TRANSFERENCE NUMBERS OF POTASSIUM CHLORIDE IN FORMAMIDE AND IN N-METHYLACETAMIDE
Dissociation Studies in High Dielectric Solvents. VII.
197
Transference Numbers
of Potassium Chloride in Formamide and in N-Methylacetamide
by Gyan P. Joharil and P. H. Tewari2 Department of Chemktry, University of G'oralchpur, Gorakhpur,
U.P., India (Received July 8, 1966)
The transference numbers of KCI in formamide at 25' and in N-methylacetamide at 40' have been measured by the modified Hittorf method.
Introduction Precise data on transference numbers in nonaqueous solvents, particularly in amides, are scarce in the literature. The problem of their measurements has been less tractable on account of a number of difficulties involved either in the handling of these solvents, which are highly susceptible to the absorption of moisture, or in the chemical determination of the concentration changes. This has seriously hampered the interpretation of the conductance data relating to ion-solvent interactions. The essential need for such data is exempli6ed further by the recent interest in the behavior of electrolytes in these solvents. However, reasonable estimates of transference numbers have been made on the basis of the behavior of very large ions in relation to the solvent viscositya; the extent and the pattern of solvation of anions and cations in a given solvent may, in some cases, be far from identical and therefore an estimate on this basis sometimes may be misleading. In previous communications4of this series we have reported the conductance data of several high-charge salts in formamide and in mixed solvents. We report in this paper the transference number data of KC1 in amide solvents. The purpose of this study was to have an insight into the nat,ure of ion-solvent interactions as well as to supplement our conductance results for unsymmetrical salts. Because of the difficulty involved in the chemical determination of the concentration changes in the very dilute range, Hittorf's method, aa modified by Steel and Stokes16was used for the measurements. Experimental Section Materials. N-Methylacetamide (B.D.H., London) was shaken with PzOs(10 g./l.), filtered through glass
wool, and distilled fractionally at a pressure of 1 mm. when the temperature remained between 61 and 65'. This was repeated twice and was followed by three more fractional distillations through a 100 X 4 em. column, packed with glass helices, at reduced pressure, without previous PzOS treatment. The solvent used for the measurements had the following properties at 40': specific conductance of less than 3 X lo-' ohm-l em.-'; density, 0.9423 g./ml. (k60.9421) ; and viscosity, 0.0303 poise (lk6 0.03019). Formamide was purified by the method described earlier.'b Solvent with a specific conductance of less than 2 X ohm-l em. was used for experiments. Potassium chloride was the same as used in the previous studies.' Apparatus. The form of the apparatus was essentially the same as described by Steel and StokesS6 Electrodes (cathode and anode) were made of platinum wire (20 X 0.05 cm.), shaped into a helix. These were coated heavily with silver from a solution of potassium argentocyanide. The cathode was electrolytically coated with silver chloride as re~ommended.~ (1) This work was supported in part by a research grant from the Council of Scientific and Industrial Research and in part by funds from the University Grants Commission, New Delhi, India. Grateful acknowledgments are made to the sponsors. (2) On leave of absence from the University of Gorakhpur, Gorakhpur, India. (3) L. R. Dawson, E. D. Wilhoit, R. R. Holmes, and P. G. Sears, J. Am. Chem. SOC.,7 9 , 3004 (1957). (4) G. P. Johari and P. H. Tewari (a) J. Phys. Chem., 67, 512 (1963); (b) ibid., 6 9 , 696 (1965); (0) ibid., 69, 2857 (1966); (d) ibid., 6 9 , 2862 (1965); (e) ibid., 6 9 , 3167 (1965); (f) J . Am. Chem. Soc., 87, 4691 (1965). (5) B. J. Steel and R. H. Stokes, J . Phys. Chem., 62, 450 (1958). (6) L. R. Dawson, P. G. Sears, and R. H. Graves, J. Am. Chem. SOC.,7 7 , 1986 (1955). (7) W.R. Carmody, ibid.,51, 2901 (1929); 54, 188 (1932).
Volume 70,Number 1 January 1966
GYANP. JOHARIAND P. H. TEWARI
198
Measurements. The electrolysis current was obtained from storage cells (8-10 ma. for 2-3 hr.). This was measured by the potential drop across a 100-ohm resistance coil of an L &, N resistance box, which was standardized previously. The potential drop was measured by an L & N potentiometer. The electrolysis current, which changed steadily during the experiment (the change never exceeded 0.5%), was measured at frequent intervals (to within k O . 1 sec.). The total quantity of electricity passed in coulombs was obtained by a tabular integration. The specific conductances of solutions were measured by the bridge assembly described earlier.4b The change in the concentration of the cathode solution, Act was measured by a rather direct method. The initial concentration of the cathode solution was known to within =tO.Ol%. After the completion of the experiment, the specific conductance of the final solution was also known. I n the method followed, the original solution was taken in a flask-type conductance cell, and the specific conductance was measured. Weighed aliquots of more concentrated solution were added to it by a weight pipet until the specific conductance of the solution was close to that of the final concentration of the cathode solution in a transportcell run. The specific conductance in the narrow range of concentration (within 3=2% of the specific conductance of the final cathode solution) was measured and plotted against C. The final concentration was interpolated from this plot. Since the equivalent conductance of KC1 in these solvents is comparatively less sensitive to changes in the concentration, the specific conductance-C plot, within the above concentration range, was linear. This method could not give AC with the needed accuracy of 0.1% for two reasons: (i) the specific conductance of solutions, owing to the lower mobility of ions, is very small and is thus far less sensitive to AC; (ii) solvents are too hygroscopic and undergo decomposition on keeping. However, the method still affords a better way of determining the concentration changes. The number of figures for C1,C2, and AC values given here are 1 unit greater than would be significant considering the probable error in the concentration calculated from the resistances. The procedure for a transport-cell run was the same as described by Steel and Stokes. However, a difference of +0.02% in the specific conductance of the initial and final solution in the cell was consistently observed in all experiments. This may be due to the slight instability of these solvents. The correction to the quantity of coulombs due to the conduction of a fraction of current by the solvent
was disregarded on the grounds that the error introduced by it would be well within the error in the final values. All measurements were made in an oil bath at 25 k 0.005' (in the case of formamide) at 40 0.01' (in the case of N-methylacetamide). The usual precautions were observed to ensure tight seals through 10/30 Teflon sleeves and absence of air bubbles in the solution.
