SODIUM ELECTRODE POTENTIALS IN METHANOL-WATER
Oct., 1958
1319
THE ELECTRODE POTENTIAL OF SODIUM I N METHANOL-WATER SOLVENTS BY G. N. MALCOLM AND H. N. PARTON University of Otago, Dunedin, New Zealand Received May 6 , 1068
Electromotive force measurements are reported for the cell Na/NaC1(0.05 and 0.1 m ) in methanol-water solutions/AgClAg at 15, 25 and 35”. The activity coefficients of Akerlof are found not to agree with the results, or with others in the literature, and new values of the activity coefficients are estimated. The free energies of solution of sodium chloride in methanol-water solutions are calculated from the e.m.f. results and are compared with values obtained from solubility measurements.
Introduction The study of the effect of solvent on the standard potentials of galvanic cells has largely been confined to the cell H2(Pt)/HC1 (solution)/AgCl-Ag.’ I n this paper we present some results for the cell Na/NaCl (solution)/AgCl-Ag in which the solvent is a series of methanol-water solutions. Electromotive force measurements were made on the cells Na,Hg/NaCl (0.05 and 0.1 m )in water/AgCl-Ag
(1)
and Na,Hg/NaCl (0.05 and 0.1 m ) in mixed solvents/AgCl-Ag
(2)
The e.m.f. of the cell ’
Na/NaC1(0.05 and 0.1 nz)/AgCl-Ag
(3)
is accurately known12so that the difference between the e.m.f.’s of (1) and (3) gives the e.m.f.of the cell Na/NaCl (solution)/Na,Hg
(4)
which is independent of the solvent. The e.m.f.’s of the cells (2) and (4) give the e.m.f. of the cell Na/NaCl (0.05 and 0.1 m ) in mixed solvents/AgCl-Ag (5)
Amalgam cells such as (1) and (2) have been found unreliable a t electrolyte concentrations less than 0.05 m, so that their standard e.m.f.’s cannot be obtained by the usual extrapolation techniques. An alternative means of evaluating standard e.m.f.’s is by calculation from e.m.f. measurements at finite electrolyte concentrations using independently determined values of the activity coefficients. Activity coefficients of sodium chloride in a range of methanol-water solvents have been reported by Akerlof. When these values were applied to the measured potentials of cell ( 5 ) the Eo values calculated from the two different electrolyte concentrations were not in agreement, It also was noticed that Akerlof’s activity coefficients were considerably different from the values obtained by Butler and Gordon6 for a 50 mole yo methanol-water solvent. Butler and Gordon’s results suggest that the variation of the activity (1) I. T. Oiwa, THIBJOURNAL, 61, 1587 (1957), for summary. (2) E. R. Smith and J. K. Taylor, J . Research Natl. Bur. Standards, 20, 837 (1938). (3) H. S. Harned and B. B. Owen, “The Physical Chemistry of Electrolytic Solutions,” 2nd Ed., Reinhold Publ. Corp., New York, N. Y., 1950, p. 301. (4) G. Akerlof, J . A m . Chem. Soc., 62, 2353 (1930). (5) J. P.Butler and A. R. Gordon, ibid., 70,2276 (1948).
coefficient of sodium chloride with solvent composition is closely parallel to the variation of the activity coefficient of hydrochloric acid in methanolwater solutions. The activity coefficients of hydrochloric acid in methanol-water have been measured by Parton6 and by Oiwa.’ It was decided, therefore, to attempt to estimate the activity coefficients of sodium chloride jn the mixed solvents by assuming that the variation of the activity coefficient of sodium chloride with solvent composition a t each electrolyte concentration was parallel to that of the activity coefficient of hydrochloric acid a t the same electrolyte concentration. The usefulness of this approximation is demonstrated by the fact that the Eovalues calculated by this means from the measured potentials of cell ( 5 ) for the different electrolyte concentrations were in all cases in agreement to within the experimental error of 0.2 mv. Furthermore, the estimated coefficients are in close agreement with Butler and Gordon’s measured values (see Table 11). Although the Eo values presented here are subject to some uncertainty because of the assumption made to obtain the activity coefficients the measured values of the cell potentials at finite electrolyte concentrations are of value in themselves. Experimental Apparatus.-The amalgam cell designed by Smith and Taylor2 was used for all the measurements. Stationary amalgam drops gave satisfactory results in pure water as solvent, but in the methanol-water solvents it was necessary to use flowing amalgams. For this purpose the cupshaped tip of the amalgam electrode was replaced by a fine jet tip. The sodium amalgam was prepared by the electrolysis of sodium hydroxide solution with a mercury cathode, using the apparatus described by Smith and Taylor.2 The amalgam contained 0.06% by weight of sodium and was stored in an atmosphere of nitrogen. The silver-silver chloride electrodes were of the thermalelectrolytic type and were prepared according to the directions of Owen.8 The electrodes were aged by standing them in cell solution with continuous passage of nitrogen for several days. The aging process took longer in the methanol-water solvents than in water itself. Successfully prepared electrodes remained constant for several weeks, and agreement to 0.05 mv. between aged electrodes from the same and from different batches was obtained. The electrodes were used in pairs, and measurements for each cell were made with two or more different electrode pairs. In all cells, measurements were made at three temperatures in the order 25, 35, 15,25” to provide a check for reproducibility and hysteresis. Reproducibility to within 0.1 mv. was observed. The cell was immersed in a water thermo(6) Conway, ”Electrochemical Data,” Elscvier Press, New York, N. Y., 1952,p. 97, for values at 25O and unpublished work for values at 15 and 35’. (7) I. T. Oiwa, THIEJOURNAL, 60,754 (1956). ( 8 ) B. B. Owen, J . A m . Chena. Soc., 60, 2229 (1938).
