SOLVENTS HAVINGHIGHDIELECTRIC CONSTANTS
775
Solvents Having High Dielectric Constants. XVII.
Electromotive Force of the
Cell Pt, H,; HCl(m); AgC1-Ag in N-MethylacetamideDioxane Mixtures at 40"'
by Lyle R. Dawson, K. Hong Kim,and Hartley C. Eckstrom Department of Chemistry, University of Kentucky, Lexington, Kentucky
(Received September I S , 1965)
The emf of the cell Pt, Hz; HCl(m); AgC1-Ag has been measured at 40" in N-methylacetamidedioxane mixtures ranging from 29.98 to 73.85% dioxane. Solute concentrations varied from 2 X to 2 X lo-' m. In these mixtures the standard electrode potential varied from 0.16772 t o 0.02727 v as the dielectric constant became smaller. Absence of regularity in the relationship of E" and dielectric constant is attributable to differences in basicity and other chemical properties of the solvent mixtures. Activity coefficients of HC1 in the solvent mixtures have been calculated.
Thermodynamic properties of solutions of hydrogen chloride in N-methylacetamide (NRIA) have been investigated in this laboratory through electromotive force studies using platinum-hydrogen and silversilver chloride electrode^.^*^ These electrodes are stable and reversible in this solvent which has a very high dielectric constant (165.5 at 40'). The mean molal activity coefficients of HC1 in NMA a t given concentrations are greater, and these values decrease less rapidly with increasing concentration than in water or in ethanol. The present investigation was designed to study the electromotive force of the hydrogen and silver-silver chloride electrode systems in NMA-dioxane mixtures. The dielectric constant of dioxane is approximately 2. The resulting data were sought for use in determining some properties of solutions of hydrogen chloride in these solvent mixtures with which a wide range of dielectric constants can be obtained.
Experimental Section The apparatus which was used has been described adequately ea~-lier.~-~ Electrical leaks were minimized by shielding. Measurements were made at 40 f 0.03' in a mechanically stirred mineral oil bath. The conductivity of the NMA was 3 4 x lo-* ohm-1 cm-1. Hydrogen - - chloride solutions in the NMA-dioxane mixture were prepared conveniently by using HC1* This was made by NMA in anhydrous ether and bubbling hydrogen chloride gas slowly
into the solution. The white precipitate of (NMA)2. HC1 was recrystallized three times from acetone and dried several hours in a vacuum desiccator over anhydrous magnesium perchlorate. Since the compound is quite hygroscopic, it was prepared and purified under 1 atm of nitrogen. The chloride content of the salt was found to be 19.37-19.40y0 (theoretical value 19.41) by the Volhard method. Commercial dioxane was purified by the method described by Kraus and Vingeee in which it was first refluxed oyer NaOH for 4 or 5 hr and then fractionally distilled. The middle portion of the distillate was retained and was refluxed over metallic sodium and then fractionated. The latter process was repeated twice. Each cell solution in each solvent mixture was prepared independently in a 100-ml volumetric flask. For the stock solutions of HCl in each solvent mixture, freshly prepared, purified, and analyzed samples of (NMA)2*HC1were used. (1) This work was supported in part by a. research grant from the
U. S. Atomic Energy Commission. (2) L. R. Dawson, R. C. Sheridan, and H. C. Eckstrom, J . Phys. Chem., 65, 1829 (1961). (3) L. R. Dawson, W. H. Zuber, Jr., and H. C. Eckstrom, ibid., 69, 1335 (1965). (4) L. R. Dawson, E. D. Wilhoit, and P. G . Sears, J . Am. Chem. Soc., 7 8 , 1569 (1956). ( 5 ) R. G . Bates, "Electromotive PH Determinations," John Wiley and Sons, Inc., New York, N. Y . , 1954. (6) C . A. Kraus and R. A. Vingee, J . Am. Chem. SOC.,56, 511
(1933).
Volume 70,Number S
March 1966
776
L. R. DAWSON, K. H. KIM, AND H. C. ECKSTROM
The emf cell was provided with a stopcock6which was kept closed during the time between series of readings to prevent poisoning of the hydrogen electrode by the trace amount of AgCl dissolved in the cell solution. Four NXA-dioxane solvent mixtures ranging from 25 to 7 5 wt % NMA were prepared, and their dielectric constants were measured using a General Radio Type 821-A Twin-T impedance measuring circuit. A plot of dielectric constant us. weight per cent of NMA was prepared. Dielectric constant values for the various solvent mixtures used were obtained from this graph. Densities and vapor pressures of the various solvent mixtures were obtained in a similar manner.
equals 0.15465; @ is an empirical constant. The terms making up E' involve only experimentally determined quantities. Values of E' were plotted us. m to obtain a line with a slope of 2(2.303)RT/F and an intercept EO. In each case a least-squares fit of the calculated values of E' was obtained, and from this the intercept, EO, was determined. With the standard potential, E O , for a given mixture
Table 111: Summary of Emf Data for HCI in NMA-Dioxane Mixtures at 40" Molality of HC1
Results Properties of the solvent mixtures used for emf measurements as well as data for those used to establish the graph for dielectric constants are shown in Tables I and 11. Table I : Properties of Solvent Mixtures a t 40" Mixture no.
I I1 I11
O/b
70
NB1.4
dioxane
70 02 56 90 26.15
29.98 43 10 73.85
-
Vap press, mm
Density,
44.0 54.2 78.2
0.9632 0.9729 0.9956
Die1ec tric oonstant
g/cc
89.5 65.7 24.8
Table I1 : Experimental Values for Dielectric Constants of NMA-Dioxane Mixtures a t 40' Mixtu19 no.
