.But there may be other reactions which render it possible to measure by means of a potential the influence of composition and constitution on the affinity VOL. XIX-T26690 OXIDATION AND REDUCTION POTENTIALS OF of reactions with double linking. For instance, between ethylene derivatives and mercury salts we have a series of reactions of the type ) C : C ( + Hg++ + OH + 2 > C(0H) . C<Hg+, that is to say, formation of organic mercury compounds, the complexity of which may be supposed to depend on the constitution of the organic molecule. Some years ago I studied the affinity of processes of this kind by measuring the potentials of elements containing the mercury compounds of allylalcohol, allylacetic acid, crotonic acid and maleic acid (Biilmann and Hoff, 1917).We are now continuing these investigations in my laboratory. For many years past synthesis has been at the basis of the development of organic chemistry. I think we are now entering an epoch in which the study of the laws governing the transformations of organic compounds and their properties may prove to be as important in organic chemistry as the still very important synthetic work and study of constitutions. As a matter of fact, since Kopp’s investigations on the relations between boiling-point and composition a great deal of useful work has been done dealing with problems of this kind. But just this important problem of affinity has until recent years been but little studied by means of organic compounds, and in most cases the term “affinity ” has been applied in organic chemistry in rather a confusing manner, quite different from the classical definition of affinity.In many cases it may surely be difficult or impossible to determine the the affinity of organic reactions by the electrometric method. But the examples treated in the preceding pages show that the measurement of the affinities of important types of transformation is possible using one of the best working methods of physical chemistry. As, moreover, we also get- by very simple calculations-the equilibrium constants, the heats of trans- formation and in special cases (as shown here for an azo compound) a velocity constant, I think it useful to draw the attention of organic chemists to this problem and this method. BIBLIOGRAPHY.Baker, Lillian E. : The effect of substitution on the free energy of reduction of benzo- Biilmann, Einar and Agnes Hoff : Sur la complexit6 de quelques combinaisons organiques Biilmann, Einar : Kinhydroners Brintning, Annual ofthe University of Copetrhagetz, 1920. Biilmann, Einar : Sur I’hydrogCnation des quinhydrones, Ann. de Chimie, g’s., IS, ~og, Biilmann, Einar and Hakon Lund : Sur I’Clectrode A quinhydrone, Ann. dc Chimic, Biilmann, Einar and Hakon Lund : Sur le potentiel d’hydroghation des alloxanthines, Bodforss, Sven : Ueber die Beeinflussung von verschiedenen chemischen Reaktionen Conant, J. B., Kahn, Fieser and Kurtz: An electrochemical study of the reversible re- Conant, J. B., and Fieser : Reduction potentials of quinones, ibid., 45, 2194, 1923. Clark, M. W. : The determination of hydrogen ions, Baltimore, 1922. Granger, F. S., and J. M. Nelson : Oxidation and reduction of hydroquinone and quinone from the standpoint of electromotive-force measurements, Am. Chem. SOC., 43, 1401, 1921. Haber, F., and R. Russ: Elektrische Reduktion, 2. phys. Ch., 47, 257, 1904. Harris, Leslie J.: Use of the Quinhydrone Electrode for the Estimation of Amino- acids and of Acid and Basic Functions, your. Chem. SOL, 123, 3294, 19-23. Kolthoff, F. M. : Die Verwendung der Chinhydron-statt Wasserstoffelektrode bei potentiometrischen Aciditatsbestimmungen, Rec. Trav. Chim. &s Pays-Bas, XLII., quinone, Dissert, New York, 1922. de mercure, Rcc. Trav. Chim. Pays. Bas., 36, I, 1917. 1921. g’S., 16, 321, 1921. Ann. & Chimie, g’s., 19, 137, 1923. durch Substituenten, 2. phys. Ch., 10% 51, 1922. duction of organic compounds, Am. Chem. SOC., #, 1382, 1922. 186, 1923.ORGANIC COMPOUNDS. 691 La Mer, V. K., and L. E. Baker : The effect of substitution on the free energy of oxida- tion and reduction reactions, Am. Chem. Soc., 44, 1954, 1922. La Mer, V. K., and T. R. Parsons : The Application of the Quinhydrone Electrode to electrometric Acid-Base Titration, The yourn. o Biol. Chemistry, LVII., 613, 1923. Larsson, Erik : Zur elektrolytischen Dissoziation d zweibas. SPuren, 2. anorg. Ch., 125, 287, 1922. Schreiner, E. : Die Hydratation des Wasserstoffions, 2. phys. Ch., 121, 321, 1922. Id. ; Der Dissoziationszustand von Mittelstarken Sauren, besonders der Dichloressig- saure, in Wasser und in Salzlosungen, 2. phys. Ch., 122, 201, 1922. Soerensen, S. P. L. S., Margrethe Soerensen and K. Linderstroem Lang : Sur l’erreur de sel inherente A 1’Clectrode de quinhydrone, Ann. de Chimie, g’s., 16, 283, 1921. Veibel, Stig : The quinhydrone electrode as a comparison electrode, ywr. Chm, SOC., 123, 2203, 1923.

ISSN:0014-7672    

DOI:10.1039/TF9241900676

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. THE PROCESSES AT THE MERCURY DROPPING CATHODE. PART I. THE DEPOSITION OF METALS. By JAROSLAV HEYROVSK$, D.Sc., PH.D., CHARLES’ UNIVERSITY, PRAGUE. Received October I Ith, I 923. In froduction. The methods hitherto employed of following electrode potentials during polarisations contrast markedly-as regard exactness-with the E.M.F. determinations of primary galvanic cells : this is due to the circumstance that for polarisations more or less considerable current densities are used, whereas galvanic cells are studied in a ‘‘ currentless ” condition.Thus the polarisation ‘‘ current-voltage ” diagrams bear the character of a technical research rather than of exactly reproducible measurements, which would show strict thermodynamic relationships. To approach nearer to the stage of reversibility in investigating the polarisatioq phenomena the author has applied to the study of electrolytic processes a dropping mercury cathode as used by B. KuEera in the deter- mination of the interfacial tension of polarised mercury.2 Fig. I shows a suitable form of the electrolytic vessel. Through a rubber or cork stopper A, placed at the top of a long-necked conical flask so as not to contaminate the solutions, the mercury cathode B is inserted, consisting of a thick-walled capillary, the lower end of which is drawn into a tip allowing mercury to drop out slowly.The upper end of the capillary is connected by a rubber tubing, ca. 5 0 cms. long, to a mercury reservoir, by the level of which the rate of dropping can be regulated ; the usual drop time was ca. 3 seconds. In order to expel air and remove oxidising impurities from the solution in the vessel pure hydrogen is let in before electrolysis through the glass tube D, it passes the solution and escapes through the syphon C. The large bottom mercury layer Hg serves as anode, from which a platinum contact leads to the connection in F. Through the tap H the solution or mercury may be let out, and through E fresh mercury can be poured in, if required. When the solution is properly reduced by hydrogen, which is effected after two or three hours of slow passage, the siphon C is lowered and filled with the solution, thus establishing a connection with a normal calomel electrodk, to which the potential of the bottom mercury layer is referred.Phil. Mag., 45 (1gz3), p. 303. * Drud. Ann., 11 (I903), p. 529. 692PART I. THE DEPOSITION OF METALS 693 The negative pole of the polarising source is then connected to the mercury reservoir, making the drops cathode, whilst the mercury layer is joined with the positive pole ; the polarising E.M.F. is then slowly increased and the current which passes through the solution is determined by a sensitive galvanometer G (a d’Arsonva1 with 4 mm deflection corresponding to 10 - amp.).FIG. I. The constant renewal of mercury surface at the dropping cathode, which prevents ‘‘ concentration-polarisation ” and hydrogen evolution, effects slight fluctuations due to the charge acquired by the spreading of the mercury surface (the electrocapillary phenomenon), which causes the galvanometer mirror to oscillate I -2 millimeters of the scale-division, corresponding to changes in intensity of z - 5 x 10-9 amp. ; the current which passes the cell before electrolysis of the solution sets in is of the order of 10 - 8 - 10- amp. Electrodeposition in Mercury. If a properly reduced solution be polarised, the current must all be due to the ionic deposition in the mercury drop ; it is therefore important to avoid in the solution traces of mercury salts, which would deposit alreadyat very small polarisations.Even very sparingly soluble salts such as mer- curous chloride or mercuric oxide cause by their presence a large increase of current; this is especially marked in more concentrated (ca. normal) solutions of alkali hydroxides or chlorides, showing that these solutions then contain mercury in complex ions. Solutions of bromides and iodides, when standing longer over mercury, show the presence of such complexes so strongly, that only concentrations from decinormal downwards can be used694 PROCESSES AT THE MERCURY DROPPING CATHODE for electrolysis in this method. A trace of cupric hydroxide when added to an alkaline solution brings the galvanometer mirror off the scale even at 0.2 - 0.3 volt polarisation.If all such impurities of nobler metals are absent, the current passing the solution is very small and the oscillations of the mirror are slight, vanishing entirely at the potential at which the interfacial tension of mercury in the solution is maximal (i.e., ca. - 0.56 volt from the calomel electrode) ; this, then, tests the purity of the solution and furnishes a check for the right adjustment of the arrangement. Fig. 2 shows some of the characteristic ‘‘ current voltage ” curves as obtained by this method. All measurements were carried out at room temperature