JOE SMISKOAND LYLER. DAWSON
84
the stability reported by P e r k i n ~ ~exchange 3 . ~ ~ with Zn++ and Cd++ should readily occur. Bryant26 found that there was no coordination between the Be2+ion and 2-aminotroPone in 50% HzO-dioxane solution as evidenced by the formation of beryl(26) B. Bryant, J . A m . Chem. Soc., 7 6 , 4864 (1954).
VOl. 59
liurn hydroxide a t the same pH whether the 2aminotropone was present or not. Acknowledgment.-The authors gratefully acknowledge financial support furnished for this work by the United States Atomic Energy Commission through Contract AT (30-1)-907.
TRANSFERENCE NUMBERS OF POTASSIUM ION IN SOLUTIONS OF POTASSIUM BROMIDE IN METHANOL AND POTASSIUM THIOCYANATE IN METHANOL AND IN ETHANOL AT 25' BY JOE SMISKO' AKD LYLER. DAWSON Contribution from the Department of Chemistiy , University of Kentucky, Lexington, K y . Received AWU.81 8, 1864
Cation transference numbers have been determined at 25" by the autogenic boundary method in solutions of potassium bromide in methanol and potassium thiocyanate in methanol and in ethanol at concentrations ranging from 0.0015 to 0.01 N . Plots showing the concentration dependence of the Longsworth function are linear up to approximately 0.01 N . The results indicate that for these two potassium salts the tendency for the cation to solvate, relative to the anion, is greater in methanol than in ethanol or water.
Relatively few transference data in non-aqueous media can be found in the literature, and many of those which are available may be not highly accurate. Most of the published results have been obtained by the Hittorf method. The autogenic boundary method was used first by Franklin and Cady2 in liquid ammonia. Recently Davies, Kay and Gordon3 have determined values for sodium and potassium chlorides in methanol using the moving boundary method. Numerous transference data are needed for developing the theory of electrolytes in non-aqueous media, especially for calculating ionic conductances and applying the limiting Onsager equation to unsymmetrical electrolytes. The present investigation was initiated in this Laboratory as a part of a program of study of the electrochemical and thermodynamic properties of some non-aqueous solutions. Experimental Apparatus.-A typical autogenic boundary cell was used. The anode was constructed from a pure cadmium rod which had been ground cone-sha ed and sealed into position with Apiezon. A coat of shelkc provided electrical insulation and protected the Apiezon from the bath liquid. A silversilver chloride electrode was used for the cathode. The graduated boundary tube, which was 11 cm. long, was made from 4 mm. Pyrex tubing having an internal diameter of approximately 2.5 mm. It was calibrated carefully before it was sealed to the cell. A similar boundary tube, 7.5 cm. long, was used with the ethanol solutions which have higher electrical resistances. The cell was held in a thermostated kerosene-bath so that the boundary tube was between arallel plate glass wall sections about four inches apart. 8 g h t from a slit illuminated the boundary which was observed with a magnifying lens. Its motion was timed with a mechanical timer. By the use of an efficient current regulator, variations of the current through the cell were limited to 0.05% for the methanol solutions and O . l O ~ ofor the ethanol solutions. (1) Based on 8 diasertation submitted by Joe Smisko in partial fulfillment of the requirements for the degree of Doctor of Philosophy. (2) E. C. Franklin and H. P. Cady, J . Am. Chem. SOC.,26, 499 (1904). (3) J. A. Davies, R. 749 (1951).
