July, 1959
CATHODE CURRENT DISTRIBUTION IN SOLUTIONS OF SILVER SALTS
1147
CATHODE CURRENT DISTRIBUTION IN SOLUTIONS OF SILVER SALTS. I. THE EQUIVALENT CONDUCTANCE OF SOLUTIONS OF SILVER NITRATE AND OF POTASSIUM ARGENTOCYANIDE AS A FUNCTION OF VOLUME CONCENTRATION AND TEMPERATURE BY CHARLES H. ORR' AND HENRYE. WIRTH Contribution from the Department of Chemistry, Syracuse University, Syracuse, N . Y . Received December 16, 1968
The equivalent conductance of solutions of silver nitrate and of potassium argentocyanide were determined at 25, 35 and 45" in the concentration range from 0.01 to 0.5 N . For the solutions of silver nitrate an empirical equation of the form log A = log A0 U1C1/9 anc U ~ C ' / Z a4ca was used to re resent the data and for the solutions of potassium argentocyab2c 4-b& b4c2 was used. The densities of the solutions of otassium nide an equation of the form A = AO blcl/g argentocyanide were determined as a function of concentration and these data were represented by equations of t f e form of dt4 = do elc e2c8/2. The values of the constants in these equations a t 25, 35 and 45", respectively, are: log Ao, 2.12503, 2.20496, 2.27852; al, -0.28746, -0.29307, -0.30733; as, -0.04335, -0.03869, -0.05265; a3,0.35309, 0.36286, 0.45853; ~ 4 ,-0.2980, -0.3161, -0.4034; Ao, 123.7, 146.4, 175.9; bl, -106.8, -92.8, -135.5; b2, 217.8, 101.1, 194.5; br, -277, -66, -173; br, 132, 11,55; do, 0.9971,0.9941,0.9902; e l , 0.1348,0.1330, 0.1312; e2, -0.0153, -0.0150, -0.0147.
+ + +
+ +
++
+
Introduction As part of a problem on current distribution in plating baths it was necessary to know the equivalent conductance of silver nitrate and of potassium argentocyariide as a function of volume concentration and temperature. Data for silver nitrate solutions a t 25" are readily available, but no data for 35 and 45" over the concentration range of interest could be found. The data at 25" were determined as a check on the equipment and procedure, after which the equivalent conductances a t 35 and 45" were determined. There are very few data available in the literature on the equivalent conductance of potassium argentocyanide. Walden2 presented data for the range 0.00098 to 0.031 N at 25" with no indication of the method of measurement or of preparation of the salt. Britton and Dodd3 presented data for the range 0.0025 to 0.01 N a t 25". Although the agreement with the data of Walden is fair, their values were derived from data on the conductance of mixtures of salts in solution. In view of the lack of suitable data, a determination of the equivalent conductance of the solutions of potassium argentocyanide over a sufficiently wide concentration range to embrace most electroplating solutions and having moderate precision was undertaken. It was desired to make up the solutions by weight and to calculate the volume concentration and since very few data concerning the density as a function of volume concentration were available, densities also were determined. Experimental A. Apparatus.-The conductance bridge was constructed from the plans of Edelson and Fuoss4 and the balance point was determined with earphones and with a Heath Kit PushPull, Extended Range Oscilloscope. The bridge resistance decade box, General Radio Type 1432M, was calibrated i n situ against National Bureau of Standards certified resistors by a substitution method at a frequency of approximately 500 C . P . S . The two conductance cells of the design of Jones and Bollingers had constants of 1.6 and 22.9. The (1) The Prooter & Gamble Company, Miami Valley Laboratories, Cincinnati 31, Ohio. (2) P. Walden, 2. anorg. Chem., 23, 373 (1900). (3) H.T. S. Britton and E. N. Dodd, J . Chem. SOC.,1932 (1940). (4) D. Edelson and R. M. Fuoss, J . Chem. Education, 27, 610 (1950).