varying between 18O-20~ C. During polarisation ions are discharged at the drops, causing thereby a back E.M.F.equal and opposite to the polarising force ; the current due to this deposition is minute, 10-8 - 10-7 amp. per sq. mm., as will be observed on the horizontal part of the curves. However, at a certain potential of the polarised drop, termed the ‘‘ deposition potential,” and denoted on the diagrams by arrows, a sudden increase of current begins. Three processes might cause this increase, provided oxidising agents are absent and the ions in the solution are at their lowest stage of oxidation :- ( I ) Combination with mercury and diffusion into the drops. (2) A new phase formation at the mercury drop surface. (3) Diffusion of a deposited volatile product into the surrounding The first case occurs with all metals which possess chemical affinity for mercury, i.e., form amalgams ; their deposition potential into mercury, rN, is therefore much more positive than the electrolytic potential E.P.of the pure metal, the difference of these two potentials being a measure of the free energy A of amalgam formation. As this has been discussed already in the previous communication (1.c. PhiZ. Mag.), only some recent values are added here :- solution. A = rN - E.P. The metal E.P. =N Zn - 1’043 - -865 -178 Cd - -683 - ‘348 ‘335 Pb - ‘415 - -264 -151 Again, the affinity is expressed in volt-faradays (per gram equivalent). The values previously derived for alkali metals are about I volt-faraday (23,000 cals.) larger ; this agrees with the fact that alkali metals combine strongly with mercury, whereas zinc and lead dissolve simply ; the cadmium amalgamation, however, is of a more complicated nature, as is evident from the electrode potentials of such alloys.The second case will occur if the atoms of the metal in solution have a limited solubility in mercury or are insoluble, ie., have no affinity for mercury. Here it must be borne in mind that the deposit of such a metal forms on the mercury surface by no means a coherent piece of a metallic lamella, held together by the interatomic bonds of crystalline structure, but must resemble more a concentrated metallic vapour, with a much greater vapour tension and activity than that of the metallic rod. Consequently the “ solution tension ” of this-atomic-form of the metal must be much greater and the electrolytic potential more negative than in the compact form.0.2 0'3 0'4 0.5 0'6 0.7 0.8 0.9 1.0 1.1 1.2 1.3 1.4 1.5 1.6 1.7 1.8 1.9 2.0 Volts.P.D. from the calomel electrode. Fro. 2.696 PROCESSES AT THE MERCURY DROPPING CATHODE In such cases we must expect the electro-deposition in mercury to take place at a potential even more negative than is the E.P. of the metallic rod (an instance of, as it is sometimes denoted, (‘ cathodic passivity ”). This has been examined in the cases of iron and arsenic, which are known to possess little affinity for mercury. To investigate the process of iron deposition at the dropping electrode, solutions of n, o*In, and o - o ~ n hydrochloric acid, and O - I ~ hydrobromic acid, were saturated by dissolving in them (Merck’s) pure iron in the form of filings, or piano-wire, in an atmosphere of hydrogen.Repeated elec- trolysis with solutions prepared at different times gave uniform results, viz. the deposition from - FeCl, at - 1.1 I 5 v., from - FeCl, and -FeBr, at - 1-145 v,, and from the most dilute ferrous solution, to which platinised platinum had to be added to facilitate the dissolution of iron and potassium chloride to increase the conductance. The deposition started at - 1-167 volt from the n calomel potential. During these experiments it happened often that some of the iron filings fell upon the bottom mercury layer changing at once its potential from the ordinary calomel value to - 0.714 volt in - FeCl,, and to - -744 in the 0 . 1 9 ~ FeC1, or FeBr,. This value remained constant during electrolysis, and must be regarded as the potential of pure iron in the corresponding solution of ferrous ions ; it agrees with the values obtained by Richards and Behr and by Foerster.? I t is well known that even on metallic iron ferrous ions are deposited at a potential much more negative than that corresponding to its electrolytic potential ; however, the value was never found as negative as - 1.1 volt.The explanation of this ((cathodic passivity” is to be sought in an alloy formation with hydrogen (see Foerster, Z.G.) ; however, such a process seems improbable at the dropping cathode, since an increase of acidity of the ferrous solution, which would enrich the hydrogen deposition, had no marked effect upon the deposition potential of iron. The value - 1-1 15 v. (from the n calomel electrode) represents the deposition of iron forming a solution in mercury, whereas - 0.714 relates to the potential of metallic iron.The exceptionally small vapour tension of iron and its correspondingly very great energy of evaporation-which is estimated by Gruneisen * to be I I 3 kg.-cals., ie., z volt-faradays per equivalent at absol. zero-and, again, the well-known unstability of iron amalgam enable us to understand, that the potential at which single atoms are deposited in mercury must be more negative than the potential of solid iron. Applying the affinity relationship, we obtain for the affinity of metallic iron towards mercury a negative value-0.401 volt-faradays ; this means that metallic iron does not combine with mercury and that the iron amal- gam is metastable, it., unstable when in contact with metallic iron.Only when in a state corresponding in activity to a much greater vapour tension can the reaction of iron with mercury proceed spontaneously ; in fact, the amalgam can only be prepared through atomic dispersion, e.g., by elec- trolysis, and is very unstable. The case of arsenic is simpler, since it does not form an amalgam ; the sudden increase of current should therefore appear, when an amorphous n n n I I 0 I0 n I Zeitsch. f. p . Chem., 9 (Ig07), p. 301. Abhandl. Bunsew-Ges., Nr. 2 (1909). Coffetti and Foerster, Ber. d. Deutsch. clcem. Ges., 38 (1905), p. 2940. J Vcih. d. D.phys. Ges., 10 (1912), p. 327.PART I. THE DEPOSITION OF METALS 69 7 layer of arsenic begins to be deposited. Since such a thin layer of arsenic must have a greater vapour tension than the solid crystal, the arsenic deposition potential in mercury should be more negative than the electrode potential of metallic arsenic in the same solution.The latter is, however, very un- stable; measurements were carried out in that respect by Mr. Bayerle in this laboratory with a platinum wire upon which arsenic had been electri- cally deposited from arsenite solutions. The potential of this electrode, dipping into alkaline arsenite solutions in an atmosphere of hydrogen, is very variable in n sodium hydroxide solutions, but more stable in deci- normal alkaline solutions. Thus in o*og5 n sodium hydroxide with 0.00945 mols. of As,O, the potential fluctuates between - 0.893 and - 0.900; with 0.0312 mols.of arsenious oxide from - 0.882 to - 0.887 volt. In a decinormal hydrochloric acid with 0.02 mols. of AsZ03, the value varied between + 0.046 and + 0*091u. The deposition potentials of arsenic are much more negative than these values ; thus the affinity of arsenic for mer- cury again appears negative. I t will be noticed from the ‘( current voltage ” curves (in Fig. 2) that in the case of arsenic the turning points are indistinct and show too great shifts, which cannot be accounted for if the process which furnishes arsenious cations, As *, proceeds with great velocity ; this suggests a retardation due to imperfect ionic splitting. The third process which may occur at the polarised drop and is real- ised in the case of hydrogen deposition, will be-since closely connected with the overpotential phenomenon-discussed in Part 11.of this com- munication. Applications. Ionic EquiZibrtir.-The reversible ionic deposition at the mercury drops, which has been proved previously to proceed at potentials changing according to the formula- log c, where c denotes the concentration of metallic ions, and n their ionic charge, enables us to investigate by this method ionic equilibria. I t is applicable also in cases when the potentials of ordinary metallic electrodes cease to be sensitive to the presence of their ions, as in cases of passivity or other solution actions on the surfaces, be- cause the freshly deposited atoms in the mercury surface can never be passive nor are they attacked by the solution. To show the sensitivity of the current towards ions, a diagram of “current voltage’’ curves is given in Fig.3 on the electrolysis of lead solutions, which has been carried out by Miss H. Kadlcovii, and from which various conclusions can be drawn. The first curve shows the way the readings were followed, viz. by in- creasing in the critical region the voltage by 5 millivolts and thus finding the point at which decomposition begins. We observe that the change of the deposition potential of lead ions from a saturated solution of plumbous chloride to one diluted tenfold is 25 millivolts, just as found in con- centration cells with a divalent cation. From the values we can deduce, taking into consideration the solubility of lead chloride, that the deposition potential of a solution normal in plumbous chloride would be - 0.273 volts.The deposition potential of lead from a n sodium hydroxide solution sat- urated with yellow plumbous oxide is - 0.669. We thus obtain for the RT n698 PROCESSES AT THE MERCURY DROPPING CATHODE solubility product S = [Pb ‘1 . [OH‘]2 with concentrations expressed in gram-equivalents per litre, the formula : RT - log,, S = - 0.669 + 0.273 = - 0.396, 2 or S = 2-2 x 1 0 - l 4 ; taking into account the degree of dissociation of the n alkali as 7 2 per cent., S becomes 1.1 x 1 0 - l ~ . A similar value will be obtained from the 0.01 n sodium hydroxide saturated by lead oxide, since the shift of the deposition potentials is from - 0.669 to - 0.562 = 0.107 volt, which is very nearly RT log,, roo = 2 x 0.058 volt. We observe further that to a tenfold dilution of the lead content in the same alkaline solution corresponds a shift of about 2 8 millivolts, as would be expected from the equation of the ionic equilibrium Pb ’ + 3 OH‘ Pb(OH)‘,. That only such monovalent plumbite ions are formed, in other words that “plumbous acid” is monobasic, is evident, e.g.from the shift of potentials between the normal solution with 3 x 10-4 gr.