Abstract
Two types of ultrasonic interferometer have been used to measure the wavelength of compressional waves in aqueous, methanol and ethanol solutions of electrolytes; densities of the solutions studied have been measured. Velocities of sound, compressibilities, and apparent molal compressibilities have been calculated from the density and wavelength data over a range of concentrations up to saturation. Velocities of sound usually increase with increasing solute concentration; however in a few cases where heavy, or complex ions occurred, the velocity decreased with increasing solute concentration. The compressibility of a solution is always less than that of the pure solvent, and the extent of the decrease has been used to determine solvation numbers, using an empirical equation which is related to a theoretical equation derived by Passynski. Apparent molal compressibilities ϕ[2] were calculated, and the extrapolated values at infinite dilution determined, these were found to be related to the solvation numbers. The variation of ϕ[2] with C[2] [1/2] is linear as can be shown from the Debye Huckel theory; deviations from the theoretical slope have been accounted for by an equation analagous to that used by Onsager in electrical conductance work. The solvation numbers at infinite dilution of a series of solutes have been determined in water, methanol, and ethanol; individual ionic hydration numbers have been suggested. The solvation of anions in alcohols follows the reverse order to their hydration, whereas the orders of solvation of cations in water and alcohols are the same; this has been explained in terms of orientation of the solvated solvent molecules. Acids and tetra-alkyl ammonium salts show unexpected solvation numbers, it is suggested that this is due to opposing effects of "structure making" in the solvate shell, and "structure breaking" at the boundary of the solvate shell; the greater effect determines the overall order of the solution, and thus the observed solvation number.