+
bath temperature was maintained constant to within =t0.005". The density of the potassium argentocyanide solutions was determined by measuring their expansion in a dilatometer similar to a weight dilatometer,e except that closure was made with a three-way stopcock. The precision ty e cathetometer used for measuring the displacement of tge liquid meniscus could he read to =k0.005 cm. B. Materials .-The conductance water was prepared by double distillation, first from alkaline permanganate solutions and then from dilute phosphoric acid. Both distillations were carried out in a nitrogen7 atmosphere. The terminal condenser and the receiving flasks were made of Vvcor The average suecific conductance of the water used i n t h e determinatiGs was: 25", 0.97; 35", 1.36; and 45", 1.89 X 10-8 mhos. cm.-1. J. T. Baker Analyzed potassium chloride was recrystallized once from a saturated boiling solution in water, then dried in porcelain evaporating dish& in a muffle furnace at about 500'. Baker and Adamson reagent grade silver nitrate was used without further purification. The salt, potassium argentocyanide, was prepared according to the method of Frary and Porter.8 Since it was discovered afterward that the salt contained excess silver cyanide, corrections were applied and check determinations were made with a sample of potassium argentocyanide from Fisher Scientific Com any which analyzed 99.59% pure. C. Procedure.-TEe solutions were made up on a weight basis. Using the density of the solutions at 25, 35 and 45" the volume concentrations were calculated from the weights. The density of the solutions of potassium chloride was calculated from the data of Geffcken.8 The equations for the density of the solutions of potassium chloride are d254 = 0.99707 0.048270~- 0.002403~"~ da54 = 0.99406 0.047494~ 0.002187~'/0 d454 = 0.99025 0.047321~- 0.002129~'/~ The equations for the density of the solutions of silver nitrate as a function of the volume concentration a t the three temperatures were derived from the equation of Jones and Colvinlo and the data of Bousfield and Lowry.11 The derived equations at 35" and 45" are
.
+ + +
-
( 5 ) G . Jones and G. M. Bollinger, J . A m . Chem. Sac., 5 3 , 411 (1931). (6) N. Bauer, "Determination of Density. Physical Methods of Organic Chemistry," Vol. I , Part 1. ed. A. Weissberger, 2nd ed., Intersoience Publishers Inc., New York, N. Y.,1949,Fig. 4, p. 277. (7) L. B. Rogers, D. P. Krause, J. C. Griess, Jr.. and D . B. Ehrlinger, J . Electrochem. Soc., 95, 34 (1949). (8) F. C. Frrtry and R . E. Porter, Trans. A m , Electrockem. Soc., 28, 307 (1915). (9) W. Geffcken, 2. physik. Chem., 8155, 1 (1931). (10) G. Jonea and J. H. Colvin, J . A m . Chem. Sac., 62, 338 (1940). (11) W. R. Bousfield and T. M. Lowry, Trans. Faraday Sac., 6 , 85 (1910).
1148
CHARLES H. ORR AND HENRYE. WIRTH d354 = 0.99406 d454 = 0.99025
+ 0.14154~- 0.00298~*/~ + 0.14131~- 0.00322~~/2
TABLE I11
Results The average cell constants and their standard deviations are listed in Table I. TABLE I Cell constant (cm. -1) Cell I Cell I1
OC.
25 35 45
1.5808&0.0005 1 . 5 8 0 1 1 ,0004 1 . 5 8 1 1 f ,0006
22.92rt0.02 22.91 & .02 2 2 . 9 3 f .02
The average values of the equivalent conductances of the solutions of silver nitrate a t the three temperatures are listed in Table 11. The smoothed density, volume concentration and average equivalent conductance of the solution of potassium argentocyanide a t the temperatures indicated are presented in Table 111. Those values tabulated without density data are duplicate determinations. At the highest concentration the densities were obtained by extrapolation of the density equations.
Tyw.,
Density, g./ml.
25 35 45 25 35 45 25 35 45 25 35 45 25 35 45 25 35 45 25 35 45 25 35 45 25 35 45 25 35 45 25 35 45
0.998 .995 .991 .998 .995 .992 1.002 0.999 .995
C.
