Feb., 1956
CONDUCTANCES OF COMPOUNDS IN ACETONE
equal to about 6 X lo-' mole/liter while in formic acid ( D I ~= 58.5) the dissociation of triphenylcarbonium chloride is detectable but its "extent is too small for quantitative estimation." Acknowledgment,-This work was supported by a Frederick Gardner Cottrell Grant from The Re-
169
search Corporation and by the National Science Foundation under Grant NSF-G 436. The diand tri-p-phenyl derivatives of trityl chloride were prepared and analyzed by Mr. Michael J. Vignale. "Super-pure" magnesium was a gift of the Dow Chemical Company.
CONDUCTANCES OF POTASSIUM THIOCYANATE AND TETRA-n-BUTYLAMMONIUM IODIDE IN ACETONE AT SEVERAL TEMPERATURES WITHIN THE RANGE 25 TO -50°'a'b BY PAUL G. SEARS,EUGENE D. WILHOIT AND LYLER. DAWSON Department of Chemistry, University of Kentucky, Lexington, Kentucky Received August 17, 1866
The conductances of potassium thiocyanate and tetra-n-butylammonium iodide in acetone have been measured at 25' and at ten-degree intervals between 20 and -50". Limiting equivalent conductances and dissociation constants have been determined by the Shedlovsky extrapolation method. The results for each salt in acetone indicate that dissociation increases with decreasing temperature and that the logarithm of the limiting equivalent conductance is a linear function of the reciprocal of the absolute temperature.
Introduction Acetone is among the few solvents for which dielectric constant data are known for temperatures as low as -50". Since the availability of this information is necessary for a theoretical treatment of conductance data, it was selected as an electrolytic solvent for a conductance study a t low temperatures. Acetone is a volatile liquid of low viscosity having a dielectric constant of 20.7 which has been used as a solvent in several earlier conductance studies at 25". These studies have shown that electrolytes in this solvent generally have dissociation constants of approximately The purpose of this investigation has been to study the variations of the conductances and the dissociation constants ,of potassium thiocyanate and tetra-nbutylammonium iodide in acetone a t lower temperatures ranging to - 50". Experimental 1. Purification of Solvent.-J. T. Baker Analyzed reagent acetone was refluxed for several hours over Activated Alumina and then was fractionated through an efficient column a t atmos heric pressure. The middle fractions which were retained gad the following properties a t 25": conductivity, 2 x 10-8 ohm-' cm.-l; density, 0.7845 g./ml.; viscosity, 3.02 X IO+ poise; and dielectric constant, 20.7. 2. Purification of Salts.-Reagent grade potassium thiocyanate was recrystallized twice from distilled water. The best commercial grade of tetra-n-butylammonium iodide obtainable was recrystallized twice from ethanol-ether mixtures. Both salts were dried to constant weight in a vacuum oven at 70". 3. Apparatus and Procedure.-Four conductance cells (two each of numbers 4943 and 4944, Type A, b e d s and Northrup Catalog EN-95) were used in the conductance measurements. The constants of these cells with lightly platinized electrodes were based upon the intercomparison of resistances with a cell for which the constant was determined using 0.01 demal potassium chloride solutions according to the method of Jones and Bradshaw.2 A Jones conductivity bridge with an oscillator and an amplifier capable (1) (a) Presented a t the 128th Meeting of the American Chemical Society in Minneapolis. September, 1955; (b) based in part on research performed under a contract with the U. 8. Army Signal Corps. (2) G. Jones and B. C. Bradshaw, J . A m . Chem. SOC.,65, 1780 (1933).
of producing 500, 1000 or 2000 cycles per secondwas used to measure resistances which were observed to show no significant frequency dependence. The temperature of the solutions in the conductance cells was regulated by a manually-controlled thermostat consisting of a five-gallon Dewar flask filled with denatured alcohol which was cooled by the addition of powdered Dry Ice. Temperatures were measured with three total-immersion thermometers which are compared frequently against two thermometers calibrated by the National Bureau of Standards. Measurements were made at 25" and a t each tenIn all cases the degree interval between 20 and -50". temperature control was effective within 0.2 degree. Two independent series of solutions covering the concentration range of 1-30 X IO-' N were prepared for each salt by the weight dilution of stock solutions with all material transfers made in a dry box. Suitable buoyancy corrections were applied to all weights. The concentrations were converted from a weight to a volume basis on the assumption that the densities of the dilute solutions were equal to that of the solvent at a given temperature. Three size-25 Ostwald-Cannon-Fenske viscometers were used to make triplicate determinations of the viscosity of acetone a t each temperature. Calibrations of the viscometers were based upon the viscosity of water as 1.002 cuntipoise a t 2 0 O . 8 The change of a viscometer constant with temperature was calculated using the equation of Cannon and F e n ~ k e . Kinetic ~ energy corrections were considered to be negligible. At each temperature the conductivity of the salt was obtained by subtracting the conductivity of the solvent from that of the aolution. Data pertinent to the density, viscosity and dielectric constant of acetone which were used in the calculations are presented in Table I. The values of the necessary fundamental constants were taken from the lateat report of the Subcommittee on Fundamental Constants.s
Results Corresponding values of the equivalent conductance, A, and the concentration in gram equivalents per liter, C, for one of the two independent series of solutions for each salt are presented in Tables I1 and 111. Data for the confirmatory series of solu(3) J. R. Swindells, J. R. Coe and T. B. Godfrey. J . Research Natl. Bur. Standards, 4 8 , 1 (1952). (4) M. R. Cannon and M. R. Fenske, I n d . En& Chem., Anal. Ed., 10,297 (1938). (5) F. D.Rosaini, F. T. Gncker, Jr., H. L. Johnston, L. Pauling and Q. W. Vinal, J . Am. Chem. Soe., 74,2699 (1952).