,
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G. N. MALCOLM AND H. N. PARTON
1320
Vol. 62
TABLE I THEELECTROMOTIVE FORCE IN VOLTSOF Mole % methanol
20
40
60
T,
"C.
0.1 m
15 25 35 15 25 35 15 25 35
3.0105 3.0157 3.0205 2.9678 2.9715 2.9750 2.9321 2.9334 2.9351
E
THE
CELLNa/NaCl ( m ) IN METHANOL-WATER SoLuTIoNs/AgCl-Ag - 2 R T / F In my 0.1 m 0.05 m
0.05 m
...
0.1300 .1349 .1399 0.1339 .1391 ,1452 0.1383 .1439 .1511
3.0474
*.. 2.9983 2.0025 2.0072 2.9616 2.9642 2.9662
stat maintained at constant temperature to within &O.0lo. The e.m.f.'s were measurcd with a Cambridge Instrument Co. slide wire potentiometer. The amalgam cell was connected in opposition to an Eppley unsaturated cell of 1.OB9 volts in order to bring the over-all e.m.f. within the range of the potentiometer. The e.m.f. of the Eppley cell was determined before and after each series of measurements. Materials.-Commercial methanol was purified by the method given by Weissberger and Proskauer .g Methanolwater mixtures were made up by weight. A. R. sodium chloride was fused before use and the solutions of each concentration were made up by weight. Mercury was distilled under oxidizing conditions, washed in 10% nitric acid, and finally distilled under greatly reduced pressure. A. R. sodium hydroxide was used to prepare the sodium amalgam. Nitrogen gas was passed thr:ugh sodium hydroxide solution, over copper auze a t 400 , and finally through Fieser's reagent.10 d e n necessary the gas was dried with anhydrous magnesium perchlorate.
From 0.1 m
0.1603 .1666 .1728 0.1643 -1702 .1772 0.1680 .1745 .1821
2.8805 2.8808 2.8806 2 8339 2.8324 2.8298 2.7938 2.7895 2.7840 I
ED
From 0.05 m
... 2.8808
...
2.8340 2.8323 2.8300 2.7936 2.7897 2.7841
tween the measured values of AE, the values predicted by Akerlof's activity coefficients, and the values given by the estimated activity coefficients. TABLE I11 COMPARISON BETWEEN OBSERVED AND CALCULATED VALUES OF Al3 = E (0.05 m ) - E (0.1 m ) AT 25" Moles % methanol
Obsd. value
Calcd. from estimated coefiioients
Calcd. from Akerlof's coefficients
20 40 60
0.0317 .0310 .0308
0.0317 .0311 .0306
0.0313 .0302 .0292
Discussion The effect of solvent on the standard e.m.f. of cell (s) is a measure of the change in the free energy Results of solution of sodium chloride with solvent compoThe e.m.f.'s of cell ( 5 ) a t the two electrolyte con- sition. The results in Table I have been used t o centrations are shown in Table I. The Eo values calculate the free energies of solution of sodium which have been calculated from the two different chloride in the various solvents. Latimer's vale.m.f. values using the estimated activity coeffi- uesl' for the free energies of formation of AgCl(s) cients for the methanol-water solutions are com- and NaCI(s) were used. The results are shown in pared in the last two columns of the table. The Table IV. experimental error in the measured e.m.f.'s is TABLE IV thought to be within *0.2 mv. In Table I1 a comparison is made between the THE FREEENERGIES OF SOLUTION OF SODIUM CHLORIDE different activity coefficients for sodium chloride in I N METHANOL-WATER MIXTURESAT 25" Mole 5% AGO, oal. mole-' a 50 mole yo methanol-water solution obtained by This work L. and S. methanol AkerlofY4by Butler and C ~ r d o n and , ~ by the ap-2160 -2156 0 2 proximation used in this work. TABLE I1 THE ACTIVITYCOEFFICIENTSOF SODIUMCHLORIDEIN
A
50 MOLE yo METHANOL-WATER SOLUTION AT 25" ?la
Akerlof
Butler and Gordon
Estimated values
0.05 0.1
0.580 0.520
0,695 0.632
0.697 0.635
The difference, AE, between the cell potentials for the two different electrolyte concentrations in each solvent should be equal to the quantity 2RT/ F In ~ ~ o . I / ~ o . oTable ~ . 111shows a comparison be(9) A. Weiasberger and E. Proskauer, "Organic Solvents," Clarendon Press, Oxford, 1935, p. 114. (10) L. F. Fieaer, J . Am. Chem. Soc., 46, 2039 (1924).
30 40 60
-
890
+ 250
+1240
- 770
+ 300 +1470
The free energies of solution of sodium chloride in methanol-water solutions also have been calculated by Latimer and SlanskyI2 using solubility measurements and activity coefficients in saturated soliitions estimated from the activity coefficients of xkerlof. A comparison between the two sets of results in Table IV shows some significant differences. (1 1) W. M. Lntimer, "Oxidation Potentials," Prentioe-Hall, New York, N. Y . , 1983. (12) W. ill. Latimer and C. M . Rlansky, J . Am. Chem. soc., 62, 2019 (1940).