Due to the presence of excess silver cyanide the solutions of potassium argentocyanide were analyzed for silver content and the concentration of the double salt calculated. Several solutions were analyzed, at first volumetrically by the method of Blum and Hogaboom,lz and later by deposition of silver on a platinum gauze cathode. The concentrations of all solutions were corrected by the average value of the per cent. of the salt weighed out that was potassium argentocyanide since the contaminant, silver cyanide, was essentially insoluble. T h e constant of cell I was determined by measuring the resistance of solutions of potassium chloride in that cell at 25, 35 and 45' and calculating the cell constant from these data and the equations of Shedlovsky, Brown and MacInnes,13 and Benson and Gordon.14 The constant of cell I1 was determined by comparison of the resistance of a solution of silver nitrate run simultaneously in both cells a t the three temperatures. The conductivities of the silver nitrate solutions were determined at four frequencies and a t temperatures of 25, 35 and 45". The conductance of the potassium argentocyanide solutions increased with time, so measurements at each temperature were extrapolated back t o the time of filling of the cell. Electrolytic polarization was corrected for by plotting the zero time resistance versus the reciprocal of the square root of the frequency and extrapolating to infinite frequency.
Temp.,
Vol. 63
1.009 1.006 1.002 1.017 1.014 1,010
1.028 1.025 1.020
1.042 1.038 1.033
Volume concn., N
A
0.004944 .004929 ,004910 .01025 .01022 .01018 .039G6 .03954 ,03938 ,03946 .03934 ,03918 ,0899 .0896 .0892 .1581 .1576 ,1569 .2443 ,2435 .2424 ,2458 .2449 .2439 .3520 .3507 .3490 ,3531 .3518 ,3502 .4805 .4787 .4765
117.0 140.6 168.2 115.0 138.5 163.0 109.2 129.4 152.6 109.3 131.5 154.9 104.7 126.0 148.5 101.5 121.8 143.5 98.3 117.7 138.4 98.6 118.3 139.2 95.3 114.2 134.3 95.2 114.5 134.1 92.5 110.9 130.3
Discussion An empirical equation was found for the representation of the data on silver nitrate which represented that data with a minimum of deviation. Application of the method of least squares yielded these three equations for the equivalent conductance of the solutions of silver nitrate as a function
TABLE I1 25'
450
359 A
C
133.4l5 124.8 117.0 110.2 104.3 98.9 94.3 89.3
0
.010068 .040073 .08994 .16032 .25261 .35992 ,49881
A
C
160,316 149.8 140.3 132.2 125.0 118.5 112.9 106.7
0
.010037 ,039971 ,08966 ,15982 .25181 .35875 .49713
(12) W. Blum and G. B. Hogaboom, "Principles of Electroplating arid Electroforming," 3rd ed., McGraw-Hill Book Co., Inc., New
C
0
,009999 ,039818 ,08932 .15920 .25082 .35733 .49513
York, N. Y., 1949, pp. 195-6.
- O.28746c1/z -
(13) T. A. Shedlovsky, A. S. Brown and D. A. MacInnes, Trans. Electrochem. Sac., 66, 165 (14) G. C. Benson and A. R. Gordon, J . Chem. Phvs., 18, 473
1%
- 0.29307~'/2 -
(1945).
(1932).
(15) T. A. Shedlovsky, J . A m . Chem. Soc., 64, 1411 (16) Obtained by extrapolation in large scale plots of the equivalent conductance veraua the square root of the volume concentration.
189.918 176.9 165.1 155.3 146.9 139.1 132.4 125.2
of the volume concentration log A25 = 2.12503
(1934).