L. K a y and A. R . Gordon, J . Chem. Phus., 19,
Salts.-Potassium bromide was pre ared from hydrobromic acid and potassium carbonate y! the method described by Jervis, Muir, Butler and Gordon.' Reagent grade potassium thiocyanate was recrystallized twice and dried for four hours a t 110' in an atmosphere of dry nitrogen. Then it was stored in the dark over magnesium perchlorate. Solvents.-J. T. Baker Analyzed absolute methanol was refluxed over calcium oxide for 12 hours, then fractionated through a ten-plate column retaining only the middle portion. Silver nitrate was added to remove aldehydes, the mixture was refluxed for 12 hours, and finally distilled again retaining only the middle fraction. The specific conductance of the purifie; methanol ranged from 3 to 6 X 10-7 ohm-' cm.-l at 25 . Less than 0.01% water was indicated by the Karl Fischer reagent. Commercial absolute ethanol was refluxed with magnesium and iodine and fractionated. To the middle portion of the distillate was added a few grams of 2,4,6-trinitrobenzoic acid to remove traces of bases, and the mixture was refluxed and distilled. The specific conductance of the fraction retained ranged from 5 to 9 X 10-8 ohm-' cm.-lat 25". The KarlFischer reagent showed approximately0.003~owater. Procedure.-Prior to a determination, the equipment was allowed to operate for 30 minutes to ensure equilibration. The solution was introduced through a capillary tube which extended to the anode, care being exercised to prevent the entrapment of air; then the cathode cup was filled and the electrode was introduced. The filling operation required about 30 seconds. The current was adjusted so that a few minutes elapsed before the boundary reached the first mark on the tube, thus allowing ample time for the electrical equipment to reach a steady state. From 800 to 5000 seconds was required for the boundary to move through the graduated portion of the tube: From 20 to 30 separate measurements, involving duplicate or triplicate determinations a t each set of operating conditions, were made for each salt. Solute concentrations varied from 1.5 to 9.5 X 10-5 mole per liter. Currents used for the various solutions ran ed from 0.04 to 0.42 ma. Evidence of the reliability o f the calibrations and the general experimental procedure was obtained by determning the transference number of potassium ion in a 0.02 N solution of potassium chloride in water. A value of 0.4897 was obtained which agrees quite well with 0.4901 reported by Longsworth.' Also the transference number of sodium ion in 0.005 N solution of sodium chloride in methanol waR measured. A value of 0.4592 was obtained; Davies, Kay and Gordon* reported 0.4595. (4) R . E. Jervb, D. R . Muir, J. P. Butler and A. R . Gordon, J . Am. Chem. Sac., 76, 2855 (1963). (5) L. G. Longsworth, ibdd., 64, 2741 (1932).
CATION TRANSFERENCE NUMBERS
Jan., 1955
The conductancw of the solvent and of the solution were measured with a Jones bridge, which was manufactured by Leeds and Northrup Company, by the method described in earlier papers from this Laboratory.' Solvent corrections were derived from these data. Corrections due to volume changes were negligible.
Results and Discussion Experimental conditions were arranged so as to provide comparisons of transference data a t different current densities in order to ascertain mixing effects at the boundary. Readings made at several graduations gave evidence of the constancy of the transference number as the boundary moved up the tube. A summary of the results is presented in Table I, where C is the concentration in moles per liter, I is the current in milliamperes, t+ is the corrected transference number, and d is the mean absolute deviation of the measurements a t the given concentration from the average value listed. TABLEI TRANSFERENCE DATA
cx 101
3.890 6.068 9.606 2.3147 2.3164 4.970 4.962 7.552 9.988 1.5094 2.3106 2.3234 4.985 5.006 7.461
No. of meaa.
I
/
0.465
I
0.464
104 0.46 I
3 3 4 3 3 2 2 3 3 6 4 4 5 5 5
+ '/4(C)1/zI/[A' + A(C)'h]
are shown in Fig. 1. In this expression A' = bo (A BAO)(C)'/~, A = 82.43/(DT)"?, B =
+
(6) L. R. Dawson and W. M. Keely, J . Am. Chrm. Soc., 1 8 , 3783 (1851).
0.4831
d X tt
KBr in Methanol 9 0.05-0.12 0,4787 11 .07- .13 .4790 9 .12- .21 .4793 KSCN in methanol 5 0.06-0.11 0.4533 5 0.12 .453 I 7 .13- .21 .4529 6 .12- .25 .4532 8 .15- -32 .4530 8 .23- .42 .4528 KSCN in ethanol 8 0.04-0.10 0.4595 7 .07- .13 .4587 3 0.10 .4586 8 .12- .18 .4588 6 .lo- .20 .4589 11 .20- .35 .4591
= [t+A'
8.203 X 106/(DT)'/*. D is the dielectric constant? (methanol = 32.63, ethanol = 24.3), q is the viscosity* (methanol = 0.00546, ethanol = 0.0109), and C is the concentration in moles per liter. The values of A and B are 153.07 and 0.8549 for methanol a t 25'; for ethanol at 25", A = 88.84 and B = 1.330.