-mol. PbO content and the decinormal solution with 5 x 10-* of PbO. The shift calculated for monovalent anions should be 92 millivolts, and for the dibasic plumbite anions, Pb(OH),”, it would be 29 millivolts larger still; the experiments show - 0.726 + 0.634 = 0’092, indicating that ‘cplumbous acid ” is mono- basic. The constant of the above reaction, which characterises the acidity of plumbous oxide can easily be calculated from any of these data; it is dis- [Pb(0H)3‘I = 1 .4 x I 0 1 2 and [Pb ‘1. [0H’l3 The more usual (( acidic solubility product,” which regarding the degree of ionisation, K, = holds for any dilution. holds only for saturated plumbites, becomes then [H ‘1. [Pb(OH),’] = 3 x 1 0 - l ~ . A similar research on zincates, done by the same student, reveals ana- logous relationships : from a normal sodium hydroxide saturated by metal- lic zinc dissolution the metal deposits at - I -330 volt, whereas for a solution normal in zinc ions at - 0.874 ; the difference leads to the solubility pro- duct = I x 10 l6. The (( zincic acid ” appears also to be monobasic, at least towards normal or more dilute alkalies, its acidic constant K, being 2.3 x 1014 and the “acidic solubility product” 4.3 x 1 0 - l ~ .The zinc oxide is therefore about as equally acidic as the plumbous oxide, It may be mentioned that it has been found impossible to obtain these relations from zinc rod potentials, since the metal becomes passive in dilute alkali. Thus this method may be suitably applied for the study of metallic complexes, such as amphoteric or acidic hydroxides, complex halides, amines, oxalates, etc. ; the available range of potentials extends from o to - 2 . 0 volts from the calomel electrode. QuaZitafive EZectro-ana&sis.-An important feature observable on the curves in Fig. 3 is the turn to the horizontal in solutions with a very small amount of the metal, down from 1 0 - 4 gram-ions per litre.Evidently the current of about I O - ~ amp., depositing every second at the cathode 1 0 - l ~ gram-ions from the few cubic mm. in the immediate neighbourhood of the Glasstone, y. Chem. SOL., 119 ( I Q ~ x ) , p. 1924, deduced from E.M.F. measurements S = 1-17 x 1 0 - l ~ in gram-ions per litre.PART I. THE DEPOSITION OF METALS 699700 PROCESSES AT THE MERCURY DROPPING CATHODE dropping cathode, which contain at this dilution some 1 0 - l ~ gram-ions of the metal, must soon exhaust the space round the drop, and the current intensity then depends chiefly on the amount diffused to. The metal in solution reveals itself in the form of a “ wave ” on the current-voltage curve quite distinctly when present in I O - ~ gram-mols per litre, and even smaller amounts are still detectable.The “ wave ” due to an admixture of 3 x I O - ~ mols PbO per litre of a normal sodium hydroxide solution begins at - 0-758v., and is about ten times more prominent than the “ wave ” due to a 3 x I O - ~ PbO impurity, which rises at about - 0.782 volts from the calomel electrode. The latter ‘‘ wave ” again is much more distinct than that due to 4 x IO-? gr.-mm. PbO. The same thing is observable in the case of dilute zincates in alkalis ; here, of course, the potential of polarisation at which the wave due to zinc deposition begins is much more negative than in the case of lead. It is obvious that by the determination of the position and size of such waves on the “ current-voltage ” curves traces of some metallic impurities in solutions can be identified ; in other words, that this method can serve as a means of qualitative electro-analysis.Some instances of this are graphically represented in Fig. 4. The curve (1)Zn was obtained from electrolysis ot a normal zinc chloride solution, which was prepared by the solution of the equivalent amount of pure (Merck’s b b pro analysi ”) zinc in normal hydrochloric acid, so that no metallic residue was left. Two big waves appear (at - 0.41 and - 0.59 volt), which bring the galvanometer deflexion off the scale before the actual deposition of zinc ions in mercury can be reached. The proper value of the zinc deposition potential is obtained only when an excess of the metal is treated with hydrochloric acid, and the presence of the metallic phase does not aIlow impurities of a more noble character than that of zinc to enter the solution.That - 0.874 is the true value of zinc deposition, and that it does not relate to another substance, is ascertained on subsequent dilutions, when only a regular shift (by about 28 millivolts per tenfold dilution) is observable, until the curves turn into the form of “ waves ” at about I O - ~ gr.-mols of Zn per litre. Such extreme dilutions have to be made up with a conducting solution, e.g. in a pure o - ~ n potassium chloride, which does not interfere with the deposition of zinc. If the resistance of the solutions is large, the slope of the ascending branch of the curve is lessened and the turning-point becomes indistinct. A tenfold dilution of the solution (1)Zn causes a great diminution of the two waves, so that the determination of the zinc deposition at - 0-9aov appears within the galvanometer scale.Evidently the very pure zinc contains at least two impurities of nobler metals in an amount of about I part in IOO,OOO, i e . 0.001 per cent. Judging from their potential positions, they are probably indium and lead. The first wave (at - 0.41) appears near the cadmium normal deposition potential ; however, sulphureted hydrogen produced only a faint yellow coloration in this solution, showing that there is so little of cadmium present that its deposition could not begin before - 0.50 volts. It has been ascertained later on, that the first wave is due to lead and the second to indium impurities. The next curve shows the electrolysis of a ferrous solution with an addition of 5 x I O - ~ mols of zinc chloride per litre.Zinc, of course, de- posits before iron, behaving at the mercury electrode as a more noble metal. In this way the curve (2)Zn was obtained.PART I. THE DEPOSITION OF METALS 701 d i4702 PROCESSES AT THE MERCURY DROPPING CATHODE The curve denoted ‘‘ Cd(r) ” shows the polarisation current obtained in a normal sodium hydroxide solution, which was allowed to act for three months on a thin strip of pure (Kahlbaum’s “ pro analysi ”) cadmium. Al- though cadmium apparently does not dissolve in alkaline solutions, yet it gives rise after some weeks to a precipitate of cadmium hydroxide, and the metallic surface turns black and seems corroded. The alkaline extract con- tains zinc, as it is evident from the large wave coinciding in size and posi- tion with an artificial zinc addition to normal sodium hydroxide (see curve below “Cd(2) ”).The alkaline extract when filtered and neutralised by pure hydrochloric acid gave the curve denoted “ Cd(2).” In this the zinc impurity is found to deposit in the expected place at - I-oov. (it has been also ascertained as zinc micro-analytically). The other impurity, producing a wave at - 0.810~. in the alkaline and at - 0.61~. in the neutralised solution, must be due to an amphoteric substance, perhaps cadmium hydrox- ide itself, if it possesses a faintly acidic character. The more markedly pronounced acidity of bismuth hydroxide, which manifests itself in dissolving in very concentrated alkalies is easily shown from the polarisation curve of normal sodium hydroxide to which some bis- muth hydroxide has been added; the increase of current due to bismuth deposition starts near - 0-5 volt. These examples suggest the possibility of working out a systematic method of analysis by means of the dropping cathode. Co ~ulusions. The experimental results given above show that a polarised drop of a mercury capillary cathode represents a reversible state of equilibrium. On each polarised drop the number of ions is instantly deposited sufficient to charge it by their solution tension to the balancing back E.M.F. The almost streamless condition of polarisation excludes the secondary effects of the current so far that the potentials at which ions are deposited from various concentrations change like the potentials of concentration cells with reversible metallic electrodes. Thus this arrangement allows an extension to the study of tri- or tetra-valent ionic concentration cells. Further, since the freshly deposited atoms are always in an active condition, the conclu- sion seems justified, that any retardation phenomena observed in the deposi- tion cannot be due to surface conditions, but must be rather sought in an imperfect ionic equilibrium of the solution (e.g. in the case of arsenic). This circumstance enables us also to decide whether some metallic com- pounds exist as a true or as a colloidal solution. The applicability of this method is limited on one side by the most ‘ unnoble ’ lithium deposition-potential at about - 2.1 volts from the normal calomel electrode, on the other it stretches to + 0.2 volt, where the oxida- tion of mercury begins. Finally it may be pointed out that the method can be equally adapted for the study of non-aqueous solutions (see Dr. Shikata’s subsequent com- munication). 1 E.g. Grube and Schwe;gardt, 2.f. Elektrochemie, 3 (I923), p. 257. The Physico- Chemical Institute Q/ the Charles’ Universig, Frague.