A
+
0.04335~ 0.35309~*/2- 0.2980~' A36
=
2.20496
0.03869~f 0.36286ca/2 - 0.3161~2
log A45 = 2.27852
- ~ ~ 3 ~ 7 3 3~1'z
+ 0.45853ca/2 - 0.4034~2
0.052G5~
.-
CATHODE CURRENT DISTRIBUTION IN SOLUTIONS OF SILVERSALTS
July, 1959
The equations for the density of the solutions of potassium argentocyanide as a function of the volume concentration are 0.9971 0.9941 d464 = 0.9902
da54
d354
= =
+ 0.1348~- 0.0153cS/2 + 0 . 1 3 3 0 ~- 0.0150ca/a + 0 . 1 3 1 2 ~- 0.0147c8/2
Application of the method of conductance data yielded these for the equivalent conductance potassium argentocyanide as a volume concentration A26
123.7 106.8~'/2f 2 1 7 . 8 ~- 277ca/z f 132~2 146.4 92.8c1/2 1 0 1 . 1 ~- 66cS/2 f l l c 2 = 175.9 135.5~~12 f 1 9 4 . 5 ~- 1 7 3 ~ * /f~ 55ca
=
A36 A46
-
averages to the three equations of solutions of function of the
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One of the limiting factors in these determinations is the precision with which the cell constant was determined. The maximum deviation from the average was one part in three thousand for cell I and one part in one thousand for cell 11. In the case of the solutions of potassium argentocyanide the limiting factor is the determination of the concentration. The fact that there must be present a very small amount of potassium cyanide in excess of that required to make the potassium argentocyanide from silver cyanide and potassium cyanide complicates the matter a little when temperature variations are involved since there is no information available as to the temperature dependence of the solubility product of silver cyanide and the instability constant of the argentocyanide ions. To check the procedure used to correct the volume concentrations, a sample of potassium argentocyanide was purchased. This sample, from Fisher Scientific Company, catalog number 5-186, lot number 522873, was analyzed electrolytically for silver content and volumetrically for free cyanide. It was found to contain 99.59% KAg(CN)* and 0.231Oj, KCN. Solutions having concentrations of about 0.04, 0.25 and 0.36 N KAg(CN)Z were made up with the Fisher salt and the weights used in making the solutions were adjusted by the factor 0.9959 in calculating the volume concentration. To one of the solutions, after its conductance had been determined, was added 0.231% KCN and the resistance of the altered solution was determined a t all three temperatures. The difference between the specific conductances of the altered and the unaltered solutions was taken as the increase in specific conductance due to the presence of an excess
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of 0.231% potassium cyanide. The increase in specific conductance was found to be 59.8% of that calculated for a solution of pure potassium cyanide as reported by Bodensiek." The specific conductances of the potassium argentocyanide solutions then were corrected for the amount of potassium cyanide present, assuming that the specific conductance of the KCN was 59.8% of that of the pure salt. A large scale plot of the equivalent conductance versus the square root of the volume concentration was made from the previous data and the best smooth curve drawn through those points. The test values obtained by the method above were plotted on the same graph. The deviations from the curve were of the order of f 0.0 to 0.3 unit. In determining the equations for the variation of the equivalent conductance of the solutions as a function of the volume concentration, these points were included with the other data. In view of the difficulties encountered in this work on solutions of potassium argentocyanide it is estimated that the values of the density and of the equivalent conductance are reliable to about 3 to 5 parts per 1000. The values of the equivalent conductance at infinite dilution a t 35 and 45" for the solutions of silver nitrate and at all three temperatures for the potassium argentocyanide were obtained by extrapolation of large scale plots of the equivalent conductance versus the square root of the volume concentration and since the concentrations were still relatively large, the extrapolated values are not considered reliable. They are, however, reasonable in terms of the expected behavior of solutions of electrolytes. The equivalent conductance a t 25" of solutions of silver nitrate calculated from the appropriate equation above agree with values15in the literature to within 4 parts in 10,000. The equations representing the data are good representations of the data for the range of concentration studied (0.01 to 0.5 N ) but should not be expected to give reliable values outside this range. Acknowledgment.-The authors wish to express their appreciation for a du Pont fellowship during the academic year 1951-1952. (17) A. Bodensiek, Dissertation, Hannover T. H., 1925, p. 10a ("Gmelin," Vol. 8, System 22K, part 27, page 885).