0.463
All boundaries for potassium bromide in methanol were rather faint. Below 0.004 N the boundary was not discernible and above 0.01 N the transference number varied with current density. This was attributed to heating effects since a greater current density was required to maintain a visible boundary. With potassium thiocyanate in methanol, the boundaries were sharp and easily observed over the concentration range 0.002 to 0.01 N . Because of the high electrical resistance of the ethanol solutions of potassium thiocyanate, reliable transference data could not be obtained at concentrations above 0.0075 N . The lowest concentration used was 0.0015 N ; in more dilute solutions the boundary was very indistinct. Plots of the Longsworth6function t+O'
85
0.460
r c
0.457 a456 2.0
0
4.0
8.0
6.0
10.0
c x io3. Fi 1 The concentration dependence of the Longswortlf; f i n t i o n for: 1, KBr in methanol; 2, KSCN in methanol; 3, KSCN in ethanol.
The Longsworth plots exhibit essentially linear relationships for the three solutions within the experimental concentration ranges. However, t+O' for KSCN in ethanol at 0.01 N would be approximately 0.468, which is evidence that the plot becomes definitely curved above 0.007 N . By the method of least squares an equation of the form, t+O' = t + O bC was obtained for each system. Values of the intercept and the constant, b, a.re shown in Table 11. The precision of the limiting
+
TABLEI1 LIMITINQ TRANSFERENCE NUMBERSFOR IONAT 25' Salt
Solvent
b
KBr KCNS KCNS
Methanol Methanol Ethanol
0.30 .42 .66
THE
POTASSIUM t+O
0.4795 .4555 .4612
transference number is estimated to be within 0.25%. The data in Table 111 correspond to the plots in Fig. 1. (7) A. A. Maryott and E. R . Smith, National Bureau of Standards Circular 614, 1951. (8) R. C. Miller and R. M. Fuoas, J . Am. Chsm. ~ o c . ,76, 3076 (1963).
W. F. GRAYDON AND R. J. STEWART
86
Vol. 59
TABLE I11 yields 23.4 and 27.4 for the limiting ionic conductTRANSFERENCE NUMBERR FOR THE CATIONIN SOLUTIONS ances of potassium and thiocyanate ions in ethanol a t 25”. Barak and Hartley reported 22.0 for the OF KBr A N D KSCN potassium ion based on the transference number of 1+, t+ 1+ KBr in cx K S C in ~ K S C in ~ the cation in hydrogen chloride (0.71 i 0.01) as 108 methanol methanol ethanol determined by the electromotive force method, in 0 (0.4795) (0.4555) (0.4612) conjunction with a A. of 83.8. It is difficult to 0.2 .4791 .4547 .4603 ascertain the reliability of their results because of 0.5 ,4790 ,4543 .4599 the uncertainty in the limiting equivalent conduct1.0 .478~ .4595 .4539 ance of hydrochloric acid for which MacInnes18 3.0 .4786 ,4531 ,4588 reports 81.8, obtaining this value from the data of 5.0 .4787 ,4528 .4588 Goldschmidt and Dahil,14 while Bezman and Ver7.0 .478g ,4527 .4590 hoek15 report 84.25. 10.0 .4793 .4528 ,4596 The ratio t + O / t - O for KBr in methanol is 0.922;
Using the conductance data reported by Gordon, for KSCN it is 0.839 and 0.855 in methanol and et u Z . , ~ for KBr in methanol and Hartley and co- ethanol, respectively. For KBr in water the ratio w o r k e r ~ ’data ~ for KSCN in methanol in conjunc- is 0.9416; for KSCN in water it is approximately tion with the limiting transference numbers re- 1.11.” From the foregoing results it is evident ported herein, values of 52.11 and 52.24 ohm-1 that the potassium ion moves slower in comparison cm.2 for the limiting ionic conductances of the po- to the thiocyanate ion in both methanol and ethtassium ion were obtained for the KBr and KSCN anol than in water, the relative decrease in mobility solutions, respectively. Other investigators have being greater in