This paper describes conductometric measurements of the influence of pressure on the molal dissociation constant Km of lanthanum ferricyanide ion pairs in water at 25°. The value of Km increases by a factor of 1.7 between 1 and 2000 atm., and the initial trend corresponds to a change in partial molar volume of -8.0 cm.3/mole for dissociation at 1 atm. The results agree quite well with the predictions of the ion-association theories of Bjerrum and of Fuoss, if it is assumed that the "contact distance," a, is independent of pressure and that the change of Km arises only from the increases in dielectric constant and density which occur when water is compressed.

The compressions at 25� of dioxan-water mixtures containing 0, 25, 50, 70, 80, and 100 wt.% dioxan have been determined at pressures up to 1500 bars. The mixtures show a minimum compression at a composition between 25 and 50 wt.% dioxan. The freezing pressure of dioxan at 25� is estimated to be 625 bars but measurements have been made on the metastable liquid at pressures up to 960 bars.

Calorimetric measurements have led to AHrO values for solid Cu(OH)z, CuCI,, C U ( I O~)~. H~O , and C L I ( I O~)~. These AHiO values have been used with data from earlier investigations in evaluating AGrO a_nd SZ9,0 values for these compounds. All of these quantities are based on "best" AGrO, AHrO, and SzO values for Cu2+(aq) derived in this paper.

The electrolytic conductance of solutions of potassium chloride and tetrabutyl- ammonium picrate over a range of concentrations have been measured in dioxan-water mixtures containing 0, 25, 50, 70, and 80% dioxan at 25� and pressures up to 1000 and 2500 bars respectively for the two solutes. The solvent concentration range corresponds to a range of dielectric constants of 78.3 (for water) to 11.98 (80% dioxan). The association of KCl in solutions of low dielectric constant is reduced by increasing pressure, so that the conductances of the solutions of finite concentration are not reduced as much by pressure as at infinite dilution. In contrast to this, the solutions of the tetrabutylammonium picrate are wholly dissociated even in very low dielectric constant solvents, as is shown by the limited concentration dependence of the conductance pressure characteristics.

Electrochemical transport calculations for ions in solutions of liquid mixtures at high pressures require some knowledge of the dielectric constant of the mixture under these conditions. If no reliable experimental dielectric constants are available, a simple linear interpolation method as described here may give useful estimates of sufficient accuracy. In the experimental work described elsewhere,l the dielectric constant of dioxanwater mixtures was of interest, and at an early stage of the work this had not been measured. Recently these data have become a~a i l a b l e ,~ and it is possible to compare the estimates with experimental values. 25% dioxan Pig. 1.-Dielectric constants