ISSN:0014-7672    

DOI:10.1039/TF9241900692

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. THE STANDARDISATION OF THE SIGN OF ELECTRIC POTENTIAL. BY ALFRED W. PORTER, DSc., F.R.S., F.INsT.P. MS. received December Sfh, 1923. The object of this note is to attempt to bring about uniformity in the sign to be attributed to the “potential” ofan electrode. Thequestion was raised in the American Electrochemical Society in I 91 7 and a committee was appointed to consider it. A report was presented in 1918 but no unanimity was obtained.With the conclusions of this report and with the greater part of a paper by Professor W. D. Bancroft which accompanied it I agree in the main though I do not always agree with the reasons brought forward. These are documents published in the Trans. Amer. Elecfroch. SOL, Vol. xxxiii. (I 91 8). Up to 1893 the convention adopted was in accordance with the funda- mental definitions universally adopted by physicists and engineers. Accord- ing to these definitions the potential of the points round an isulafed positive charge is positive. With this convention experiment shows that zinc is at a more negative potential than a solution of a zinc salt in contact with it. I t is important to emphasise that what is called a “single potential” is in every case a potential-difference between two things ; we know nothing whatever of absolute potentials. In a Zn - ZnS04 - CuS04 - Cu cell the zinc is negative to the solu- tion of ZnS04 while the solution of CuS04 is negative to the copper; so that, ignoring an uncertain difference between the two solutions the copper is positive with regard to the zinc.This is so whether we regard a path through the outside air or a path through the cell. The consequence is that when the copper and zinc are joined by a wire in which there is no further source of electromotive forces, a current flows in the wire from copper to zinc. We cannot deduce this so easily by considering a path through the cell because the cell contains an electromotive force. In the American discussion the suggestion was made that since electro- chemists are concerned with the cell itself we ought to consider primarily the current in fhe ceD, and since this flows from zinc to copper the zinc must be positive with respect to the copper.This suggestion is so ingenuous in its ignoration of scientific facts that it scarcely deserves mention. I t is equivalent in the corresponding problem of the circulation of water in pipes to regarding the water entering the pump as being at a higher pressure than at the outlet. This convention could, of course, be adopted; but only by turning the meaning of pressure upside down. 703704 STANDARDISATION OF SIGN OF ELECTRIC POTENTIAL The physical convention was disturbed by Ostwald in 1893, but it was returned to by Foerster in 1915 in correspondence with a decision of the Bunsen Gesellschaft.This would have probably ended the matter but for the strong advocacy of the opposite notation by G. N. Lewis from 1913 onwards. Owing to the great amount of important work done by Lewis and his school the adoption by him of the non-physical notation cannot fail to have an influence, and there is a danger of it spreading amongst chemists. I wish to point out clearly that it is entirely opposed to physical practice and to urge that it is exceedingly undesirable that the two conven- tions should continue to co-exist. Now I am not aware that physicists have been pressed to change their custom. If adequate reasons were forthcoming I suppose that even this might be possible though it should be recognised that it would upset the whole of physics and electrical engineering.Lewis’s argument is based on a desire to make the chemical and electric potentials have the same sign. But surely Bancroft is right in stating that they ought to have opposite signs. In a Daniel1 cell there is a diminution of free energy when the electric displacement is from zinc to copper in the cell ; and there is an equal gain in electric energy. The equation for the cell may be written as Lewis desires Zn + ZnS04 (sol) 3 CuS04 (sol) 3 Cu = + I -05 volts, indicating that there is a gain of electric potential when the electric dis- placement is from left to right along the path indicated. So written all that is indicated is that the current rises in potential on the whole in passing from Zn to Cu through the cell.Further investigation shows that ZnS04 (sol) is positive to Zn while CuS04 (sol) is negative to Cu. Hence there is a diminution of free energy in each of the left-to-right steps as well as on the whole. [Of course, the equation only applies to approximately equilibrium conditions ; it is unnecessarily complicated if we consider differences when the current is large or small as Bancroft does.] All this is perfectly well represented by the equation written out in accordance with Lewis’s practice but without introducing any innovation into the convention of sign adopted by physicists. I n conclusion there are two intimately connected questions on which I wish to make general remarks :- (i) I am of the opinion that confusion of thought is encouraged by speaking of potentials when only differences can be observed.To talk of the potential of zinc or of zinc sulphate solution is nonsense; we can only speak of their differences when in contact. Even then it is a moot point whether by difference we mean the excess of the potential of the solution over the zinc or of the zinc over the solution. (ii) I am of the opinion that confusion is also introduced by regarding the change of the energy and free energy of a system as positive when it is a decrease. This is utterly at variance with the conventions of the calculus in terms of which the changes are expressed. I t is impossible to build up a logical system until this custom is changed. Physicists have never adopted it. Chemists unfortunately are not consistent even with themselves. The gain of energy in evaporation (i.e. the internal latent heat) is taken as posi- tive whereas the Zoss of energy in a chemical reaction (i.e. the L L heat of reaction ”) is taken as positive. Much would be gained by taking the gain of energy as positive. This I have done and for explicitness have called it the latent heat of reaction, in anything I have written on the subject.