P. G. SEARS, E. D. WILHOIT AND L. R. DAWSON
170
Vol. 60
TABLE I TABLE I1 BOME PHYSICAL PROPERTIES OF ACETONE AT TEMPERATURES EQUIVALENT CONDUCTANCE OF POTASSIUM THIOCYANATE IN WITHIN THE RANGE 25 TO -50" ACETONEAT TEMPERATURES WITHIN THE RANGE25 TO Temp., OC.
25 20 10 0 10 20 30 40 50
-
Density,' g./ml.
Viscosity, poise X 1000
Dielectric8 constant
0.7845 .791 .802 ,814 ,825 .836 .846 .857 .868
3.02 3.18 3.51 3.91 4.37 4.96 5.67 6.57 7.77
20.7 21.2 22.2 23.3 24.4 25.6 26.8 28.1 29.5
-50" c! X 104
A
(a) 25"
I
I
200.
A
191.0 178.5 162.2 145.8 135.7
(d) 0" 1.035 146.2 3.387 139.4 9.100 127.3 18.52 116.5 27.57 111.8
(e) -10" 1.049 131.1 3.433 125.3 9.224 114.9 18.77 105.9 27.94 101.2
(f) -20" 1.063 116.0 3.479 111.3 9.346 102.5 19.02 95.0 28.32 89.9
-30" 101.7 97.9 90.5 84.2 81.5
(h) -40" 1.089 88.2 3.566 84.9 9.581 78.8 19.50 73.6 29.03 71.3
(i) -50" 1.103 74.7 3.612 72.1 9.704 67.1 19.75 63.0 29.40 61.1
(g)
160.
C X 104
(c) 10" 1.019 164.7 3.337 154.5 8.966 141.7 18.25 129.1 27.16 121.7
1.075 3.520 9.458 19.25 28.65
180
A
(b) 20" 1.005 181.6 3.291 169.9 8.843 155.5 18.00 140.2 26.79 130.9
0.9971 3.264 8.771 17.85 26.57
tions for each salt have been deposited with the American Documentation Institute.6
C X 10'
TABLE 111 EQUIVALENT CONDUCTANCE OF TETRA-n-BUTYLAMMONIUM IODIDEI N ACETONEAT TEMPERATURES WITHIN THE RANQE
140.
A.
120. 100.
80.
60. 1
0
25 TO C X 10'
-50"
(a) 25" 0.5440 173.9 1.748 168.0 5.034 158.5 10.41 147.3 16.48 139.7
(b) 0.5485 1.762 5.076 10.50 16.61
20" 165.4 160.2 151.4 140.9 134.0
( 4 0" 0.5644 134.7 130.8 1.814 5.223 124.1 115.9 10.80 17.10 109.9
(e) -10" 0.5721 120.6 1.838 117.4 5.294 111.4 10.95 104.3 17.33 98.0
c x 104 A (c) 10" 0.5561 150.0 1.787 145.5 5.146 137.9 10.64 128.6 16.85 122.4 (f) -20" 0.5797 106.8 1.863 103.7 5.365 98.9 11.09 92.6 17.56 89.2
-30" 93.5 90.9 86.7 81.3 78.8
(h) -40" 0.5942 80.3 1.909 78.2 5.499 74.7 11.37 70.2 18.00 66.7
(i) -50" 0.6019 67.7 1.934 66.0 5.570 63.1 11.52 59.4 18.23 57.5
C X 10'
I
2
3
4
5
6
1
IOOJT. Fig. 1.-Equivalent conductance of potassium thiocyanate in acetone as a function of the square root of the concentration. Dashed lines represent Onsager slopes.
Discussion A plot of the equivalent conductance of potassium thiocyanate in acetone as a function of the square root of the concentration is shown in Fig. 1 for each of the temperatures a t which the study was made. A corresponding figure for tetra-n-butylammonium iodide has been omitted because of similarities in the behavior of the two salts. It may be observed from Fig. 1 that plots for the various temperatures differ only in the magnitude of displacement along the ordinate and in the slope which increases with increasing temperature. The plots (13) Material supplementary to this article has been deposited as Document number 4671 with the AD1 Auxiliary Publications Project, Photoduplication Service, Library of Congress, Washington 25, D. C. A copy may be secured b y citing the Document number and by remitting in advance $1.25 for photoprints or $1.25 for 35 mm. microfilm, by check or money order payable to: Chief, Photoduplication Service, Library of Congress. (7) F. J. Shell, Dissertation, University of Kentucky, 1953. (8) A. A. Maryott and E. R. Smith, National Bureau of Standards Circular 514, August 10, 1951,
(g)
0.5866 1.885 5.429 11.23 17.77
A
A
are non-linear and have limiting slopes which are appreciably more negative than those calculated for a completely dissociated electrolyte; consequently, evaluation of the limiting equivalent conductance, A,, by direct extrapolation of the A versus
