10-10 and was negligible in comparison with that of the solution in all CONDUCTANCE OF SOLUTIONS OF SALTS IN BROMINE

Transcription

1 VOL. 38, 1952 CHEMISTRY: MOESSEN A ND KRA US 1023 CONDUCTANCE OF SOLUTIONS OF SALTS IN BROMINE By GUSTAVE W. MOESSEN AND CHARLES A. KRAus METCALF RESEARCH LABORATORY, BROWN UNIVERSITY Communicated October 1, 1952 According to Darby,I trimethylammonium chloride is readily soluble in bromine at ordinary temperatures. The concentrated solutions are very viscous and are transparent with a pale yellow color. At 18 C. and a concentration of N, the equivalent conductance, A, is only ; at a concentration of N, the conductance is The increase of conductance with increasing concentration is a very general phenomenon in solvents of low dieledric constant. However, the high value of the conductance in bromine is exceptional. In view of these circumstances it appeared of interest to investigate the properties of solutions of salts in bromine somewhat further. We have measured the conductance of solutions of tetra-n-butylammonium bromide from approximately 0.01 to 1.3 N and that of trimethylammonium chloride from 0.03 to 3.6 N at 25 C. Measurements were made with alternating current in the audio frequency range. For the concentrated solutions a Jones bridge was employed and, for the dilute solutions of high resistance, a parallel arm bridge capable of measuring 100 megohms with precision. Two conductance cells were employed having constants of and They had a volume of 100 and 150 cc., respectively. The electrode chamber of the first cell was quite small so that only a comparatively small volume of solution was required. Concentrated solutions were made up by adding, successively, known weights of bromine to a known weight of salt, these weights being determined by weighing the cell and contents. More dilute solutions were made up by adding, successively, known weights of salt to a known weight of bromine. Corrections were made for the weight of solvent in the vapor phase. The cells were placed in an oil thermostat whose temperature was controlled to 25 A Bromine was purified by a slight modification of the method of Terwogt2 in which zinc oxide, prepared from zinc carbonate, was employed to eliminate iodine.' The specific conductance of the solvent was 1-6 X and was negligible in comparison with that of the solution in all cases. At 25, the density of bromine is 3.102; its viscosity is X 10-3 Poise; the dielectric constant is and the vapor pressure is 214 mm. In order to determine the concentration of the solutions on a volume basis, their densities were determined. Darby' had shown that the

2 1024 CHEMISTRY: MOESSEN AND KRAUS PROC. N. A. S. density of solutions of trimethylammonium chloride is a linear function of molal concentration; accordingly, the density of one concentrated solution was measured for each salt. For tetrabutylammonium bromide, the density is at M; for trimethylammonium chloride, it is at M. Densities of various solutions were then found by graphical interpolation. The salts were prepared and purified according to conventional methods. Tetrabutylammonium bromide melted at ; trimethylammonium chloride sublimed in the neighborhood of The results of conductance measurements are presented in table 2 for tetra-n-butylammonium bromide and, in table 1, for trimethylammonium chloride. Concentrations, in moles per liter of solution, are given in TABLE 1 CONDUCTANCE OF TRIMETHYLAMMONIUM CHLORIDE IN BROMINE AT 250 C A C A TABLE 2 CONDUCrANCE OF TETRABUTYLAMMONIUM BROMIDE IN BROMINE AT 250 c A c A column 1 and equivalent conductances, A, in column 2. The results are shown graphically in figure 1, where the logarithm of equivalent conductance is plotted as ordinate against the logarithm, of concentration as abscissa. Both salts are poor conductors at low concentrations; at N, the conductance of the bromide is approximately 30 times that of the chloride. The conductance increases greatly with increasing concentration. The conductance of tetrabutylammonium bromide reaches a maximum of at N; the conductance of trimethylammonium chloride has a maximum of at N. The conductance decrease at concentrations above that of the maximum is due to the increasing viscosity of the solution.

3 VoL. 38, 1952 CHEMISTR Y: MOESSEN A ND KRA US 1025 The form of the conductance curve of these salts in bromine conforms to that of similar salts in solvents of low dielectric constant. However, the conductance at higher concentration is much greater than in other solvents. Considering the viscosity of the solvent and the markedly greater (but unknown) viscosity of the more concentrated solutions, it follows that these salts are highly ionized at these concentrations. In accord with observations in other solvents, dilute solutions of terabutylammonium bromide in bromine are much more highly dissociated into ions than solutions of trimethylammonium chloride, approximately 30 times at N. At high concentrations, however, the two salts are dissociated to much the same extent. For the 2 bromide, the maximum appears at lower concentration than for the chloride because - _ e of the greater viscosity of its solutions as a result of its much nbu44nbr larger positive ion. 0 - The behavior of / MNHO these salts in bromine is in accord with the T view of Strong and T Kraus,4 that the more concentrated solutions of salts in solvents of - _ S lower dielectric constant ar.e best looked.t,_- upon as solutions of lo G solvent in supercooled, FIGURE 1 fused salts. This view Conductance of salts in bromine. is strongly supported by the measurements of Seward6 with solutions of tetrabutylammonium picrate in n butanol at 910C. Seward measured the conductance and the viscosity of this system all the way from pure butanol to pure fused salt. The conductance-viscosity product was found to be the same for the pure fused salt as for an infinitely dilute solution. The conductance-viscosity product passes through a minimum at 0.08 N. Above a concentration of 0.75 N, the product increases almost linearly with composition up to that of the pure fused salt. The conductance of an electrolyte, other things being equal, is nearly proportional to the fluidity (the reciprocal of viscosity) of the medium. The increase in the conductance-viscosity product is, to a first approximation, a measure of the increase in the number of free ions present.