ISSN:0014-7672    

DOI:10.1039/TF9241900703

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. DETERMINATION OF THE AFFINITY CONSTANTS OF BASES BY THE HYDROGEN AND QUINHYDRONE ELECTRODES. BY J. N. PRING, M.B.E., D.Sc The data at present available on theaffinity constants of bases are de- rived from measurements made by a number of workers employing difterent methods such as conductivity, avidity or the competition of two bases for a limited quantity of acid, catalysis, the change of solubility produced on the addition of an acid, and the amount of hydrolysis of salts of the bases With a strong acid as measured by the concentration of free base acquired in a layer of benzene.On comparing the different results obtained, many dis- crepancies are observed. The methods employed are all more or less indirect and it is not clear that the difl‘erent properties such as solubility or catalytic relations are not affected by the operation of other factors which are not understood and are not allowed for. In cases where a series of bases has been investigated by the same method, the results may at most be expected to have a comparative value amongst themscIves which will not strictly apply when the measurements are compared with another series determined by other methods.Though basicity can be regarded as the indirect manifestation of some definite electrical feature of the molecule, it does not appear certain that an absolute standard can be determined. The hydroxyl ion concentration which normally defines the basicity cannot be measured in exactly the same medium in the different cases. The presence of the undissociated base, for instance, affects the dielectric properties of the solvent, the degree of hydration of the ions and probably such factors as association which, apart from the operation of mass action, determine the apparent concentration of hydroxyl ion and modify in a marked manner the value of the dissociation ‘‘ constant.” I n commencing the present investigation, it was hoped to be able to relate measurements of basicity of a large number of compounds to a de- finite uniform standard which can be directly applied and used to give accurate values with substances of a low degree of basicity.The procedure which has been followed consists in measurements of the H ion concentration of solutions of bases either alone or after adding a definite amount of free acid. A direct evaluation is thus obtained of the degree of dissociation or hydrolysis. Measurements have been made by the following three methods which are compared with each other (I) the hydrogen electrode, (2) the quinhydrone electrode as developed by Prof. 705706 DETERMINATION OF AFFINITY CONSTANTS OF BASES Biilmann, and (3), estimations by the colour changes given with selected indicators. In practice, two difficulties arise in determining low values of H ion concentration in a solution which is not '' buffered" (ie., the hydrogen ion of which is not stabilised). The first is the difficulty of avoiding contamina- tion by the containing vessel and the second is the high resistance of the solution which makes a sharp balance on the potentiometer impossible.The method adopted throughout the present work with weak bases consists in taking a solution of hydrochloric acid, as being highly dissociated, and of a definite concentration, adding a known excess of base and measuring the fall in the H ion value from which the hydrolytic and basicity constants are estimated as described below. The investigation has been limited to measurements with substituted amino-bodies. The equilibrium in these cases may be expressed by the equations :- X,N + H,0,3X,NHOH X3NHOHZX3NHg+ OH' The reaction constant of the first equation is given by the expression ' (1) [X3NHOHI [&N] k, = and that of the second by : [X,NH'] [OH'] [X3NHOH] Throughout this paper, the small k is used to denote constants in which the mass of water is taken as unity, while, with the capital K, the con- centration in grm.equivs. of all the reacting bodies is taken into account. As in the case of ammonia in aqueous solutions, there is no information with amines generally of the ratio of the anhydrous compound merely held in solution to the amount of the hydrated but undissociated compound. The two distinct constants which enter into the basicity of these com- pounds as measured are thus represented by kl and Kb, the former denoting the facility with which the nitrogen passes from the trivalent to the penta- valent form, and is sometimes termed the amholytic constant, and the latter expresses the true basicity in analogy with metal bases.The basicity as measured is expressed by the equation :- * (3) [XaNH'] [OH'] [X3N] + [&NHOH] ' kb =Z The determinations of the values of K O are made from the degree of hydrolysis by considering the following further relations :- H20 H' + OH', whence A, = [HI [OH']. . (4) and X3NHAzX3NH' + A whence [X3NH*] [A'j [X3NHA] * K, = Hydrolysis may, in general, be expressed by the following ionic reaction :- B' + A' + H,O 9 BOH + H' + A' where B' is the cation of the base, ie., in this case X3NH' while BOH is here X3N + X3NHOH.BY THE HYDROGEN AND QUINHYDRONE ELECTRODES 707 From this equation the hydrolysis constant as measured is given by the [HI {[X3NI + [X3NHOHl) formula :- a (7) X3NH‘ kh = In accordance with equations (3) and (4) the value of kb is then derived from the expression :- * (8) k b = z k* .Rela fion between Apparent and True Basicip Constants. Case I . Assuming complete dissociation of the acid and salt, we have, from equations (8), (4), (7), (I), and (2), the relation :- [HI [OH‘] EX$H’I PL3NH’I [OH1 + [WH’I [OH‘] Kb kb =I kb, the apparent basicity constant as quoted in tables is identical with the value which would be obtained by a conductivity method as measured by Bredig.1 The apparent ionisation constant for the base kb thus bears to the true constant Kb the ratio --!- where k1 is the ratio of hydrated to un- hydrated base.Apart from the influence of medium already referred to in determining ionisation, the determinations of kb can only be regarded as of relative value. Deductions as to variation of ionisation with constitution cannot be made with accuracy since it has not yet been found possible to determine values of Kb. I t may be noted, however, that so-called “aminolytic” constants have been obtained by Goldschmidt and Salcher by measuring the a n i t y of amino bases for acids in non-aqueous solvents. The values thus obtained may be expected to bear some relation to the indeterminate constants kl in the present work. k I + A,’ Case 2. If complete dissociation of the base is not assumed then whence If, however, acid and salt are equally dissociated, then kb = ~ Kbkl as before.I 3- kl In any other solvent when the active mass of the water can be con- sidered constant, though different from the values in pure water, a series of ‘‘ hydrolytic ” and basicity ” constants can be obtained, having the same significance for that solvent as the ordinary values have for water, the alteration in active mass of the water not being taken into account. Zeits. phys. Chem., 1894, 13, 289. Ibid., 1899, q, 89 ; 1921,w, 116.708 DETERMINATION OF AFFINITY CONSTANTS OF BASES The procedure in the present measurements in which an excess of a weak base was added to a definite concentration of hydrochloric acid was to evaluate ki, in accordance with equation (7). For instance when a 0.01 n.solution of hydrochloric acid is employed, the expression takes the form : Y represents the amount of base originally added and the H ion concentra- tion is calculated from the expression :- ' (13) Tl - =a 2'oo 4- 0-0002 T log = where 7rl is the e.m.f. of the solution containing base and x2 that of the solution with 0.01 n. acid alone. In the case of acids of other concentrations, the necessary adjustment is made in the coefficients. The basicity constant kb is derived from the hydrolysis constant by the expression kb = 5 as in equation (8). from the above formula, the assumption is made that the acid and salt present are completely dissociated. I f incom- plete dissociation is allowed for the above expression takes the form :- kh In the derivation of [He] (Y - (P3.1 + w31)) P.1 kh = Calculations have been made in a number of cases using the solvent acetone-water, where dissociation is only partial, of the correction which it is necessary to apply to the values of kh as