methanol. I n KBr solutions the reported AOK+ in methanol as follows: Hartley and cation moves slower in comparison to the anion than 52.40, and Evers in water, although the relative decrease in mobility Raikes,’” 53.8, Gordon, et and Knox,” 50.2. Using 52.2 f 0.15 as the most is not as great as in the KSCN solutions. Thus for probable value for the potassium ion, from our these two potassium salts the tendency for the data we obtain the following anion conductances in cation to solvate, relative to the anion, seems to be greatest in methanol and least in water. methanol: ha&- = 56.7, A~CNS- = 62.2. Our transference numbers applied to Barak’s and (13) D . A. MacInnes, “The Principles of Electrochemistry,’‘ ReinHartley’s12 conductance data, after extrapolation hold Publ. Corp., New York, N. Y., 1939, p , 365, (14) H. Goldschmidt and P. Dahil, Z . physik. Chem., A l l 4 , 1 to infinite dilution by the Shedlovsky method, (9) A. Unmack, D. M. Murray-Rust and H. Hartley, Proc. R o y . w o n d o n ) , ~ 1 3 7 , 2 2 8(1930). (10) H. Hartley and H. R. Raikes, Trans. Faraday SOC., 38, 393 (1927). (11) E. C. Evera and A. G . Knox, J . Am. Chem. Soc., 73, 1739 (1951). (12) M . Barak and H. Hartley, Z . p h y s i k . Chem., A165, 272 (1933).
SOC.
(1926). (15) I. I. Bezman and F. H. Verhoek, J . A m . Chem. Soc., 67, 1330 (1945). (16) H. 8. Harned and B. B. Owen, “The Physical Chemistry of Electrolytic Solutions,” Reinhold Publ. Corp., New York, N. Y., 1950. (17) C. W. Davies, “The Conductivity of Solutions,” Chapman and Hall, Ltd., London, 1933.
ION-EXCHANGE MEMBRANES. I. MEMBRANE POTENTIALS BY W. F. GRAYDON AND R. J. STEWART Department of Chemical Engineering, University of Toronto, Toronto, Canada Receiwd Augual 15, 1%64
A method of preparing homogenous ion-exchange membranes of the polystyrenesulfonic acid type is described. Measurements are reported which show that these membranes may be prepared with high tensile strength and low electrical resistance. The membrane potent.ials characteristic of these membranes are good approximations to the ideal membrane potentials for cation transfer only. Deviations from the ideal membrane potential are discussed. It is concluded that water transfer is in many cases the major cause of these deviations. Approximate water transference numbers for various membranes are estimated.
Introduction The primary requirement for an ideal cation-exchange membrane is that it be permeable to cations only and impermeable to anions and neutral molecules. Secondary requirements of considerable practical importance are chemical stability, mechanical strength and high cation mobility in the membrane. A number of methods have been described for the preparation of membranes to determine the extent to which these requirements may be met. The materials which have been studied include collodion, clay, granular ion-exchange (1) H. P. Gregor and K. Sollner, THISJOURNAL,58, 409 (1954). (2) C. E . Marshall, ibid., 81, 1284 (1948).
resins in binderg and “homogenous” ion-exchange resins of the condensation polymer type. Because of the chemical stability of ion-exchange materials of the polystyrenesulfonic acid type and the inherent advantages of essentially homogeneous membranes attempts were made to prepare membranes of this type by adaptation of the methods used for bead polymers.6 The product resulting from the sulfonation of polystyrene sheet was extremeIy fragile and no suitable membranes were obtained. (3) M. R. J. Wyllie, W d . , 58, 67 (1954). (4) W. Juda, N. W. Rosenberg, J. A . Marinsky and A. A. Kaspar, J . A m . Chem. Soc., 74, 3736 (1952). (6) Q. F. D’Alelio, U.8. Patent 2,366,007 (Dec. 26, 1946).