4 1026 CHEMISTRY: MOESSEN A ND KRA US PROC. N. A. S. Many years ago, Lewis and Wheeler6 measured the conductance of potassium iodide in iodine at 140 C. This important investigation seems to have been overlooked, probably because concentrations and equivalent conductances were expressed in arbitrary units and could not be compared with those of other systems without carrying out the necessary computations. We have computed the concentrations and equivalent conductances of these solutions. In doing this, we have assumed the density of liquid iodine at 140 C. to be 3.7, that of potassium iodide, 2.9, and, further, that the volume of these solutions is equal to the sum of the volumes of solute and solvent. The val- KI in I o14(c. at ues computed on this basis are not precise because the data for the densities of iodine and potassium iodide relate to temperatures 15) _ differing somewhat from However, the error can hardly be greater than one no0 _ / \ per cent and does not affect any conclusions that we may draw. The conductance of o potassium iodide in 5()- iodine is shown graphically in figure2, where equivalent conductances are plotted as o _ ordinates and the log- I T 0 arithms of concentra- FIGURE 2 tions as abscissae. It will be noted that the Conductance of potassium iodide in iodine. conductance nd te increases icase from a value of 22 at N to 200 at 1.14 N. Above this concentration, the conductance decreases, reaching a value of 90 at 4.6 N. There appears to be a flat minimum at N. The conductance curve--f-potassium iodide in iodine is in most respects very similar to that of salts in bromine. However, the conductance is very much greater than it is for salts in the latter solvent. We do not know the value of the dielectric constant of liquid iodine, nor do we know its viscosit,y. The maximum in iodine occurs at practically the same concentration as in solutions of tetrabutylammonium bromide in bromine.

5 VOL. 38, 1952 CHEMISTRY: SINGER AND WA UGH 1027 The viscosity of iodine must be of the same order of magnitude as that of bromine and the same holds for solutions in the two solvents. The concentrations at which the minimum conductance appears would seem to indicate a dielectric constant markedly higher than that of bromine at 250. In benzene (D. const. = 2.28), the conductance minimum for a strong salt lies at 1 X 10-1 N; for anisole (D = 4.29), it lies at 1 X 10-3 and for ethylene bromide (D = 4.76), it lies at 5 X 10-3 N. On this basis, the dielectric constant of liquid iodine at 140 C. might be expected to lie in the neighborhood of 5.0. In any case, we cannot escape the conclusion that potassium iodide in iodine solution is very measurably dissociated into free ions at low concentrations and highly dissociated at higher concentrations. If we may be permitted to repeat, to gain an understanding of concentrated electrolytic solutions, we shall have to consider them from the point of view of solutions of solvent in salt. In solvents of low dielectric constant, the state of the system is determined by the charged ions of the electrolyte; the lower the dielectric constant, the lower the concentration down to which the ions are the dominant factor. 1 Darby, Edward H., J. Am. Chem. Soc., 40, 348 (1918). 2 Terwogt, P. C., Z. anorg. Chem., 47, 203 (1905). 3 Scott, A., J. Chem. Soc., 103, 347 (1913). 4 Strong, L. E., and Kraus, C. A., J. Am. Chem. Soc., 72, 166 (1950). 5Seward, R. P., Ibid., 73, 515 (1951). 6 Lewis, G. N., and Wheeler, P., Proc. Am. Acad., 41, 419 (1906). VELOCITY ULTRA CENTRIFUGA TION OF HETEROPOL Y ACIDS By S. J. SINGER* AND JOHN L. T. WAUGH GATES AND CRELLIN LABORATORIES OF CHEMISTRY, t CALIFORNIA INSTITUTE OF TECHNOLOGY, PASADENA Communicated by Linus Pauling, October 6, 1952 This communication is concerned with some preliminary velocity ultracentrifuge experiments which we have performed with 12-tungsto-silicic acid and some other heteropoly acid salts. It was suggested to us by Professor L. Pauling that because of their large densities these compounds might sediment at measurable rates in the ultracentrifuge despite their relatively low molecular weights. Upon investigation it was found that not only were the sedimentation constants (s) easily measurable, but diffusion constants (D) could be determined from the same sedimentation diagrams. In combination with partial specific volume (V) determinations, it was possible to calculate molecular weights for the anions in question by means of the familiar Svedberg rate sedimentation equation:'