determined from equation (12).The correction is found to be negligibly small and the use of equation (12) in the form given is justified. Use of Sohenis 0 t h t h n Wafer. Most of the bases investigated have a very limited solubility in water and the possibility of the use of other solvents either alone or together with water comes into consideration. With regard to the basicity constant as derived from the expression kb = -, the relation, as shown later, is largely determined by the con- centration of water since in acetone-water solvents k , diminishes to a much higher degree than the fall in concentration of the water.In other words, water is a better dissociating medium for itself than are acetone-water kw kh - [H'l [OH'] is accordingly only constant in a mixtures. The relation K, = K O given medium. With reference to the alternative procedures of adding a weak base either (A) to water or (B) to a dilute acid solution a calculation is made below to determine the comparative change in H ion concentration which is produced by the two methods :- A. E f e c t on the H ion Concentration of adding a Weak Base to an Unbufered NeutraZ SoZution (i.e., Pure Water). Fkndamenfad Epuafions :- [B7 [OH'] (dissociation of base). BOH k, = [H'] [OH'] ; Kb =BY THE HYDROGEN AND QUINHYDRONE ELECTRODES 709 [He] + [B'] = [OH'] (electrical neutrality). or Applying this as an example to the case of a base of which Kb = 10-l~ and the solution of which is 0.02 ta.In the case of pure water we have [He] = Jz= 10-7 (at 18" C.) or pH = 7.00. With the base we have from the formula above 10-'8 [H'l d ( I o - 1 3 x 2 x 10-2) + 10-14 - - dJ0.83 X 10-14 = 0.91 x 1 0 ' ~ or pH = 7-04; the e.m.f. obtained is therefore 0.04 x 0.058 = 0.0024 volt. B. Efecf on the iY ion Conrenfration of adding a 0.02 n. Solution of a Base of K b = 10-l~ to 0.01 n. HcZ. or EH.12 + O'IIH - 10-' = o - 0'11 lk J O ~ O I 2 I + 4 x 10-3 :. [H'] = 2 - 0.11 + 0.127 - - = 0.0085 = 8.5 x I O - ~ , whence pH = 2.07 2 :. e.m.f. = 0.07 x 0.058 = 0-0041 volt. Apart from the advantages of working with solutions of higher con- ductivity and less susceptible to contamination, it is seen that a larger displacement of H' ion concentration results when a weak base is added in excess to a dilute highly dissociated acid solution than when added to pure water.7 I o DETERMINATION OF AFFINITY CONSTANTS OF BASES As a second example, in the case of a solution of 0.01 n.bass added to 0.002 n.. HCl, the displacement of potential is calculated to be 0.0069 volt. In determining the optimum concentration in order to give the largest reading, regard must be paid to the greater effect of a given error in the weighing when employing the more dilute reagents. Exjerimentad. Results are given of measurements made (A) by the hydrogen elec- trode and (B) by the quinhydrone electrode in the following solvents: ( I ) water, ( 2 ) acetone and water in the ratio of 5 0 : 5 0 by volume, and (3) acetone and water in the ratio of go : 10 by vo1ume.l A comparison of the results was then made with the values deduced from the colour change of certain indicators which had been graded and standardised by means of buffered solutions of known H ion concentration. A.Measuretnenis wiih Pt - H2 Elecirodes. The Pt electrodes were of the stationary type consisting of thin rec- tangular strips of metal 2 cm. x I cm. and were employed in the usual manner by partly submerging in the electrolyte to be measured and maintaining an atmosphere of hydrogen. In the preparation of the electrodes the platinising was carried out with a solution of H,PtCl, containing a trace of lead acetate.Though this has been sometimes condemned, the use of lead acetate was found necessary to make the platinum adhere. Electrolysis was conducted with a potential of 2 volts, the electrodes being joined in parallel and an auxiliary electrode used as anode, so that the circuit passed in one direction only. An interval of one to two minutes was found sufficient for the platinising. Electrodes with thin deposits obtained a steady potential in subsequent measurements in a much shorter interval than with thick deposits. To check the con- dition of the electrodes these were immersed in identical acid solutions and connected in the chain :- Electrode I in acid / KC1 / Electrode I1 in same acid. The acid was applied in various strengths and the e.m.f., measured by means of a potentiometer, did not, in any case, exceed 0.3 mv.Before each measurement, the treatment applied consisted in washing the clec- trodes in distilled water, exposing to cathodic hydrogen, and then im- mersing in the solution to be measured. I n cleaning the electrodes between each measurement care is required to avoid allowing the electrode to dry. After use with some bases, it was found advantageous to wash the electrodes in acetone before exposure to cathodic hydrogen. The following chain was used in the potential measurements :- N PtH,, - HC1 KC1 (saturated) 1 & HC1 + - BOH, Pt. H2. I 0 0 " I SO In order to serve as a check on secular variations measurements were repeated by removing and cleaning one electrode and renewing the solution on one side only at a time, thus two separate measurements were made with each half cell when connected with different samples of similar solutions.After several measurements with any particular solution, the electrodes were reversed and the measurements repeated. Throughout this work, these ratios represent the volume of water in 100 C.C. of the mixture.BY THE HYDROGEN AND QUINHYDRONE ELECTRODES 7 I I Care was devoted to obtain the bases used in the highest possible degree of purity. In most cases the compounds were subjected to a final distillation in vacuum and stored in evacuated tubes. It was found in instances when the solution of the base had become discoloured that the e.m.f. readings were irregular and large differences were obtained between stationary solutions and those through which hydrogen was passing, the higher e.m.f.being obtained with the former which agreed more closely with the potential given by a fresh clean solution. The results may be explained on the assumption of the discoloration being due to oxidation and subsequent reaction followed by reduction to the original base, with loss of hydrogen, in the vicinity of the electrode. I n cases where the solution remained clear and colourless, these differences of potential were not observed. I t may be noted as a generalisation in the use of hydrogen electrodes using a constant comparison electrode that, where variable potentials are given, the values are too low. A typical experiment in this series showing the time required to reach the equilibrium stage is shown below:- Arrangement of cell : The cells were in all cases mounted in a water thermostat.+ Pt . H,, 0*002 HC1/ saturated KCI / 0'002 HCl+o*oxp. amino benzoic acid / Pt . HI-. The readings in the time column were taken from the time when the hydrogen was first passed through the solutions. Time. Mins. 0 24 5 74 124 I71 e.m.f. Volts. - -0.041 + 0.015 0.020 0.0235 0.0240 Time. Mins. 224 27i 324 424 e.m.f. Volts. 0'0245 0.0250 0.0250 0.0250 of the different bases. Temperature, 25' C. The results with other bases are tabulated below under the headings B. Determination of Afinity Constants of Bases 8y the Quinhydronc In the application of the quinhydrone electrode as described by Biilmann for the determination of H ions in solutions of bases, the first consideration which arises is the possibility of chemical action between the constituents of the quinhydrone and the bases used.In aqueous solution, quinhydrone dissociates into quinone and hydroquinone and the dissociation increases with temperature. Reference to the literature indicates that both quinone and hydroquinone react with many organic bases. The reaction of quinone is generally the more mar!ted and gives highly coloured sub- stances of the induline type. I t is pointed out by Biilmann2 that hydro- quinone is a very weak acid with a dissociation constant of 1.1 x 1 0 - l ~ and in presence of basic solutions the potential relation is modified. I t must also be remembered that many aromatic bases are readily oxidisable, yielding complex bodies, and that quinone is a compound which will cause such oxidisation.The effect of such reactions will be, in the first place, to decrease the concentration of base, and secondly, to disturb the equi- librium between the quinone