Ionic solutions under high pressures VI : mobilities and hydration of monocarboxylate ions

Abstract

The electrical conductivities of potassium acetate, propionate and butyrate in water were measured in the concentration range 10^-1 to 10^-3 N up to 5, 000 atm at 25℃. The equivalent conductivity at infinite dilution of each salt is obtained by means of Onsager's limiting equation and then separated into the limiting ionic equivalent conductivities by using the conductivity and transference number data for potassium chloride. According to the same line, the equivalent conductivities of the bulky monovalent ions studied till now by the authors are recalculated. The plot of the limiting ionic equivalent conductivity against pressure at 25℃ has a maximum for acetate and propionate ions as seen in the case of simple monatomic ions, but no maximum for perchlorate, tetraalkylammonium and butyrate ions. The Walden product has a small positive derivative with respect to pressure at 1 atm only for acetate ion and negative one for all the other ions. The negative sign of the pressure coefficient of the Walden product is interpreted in terms of the higher density of water in the hydration shell than in the bulk. It is also pointed out that the compression of hydrated ions with the application of pressure is a factor of great importance that makes the Walden product increase with increasing pressure.