*
Results The experimental data were used to calculate the transference number of K+ by tg+ =
F(C2
- C~)V/lOOOp
where Cl and C2 are the initial and the final concentrations ( M ) , V is the volume (ml.) of the solution in the cathode compartment (including the conductance cell arm), F is the faraday, q is the quantity of electricity passed in coulombs, and tg+is the transference number of K+ at concentration Cl. These apparent transference numbers were converted to the conventional Hittorf values by use of the data for solution densities. The experimental data and the calculated transference numbers of K+ in N-methylacetamide at 40' and in formamide at 25' are summarized in Tables I and 11, respectively. The values presented here are the average of duplicate determinations which did not vary by more than 0.1%. Table I: Transference Numbers of KC1 in N-Methylacetamide at 40" 9,
V,
CI, M
C:, iK
coulombs
ml.
0.0195874 0.0257462 0.0323415 0.0391472 0.0428500 0.0498932 0.0634218
0.0269191 0.0343928 0.0408185 0.0469105 0.0618637 0.0584506 0.0720714
50.173 60.231 59.741 54.377 63.239 60.321 61.353
30.41 30.39 30.66 30.40 30.45 30.39 30.42
4
tt
(SPPSrat) (Hittorf)
0.429 0.421 0.420 0.419 0.419 0.416 0.414
0.429 0.421 0.4205 0.4195 0.420 0.417 0.415
Table II : Transference Numbers of KC1 in Formamide at 25" q,
el, M
0.0266793 0.0298340 0.0417630 0,0768288 0.1326320
cou-
M
lombs
0.0398761 96.515 0.0385620 64.098 0.0558145 103.46 0.0889048 * 89.823 0.1468930 107.19
v,
4
t+ (Hit-
Id
(SppfC-t)
blf)
30.40 30.54 30.55 30.38 30.46
0.401 0.401
0.401 0.4015 0.401 0.395 0.393
0.4005
0.394 0.391
TRANSFERENCE NUMBERS OF POTASSIUM CHLORIDE IN FORMAMIDE AND IN N-METHYLACETAMIDE
1
2
3
4
5
0
I
1
I
1
I
I
I
0.43
0.42 7
\+
e
0.42
0.41
.----
w
0.40
2
4
6
8
12
IO
14
1ooc. Figure 1. The concentration dependence of the Longsworth function for potassium chloride: 0,N-methylacetamide; 0, formaraide.
Owing to the instability of these solvents and the difficulty of keeping them anhydrous as well as the problems involved in the determination of the change in concentration, there may be a systematic error in the final results of 0.5-1%. The limiting transference number of K+ was derived from extrapolation of a plot of the Longsworth function? P+', against concentration. tO+'
= (&A'
+ l/&C'")/(A' + j3C'")
where A' i8 defined as
=
t"+
+ BC
199
A' = Ao - (aAo f @)d"
where no,a, and @ are the terms as defined in ref. 4, and 13 is an empirical constant, characteristic of each electrolyte-solvent system. The Longsworth function plots against C in Nmethylacetamide at 40' and in formamide at 25' are shown in Figure 1. (Ao values for KC1 used in calculations were taken from Dawson's data: Ao (in N-methylacetamide) = 20.1,'j Ao (in formamide) = 29.85.9) The best straight line passing through these points was obtained by the method of least squares. The plots give the limiting transference number of Kf in N-methylacetamide at 40' equal to 0.4292, and in formamide they give 0.4093. As seen, our value of the transference number of K + in formamide is about 1% higher than Dawson's value.I0 The difference is partly due to the variation in the experimental technique and partly t o the fact that Dawson's study lacked data on lower concentrations which are needed to obtain the extrapolated value with sufticient accuracy. The considerably lower value of to=+ in these solvents as compared to that in water suggests the existence of strong ion-solvent forces. This is because the molecules of these compounds have high dipole moments. A complete explanation of the magnitudes of the individual ionic mobilities in these solvents would involve, however, along with the short-range ion-solvent interactions, the changes in viscosity resulting from the influence of ions in modifying the solvent structure. Such an explanation must be deferred at present. Acknowledgments. The authors wish to thank Prof. R. P. Rastogi, Head of the Department of Chemistry, University of Gorakhpur, for providing the essential facilities. (8) L. G. Longsworth, J. Am. C h . Soe., 54, 2741 (1932). (9) L. R. Dawson, E. D. Wilhoit, and P. G. Sears, ibid., 79, 5906
(1957). (10) L. R. Dawson and C. Berger, ibid., 79,4269 (1957).
Volume 70, Number 1
January 1086