and hydroquinone and thus alter the e.m.f. obtained. I t was noticed, in the present experiments, that the solutions containing quinhydrone, base, and hydrochloric acid became coloured after EZecfrode. Ann. de Chintie, 1921, IS, 109 ; 16, 321. Ibid., 1921, 15, 119.712 DETERMINATION OF AFFINITY CONSTANTS OF BASES Pt. Quinhydrone 0.01 n. HCI Pt. Quinhydrone (Saturated) KC' I o*oxn. HC1 -I- 0.02M. Base In some cases the concentrations of acid and base were varied. The same solvent was used throughout the chain. The quinhydrone, which was prepared by crystallising the compound from a solution of an equi- molecular mixture of hydroquinone and quinone, was initially employed of the concentration recommended by Biilmann, i.e.0.005 M. but in some cases this was modified. The reaction vessels consisted of wide-mouth bottles of about 2 02. capacity and fitted with rubber stoppers with holes for the syphon tubes which connected the electrode solutions to two inter- mediate vessels containing the potassium chloride solution employed to annul the boundary potential. The electrodes of smooth platinum foil each about I cm. x z cm. were attached to glass tubes also leading through the rubber stoppers of the two vessels. The procedure adopted was similar to that employed with the hydrogen electrode. A solution of hydrochloric acid of definite concentration was employed to surround the comparison electrode while the second electrode was immersed in a solution containing in the same solvent definite concentrations of hydrochloric acid and the base under measurement.Each of the solutions contained quinhydrone of a definite concentration. The p.d. between the electrodes which generally became steady within a few minutes was then measured. In general the electrodes, between each measurement, were merely washed in water or acetone-water. I t was found that after more severe treatment such as heating or immersion in strong acids a longer period was required to attain equilibrium in the subsequent measurement. In cases where reaction occurred between the base and the quinhydrone, a gradual change of e.m.f.resulted. In some of the cases, approximate values were determined by noting the change of potential with time and extrapolating back to zero time. Results of this type which are only approximate are recorded in the table in brackets. The measuring vessels were placed in a water thermostat which was usually regulated to 2 5 O C. I t is convenient to classify the bases as primary, secondary and tertiary, the tertiary giving the best results as regards the constancy of e.m.f. measurements and agreement between the values of Kh and kb with those obtained by the ordinary hydrogen electrode and by the colorimetric method. The secondary and primary bases are not so satisfactory although a conclusion can be drawn in most cases. No evidence of electrical leaks was noticed.BY THE HYDROGEN AND QUINHYDRONE ELECTRODES 713 In the case of bases which gave evidence of interaction with the quin- hydrone, the measurements were much facilitated by diminishing the con- centration of the hydrochloric acid added.C. Determination of A@n@ Consfanis of Bases from N ion Vahes as This method is intended to serve mainly as a confirmatory test on the determinations made by potential measurements. I t is not, in general, considered to have the same precision as the e.m.f. methods on account of the higher degree of refinement of a potentiometer measurement compared with the gauging of a colour change. Moreover, in the case of indicators minor errors are to be expected through specific reactions between the indicator and the solutions such as protein aud colloid adsorption effects. The method as applied is a development of the work of Siirensen and of Walpole on the preparation of standard buffer solutions which contain a definite and stabilised H ion concentration.The indicators which have been applied include the following which are described by Clarke and Lubs,1 bromcresol purple with a pH range of 5-2 to 6.6, and brom- thymol blue of a range 6.0 to 7.6. In addition to these, isopicramic acid of a pH range from 4-4 to 5.6 and methyl orange of a pH range from 3.1 to 4-6 were employed. In applying this method, the procedure adopted consisted in adding a definite quantity of indicator to a solution containing a definite concentra- tion of hydrochloric acid to which a given amount of base was added as in the measurements by the hydrogen electrodes. With each indicator, a series of tubes containing buffered solutions of definite H ion concentrations was so arranged as to give variations from tube to tube of a pH value of 0.2.The tube containing the acid and base under measurement together with the same quantity of indicator as the comparison tubes was then matched against the series of standards. The basicity constant is then determined via the hydrolytic constant from the H ion concentration as in the other methods. In this way results can be reproduced to within a pH value of Estimu fed by fh Colour Change of Standardised Indrirafors. 0'1. Dissociation Constant of Wafer in Satvents other than Wafer. I t is found that the value of the dissociation constant of water is modi- fied to a large degree by the nature of the solvent.Measurements were made by R. Lowenherz2 on the influence of admixture of alcohol on the electrolytic dissociation of water. Determinations were made of the p.d. of hydrogen electrodes placed in solutions of hydrochloric acid and sodium hydroxide of definite strengths. The assumption is made that the dissocia- tion of acid and base is the same in all the solvent ratios employed. The measurements at 20' C. indicate that in water the amount dissociated is 1.075 x 10'~ gram mols. per litre, and in 99.8 per cent. alcohol, 2.88 x 10-l') gram mols. per litre. The constant K', for the expression [H'l COH'I W2OI where [H,O] is the concentration of water in gram mols. per 6.6.is such that JK', = 4.1 x 10" for water and 0.3 x 10-7 for 99-8 per cent. alcohol. In order to represent the values of K, in the usual manner in which 1 Cf. W. M. Clark, '' The Determination of Hydrogen Ions," 1922, Baltimore. 2 Zeit. phys. Chem., 1896, a, 296.7 14 DETERMINATION OF AFFINITY CONSTANTS OF BASES H, HC1 I sat. KC1 1 I NaOH ~ Pt. I H2 R T c, = F log - c2 where cl is the H ion concentration of the hydrochloric acid and c2 that of the alkali. The OH' concentration in the alkali is also known from the conductivity and the ionic product [He] [OH3 = K , can be determined. The saturated KC1 is assumed to eliminate the boundary potential. cl is known and c2 is measured. 50 : 50 ACETONE-WATXR. Conductivity of hjldrochloric acid at 20' C . Dilution. U.5 50 500 I0 I00 1000 2000 m Equivalent Conductivity. A. I 99'5 1x5 168.5 183 239'6 254'2 262'4 (270) ~ ~~~~ Degree of Dissociation. a. 0'3 7 0'43 0.63 0.68 0.87 0.95 0'97 - Ka. 4'4 x 10-2 3.2 x 10-2 2'2 x 10-2 1-5 x 10-2 1'4 x 10-2 1.8 x 10-2 1.6 x IO-~ Conductivity of NaOH at 20' C. Dilution. 20 50 500 I00 I000 00 1 Equivalent Conductivity. I-- -I I Degree of Dissociation. 62.4 70'7 76'3 81.6 80-2 (84) 0'74 0.84 O'gI 0'97 - I1 x 10-2 8.8 x IO-? 8.8 x IO-~ 7'0 x 10-2 - E.Mi? Measuremenfs.-MMeasurements with the cell described above were made with 0-01 n. HC1 around one electrode and 0.0 137 n. KOH aroundBY THE HYDROGEN AND QUINHYDRONE ELECTRODES 715 the second electrode, the solvent throughout being 50 : 50 acetone water. The potential given was 0.665 volt, whence [He] OfO*O137 n.KOH = 2.4 x 10-l4 a = 0.88 for o.0137 n. KOH (by interpolation), [OH'] = 1-21 x 10'~. whence Hencek, = [H'] [OH) = 2-4 x I O - ~ * . 1-21 x I O - ~ = 2.9 x 10-16. :. K, = 1-04 x 1 0 - l ~ . A second similar determination with 0~00154 a. NaOH gave an e.m.f. of 0.610 volt corresponding to a value of kw of 3.03 x 10-18, or K, = 1.09 x 1 0 - l ~ . go : 10 ACETONE-WATER. Cotoducfivity of HCl at 20' C. Dilution. 1 Equivalent Conductivity. 1 Dissociation. I I I I0 50 250 1250 2500 5- I0,Ooo 20,000 - 23.0 46.1 75'8 98-0 106.0 I 10.6 I 16 1x8 I20 0'1 g 0.38 0.63 0.82 0.88 0'93 0'9 7 0.98 - 4.5 x 10-3 4-8 x IO-X 4'3 x 10-3 3.0 x X O - ~ 2.7 x I O ~ 2'2 x 10-8 2.7 x 10-8 3.2 x 10-~ - I I No satisfactory measurements of the conductivity of potassium or sodium hydroxide could be made in 90 : 10 acetone water on account of their spar- ing solubility and the probable interference of residual traces of carbon dioxide.EMF. Measurements.