76
76
NMA
dioxane
Dielectric constant
1 2 3 4
74.44 60.82 50.76 25.32
25.56 39.18 49.24 74.68
98.1 72.4 55.8 23.2
A summary of emf values, corrected to a pressure of 1 atm, as a function of molality of HC1 for three solvent mixtures is given in Table 111. Values of E' were calculated from the equation2
E
2(2.303)RT log m + 7 -
-
2(2.303)RT
a+
P E'
=
Eo
-
=
2(2.303)RT @m F
For NMA a t 40°, the Debye-Hiickel constant, a, The Journal of Physical Chemistry
Emf (cor), V
E', v
0.00245 0.00905 0,00958 0.01197 0.1544 0.02334 0.02527 0.02671 0.04291 0.06000 0.09350 0.12331 0.20345
Solvent mixture no. I, 29.98% dioxane, E" = 0.16772 v 0.49506 0.42593 0.42358 0.41117 0,39735 0.37657 0.37241 0.36907 0,34460 0,32759 0.30464 0.29040 0.26401
0.16825 0.16743 0.16801 0.16707 0.16627 0.16641 0.16624 0.16567 0.16467 0,16393 0.16198 0.16048 0,15628
0.00443 0.00713 0,00991 0.01394 0.02066 0.02642 0.03203 0.04701 0.05828 0.08878 0.11599 0.12736 0,19790
Solvent mixture no. 11, 43.170 dioxane, E" = 0.14041 v 0.43646 0.41325 0.39783 0.37890 0.35906 0.34700 0,33838 0.31854 0.30705 0.28708 0.27392 0.26986 0.24737
0.13889 0.13999 0.14117 0.13923 0.13865 0.13842 0.13893 0.13689 0.13511 0,13351 0.13150 0.13123 0.12577
0.00217 0.00411 0.00632 0.00867 0.01135 0.01993 0.02383 0.03382 0.05018 0.06278 0.10089
Solvent mixture no. 111, 73.85y0 dioxane, E o = 0.02727 v 0.37157 0.02630 0.34217 0.02591 0.32421 0.02447 0,31012 0.02455 0.29871 0.02323 0,27316 0.01659 0.26693 0.01561 0.25506 0.01286 0.23841 0.00423 0.22976 -0.00113 0.20562 - 0.02190
SOLVENTS HAVINGHIGHDIELECTRIC CONSTANTS
the mean ionic activity coefficients at the various molalities were calculated from the equation
Values of the activity coefficient for HC1 at rounded molalities in the three solvent mixtures are shown in Table IV.
Table IV : Activity Coefficients for HC1 in NMA-Dioxane Solvent Mixtures Molality of HC1
0.005 0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.08 0.09 0.10
1, Y
i
0.94 0.92 0.90 0.89 0.87 0.86 0.86 0.85 0.85 0.84 0.84
11, Y*
0.91 0.88 0.84 0.82 0.80 0.78 0.77 0.76 0.75 0.74 0.73
111, Y
I
0.68 0.59 0.50 0.44 0.41 0.38 0.37 0.35 0.34 0.33 0.33
Discussion The emf data plotted against molality of HC1 for each of the solvent mixtures yield plots which are smooth curves. From these results and the general behavior of the electrodes during the study, it is assumed that the silver-silver chloride electrode is stable, reversible, and reproducible in NMA-dioxane mixtures. At 40’ the standard reduction potential of the silversilver chloride electrode in NMA is 0.20573 v ; ~in water it is 0.21208 v.’ Values of 0.16772, 0.14041, and 0.02727 for Eo found in this investigation indicate the rapid decrease in reduction potential with increasing
777
percentage of dioxane and the resulting decrease in dielectric constant of the mixture. Considering the Eo value of 0.181 v for this electrode at 40’ in formamides with a dielectric constant between those for water and NMA, it becomes apparent that, even in media of very high dielectric constant, the chemical nature of the solvent molecules is a factor of major importance. A similar dependence of E o upon the chemical nature of the solvent was found by Harned and co-workersg for several solvents having lower dielectric constants. Absence of complete uniformity in the relationship of Ea and dielectric constant and the lack of applicability of the Born equation for charging ions in a solvent mixture has been attributed1° to preferential solvation of the ions by one of the components or HC1 being incompletely dissociated in mixtures having lower dielectric constants. In the solvents studied in this investigation the decrease in E o proceeds more rapidly than the decrease in dielectric constant. It is likely that the principal protonated species is NMA.H+. Strong interionic effects and even quite incomplete dissociation at higher solute concentrations may exist in the solvent mixtures containing larger percentages of dioxane. Indeed, the basicities of the components of the solvent mixture are important. In addition, the activity coefficients of HC1 decrease much more rapidly with an increase in solute concentration in those solutions having larger percentages of dioxane and having lower dielectric constants. This would be expected as a result of the more effective interionic attraction as the dielectric constant is decreased. (7) R. G. Bates and G. E. Bower, J . Res. A’atl. Bur. Std., 53, 283 (1954). (8) M. Mandel and P. Decroly, Trans. Faraday Soc., 56, 29 (1960). (9) H. 5. Harned and B. B. Owen, “The Physical Chemistry of Electrolytic Solutions,” Reinhold Publishing Corp., New York, N. Y., 1958. (10) J. A. V. Butler and C. M. Robertson, Proc. Roy. SOC.(London), A125, 694 (1929).
Volume 70.h.’umber d March 1966