-The arrangement of cell and the potential given are shown in the following table :- Pt. H2 0.01 n. HC1 / saturated KCl / 0*00088 n. KOH, H2, Pt. 90 : 10 acetone-water. E.M.F. 0.800 volt at 15' C. The pH value of the 0.00088 n. KOH solution is accordingly 16.33. For the purpose of the approximate calculation, the dissociation of the KOH is then assumed to be the same as that of the HCl at a similar dilution, i.e., = 0.8 [OH'] = 0.00088 x 0.80 = 0~00070 hence k, = [H'] [OH'] = 0~00070 x 4-7 x 1 0 - l ~ = 3-3 x I O - ~ O . :. K, = 6.0 x IO-~'. +--- A second determination using 0.00177 n.KOH in the same solvent The mean value is, accordingly, k, = 2-0 x I O - ~ O or K, = 3-6 x I O - ~ ~ . Relation between Dielectric Constants and Dissociation Constmts.-As far as available data allow, it is of interest to consider the relation holding between dieletric and dissociation constants in the case of the solutions employed in these measurements. A possible way is thus provided of gave a value for k, of 1-4 x IO-2O.716 DETERMINATION OF AFFINITY CONSTANTS OF BASES Percentage of acetone in mixture . Dielectric constant . . . . determining the factors which control the values of k, and kb. The relation between these two constants has been investigated mainly by E. Baur,' Kruger,* and by WaldemY In all cases the relation derived can apparently be expressed in the form of the equation :- 4 - - - - CIS K, e23 where k1 and k2 are the dissociation constants, and and C~ the dielectric constants of the solutions.LowenherzS finds in the case of alcohol-water mixtures, a close relation between the dissociation constants of the water and the fourth power of the dielectric constants. In the present work values for acetone-water mixtures have been taken from the data obtained by D r ~ d e , ~ according to which the folIowing values apply at 20' C. :- -~ ~~ 0 50 90 81 53 27 VALUES OF HYDROLYSIS CONSTANTS, Rk. Base. Diethylaniline . . . p-phenylencdiamine . . Dtmethyl i-toluidine . Bcnzidine -. . . Dimethylaniline . . Ethylanilinc . . . j-toluidine . . . Methylaniline . . . Aniline . . . . p-nitroeo-dimethylaniline .Glycocoll . . . . p-amino-benzoic acid . Water. 2-3 x 10-7 8.0 x 10-7 2'5 x 10-6 6-6 x IO-~ 7-6 x 1o-O 7'0 x I 0 4 7-6 x xo4 2 x 10-8 2.5 x I O - ~ 5 x 10-5 3'5 x 10-8 5.6 x I O - ~ Solvent. go: I 0 I 50 : 50 Acetone-Water. Acetone-Water. I 5 x 10-6 (2 x 10-6)* 3 x 10-6 1'5 x 1o4' 9.0 x IO-~+ 2 x 10-** 4 x 10-6 I x 10-4 I x 1 0 4 1.5 x 10-4 I x 10-3 2 x 10-3 3-0 x 10-6 (3.0 x 10-7)* 6.0 x IO-~ 6.5 x 10-6 2.5 x I C ~ + 5'5 x 10-5 6.0 x IO-~ 2-0 x 10-4 4.0 x 10-4 9-0 x 10-4 - - 2 e i t s . j ' . Elcktrochcmie, 1905, 11, 936. Ibid., 19x1, 17, 464. Zeit. phys. Chrm., 1897, 23, 312. SLeit. phys. Chcm., 1go6,~,228; 55, 707; 1920~94. 263. 'Lot. c i t .TABLE 1. AQUEOUS SOLUTIONS. Method. Grm. Mols. per Litre. Constituents of Solution in Measuring Half-Cell. k, Mean or Probable Value.oncentra- tion of HCI in Standard 3alf-Cell. 'empera :ure OC. 17'5 25 PH. kh- kb. 3-1 x 10-l~ 3.0 x 10-l~ 1.0 x I O - ~ 1.0 x 10-9 1.0 x I O - ~ 1.0 x 10-9 1-7 x 10-9 1.45 x 10-9 1.6 x I O - ~ E.M.F. Base. Zoncentra- tion of Base. 0.010g 0'02 Concentra- tion of QH. [HCI.] 0'002 0'01 0'002 0'002 0'002 0'002 0'001 0'001 0'001 iaturated (0.018) - Aniline . . Quinhydrone electrode Hydrogen electrode . "95 x 10-5 3.1 x IO-~ 3-65 x IO-~ 3-71 x I O - ~ 1.0 x 10-6 1.1 x 10-5 3'49 x 10-6 D-72 x IO-~ 0.67 x IO-~ 0'002 0'01 1.0 x 10-10 a t 20' C. "0 x 10-9 at 20' C. saturated saturated - - Q.H. electrode , . Q.H. electrode . . Hydrogen electrode . Colorimetric . . I8 I9 25 27 5 '09 5 '74 1-98 4-95 p-Toluidine . 0'002 0'002 0'01 - 0'002 0'002 - 0'002 0'002 0'002 0'01 0'002 0'002 - 0'01 0'01 0'02 0'02 0'0011 0'00 I I 0'001 I Benzidine .. Q.H. electrode . . H. electrode . . Colorimetric . . (0'102) 0.099 I 4-46 4'37 1'4 0.005 I - saturated - 21 25 25 21 27'5 ___ 19 25 25 25 20 r.6 x 10-9 at 25O C. ~_ c.5 x 10-12 at 20' C. "0 x 10-8 at 25O C. p- Amino-benzoic acid Q.H. electrode . . H. electrode . . 1-5 x 10-12 3-8 x 10-12 0'002 0'002 3-11 3.11 __ 6-16 6-07 6-68 6-32 6'55 5-6 x 10-3 3'2 x 10-3 1-05 x I O - ~ 0.85 x 0.83 x I O - ~ 0.72 x I O - ~ 0.89 x 10-6 0'0 I 0'0 I 0'01 0'02 0'01 0'01 0.005 0'02 0'02 saturated - - - - 0.67 x 10-8 1'2 x 10-8 1'2 x 10-8 1-5 x 10-8 0.85 x 10-8 p-phenylene- diamine - Q.H. electrode . . H. electrode . . . . 19 99 9 9 9 9 . . Colorimetric . . 0'002 0'01 0'002 0'002 0'002 0'01 0'01 0'002 0'002 0'002 0'01 2.8 x 10-l~ 3'5 x 10-1: 2-56 2'54 3.0 x 10-22 at 25' C.23 25 4'7 x 10-3 3.0 x 10-3 1.3 x I O - ~ 1-4 x I O - ~ 1.9 x I O - ~ 2 x 10-t Q.H. electrode . . H. electrode . . ~- Glycocoll . . 0.005 - 0.018 0.018 - - 0.005 0.005 0.005 saturated - - 0.005 0.005 - - 0.005 0.005 0.005 saturated - - 0'01 0'01 0'002 0'002 0'002 - Methyl aniline . Q.H. electrode . . I8 I8 25 24 4'9 x 10-1c 5'5 x 10-1( 5 x 10-1( 4-6 x 10-l~ 0'01 0'01 0'0 I 0'02 5'47 5'45 5 '33 4'7 5.0 x 10-10 at 20° C, - ---- -_ __ 1'0 x 10-9 at 20' C. 4'5 x 10-8 at 25' C. _._______ 1.0 x 10-9 at 25O C. 3.0 at 20' x 10-9 C. -- I x 10-10 9 9 9 9 H. electrode . . Colorimetric . . Q.H. electrode . . 9 9 9 9 - * 19 9 9 - 9 9 9 9 * * H. electrode . . Colorimetric . . 5.16 5'7 4-48 5'78 5'0 5-15 6.9 x I O - ~ 6.0 x IO-€ 6.6 x IO-~ 1'1 x 10-: 7-1 x IO-~ 3'5 x 10-6 Ethyl aniline .1'0 x 10-9 1-15 x 10-D 1-95 x I O - ~ 1-15 x I O - ~ 1.0 x I O - ~ 1-55 x I O - ~ 4.1 x 10-* 4.8 x 10-8 4-8 x I O - ~ 4-2 x IO-~ 7.1 x 10-9 1-3 x I O - ~ 0.8 x I O - ~ 0'95 x I O - ~ 0-98 x 10-9 1.05 x I O - ~ 0'01 0'0 I 0'01 0'002 0'01 0'01 0'01 0'01 - 0'01 0'01 0'01 0'002 0'01 - 0'01 0'0 I 0'002 0'01 - 0'002 0'02 0'02 0.013 0.01 0'02 0'02 0'01 I 0'011 0'01 I 0'01 I 0'01 0.005 0'01 0'002 0'01 0'01 0'01 0'01 0'0 I 0'01 .- - Di-ethyl aniline . Q.H. electrode . . 5 '72 5 '64 5 '64 5'6 - 5.12 5 '05 5-03 5'73 4'91 5 '05 __ 5 'I7 5'35 5'99 5-17 5 '3 __ 4.46 - 20 25 25 25 1'9 x 10-5 2.3 x 10-5 2-3 x 10-5 2.5 x 10-.; 7'6 x 10-' 8.9 x 10-1 9'3 x 10-1 7'4 x 10-( 1'2 x 10-! 8.9 x 10-1 9 9 9 9 * * H.electrode . . Colorimetric . . Di-methyl aniline Q.H. efectrode . . 0'02 0'02 0'02 0'01 0'02 0'02 22-5 25 20 I9 25 23 0'01 0'01 0'01 0'002 0'01 0'0 I 9 1 9 9 > 9 9 ) * * H. electrode . . Colorimetric . . Dimethylp-Tolui- dine . . Q.H. electrode . . 3-4 x I O - ~ 4.9 x 10-9 2-7 x 10-9 3.1 x 10-9 4.0 x I O - ~ -005 a 0 0 5 saturated - - 2'2 x 10-' 2'1 x 10-' 2-5 x 10-' 3'4 x 10-' 2-5 x 10-' 5.1 x 10-! 0'01 0'01 0'002 0'01 0'01 .__ 0'00 I 0.015 0.015 0.007 0.015 0.015 - 0.0025 20 25 I9 - 20 16 9 9 9 9 * H. electrode . . 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Dicthyhniline . . . j-phenylcnediamine . . Dimethyl p-toluidine . Bcnzidine. . . . Dimethylaniline . . Ethylaniline . . . j-toluidine , . . Methylaniline . . . Aniline . . . . p-nitrosodimethylaniline . Glycocoll . . . . p-amino-benzoic acid . 4'5 x 10-8 1'0 x 10-8 3-0 x 1o-O 1.6 x 10-s 1'0 x 10-0 1'0 x xo-9 1-0 x 10-s 5'0 x 10-10 3.0 x xo-I0 1'0 x 10-10 3-0 x 1 0 - l ~ 1.5 x 10-12 50 : 50 Acetone-Water. -I 5'5 x 10-11 (1-5 x 10-l0)* 9'0 x 10-u 3'0 x 10-1y 3-2 x 10-1s~ 1.5 x 1 0 - l ~ ~ 7-0 x 10-13+ 3.0 x 10-l' 3.0 x 10-19 2'0 x 10-12 3.0 x 10-l~ 1.5 x 10-1s 90: 10 Acetone-Water. 6 3 x 10-l~ (TO x 10-l4)* 3'3 x 10-'6 3-0 x 10-l' 8.0 x 10-17 3'5 x 10-1'* 3.0 x IO-~@* 1'0 x 10-16 5.0 x 10-17 2'0 x 10-m - - The values marked with an asterisk are instances where the order of hydrolysis and basicity constants change with the nature of the solvent. Summary and Conclusions. A determination is made of the hydrolysis and affinity constants of a number of amino bases in water and in acetone-water mixtures. Both hydrolysis and affinity constants are affected by the nature of the solvent to an extent which it has not yet been possible to correlate with any other property such as dielectric constant. With a few exceptions, bases which are arranged in order of decreasing basicity in water have the same order as a similar series in the other solvents employed. A determination of the dissociation constant of water has been made in acetone-water mixtures. As in the case of the affinity constants of bases, the value of the dissociation of water falls rapidly with increasing acetone content. I wish to express my indebtedness to Mr. G. M. Westrip and to Mr. My thanks are also due to the Director of Artillery for his permission T. K. Rrownson for their collaboration in this investigation. to publish this paper.

ISSN:0014-7672    

DOI:10.1039/TF9241900705

出版商:RSC    

年代:1924

数据来源: RSC