Measurement of the temperature of maximum density of aqueous solutions of some salts and acids

Abstract

The temperatures of maximum density (θ) of aqueous solutions of alkali-metal and ammonium halides and mineral acids have been measured at low concentrations (0.002–0.08 mol kg^–1). Plots of the shift in θ(Δθ_obs) against concentration yielded smaller slopes (termed the Despretz constant, K_m or K_D) than those evaluated from literature data. In addition, K_m is found to have different values at low and high concentrations. The earlier hypothesis of a linear relationship between Δθ_obs and concentration (Despretz law) over a wide range of concentration is disproved, a finding that is supported by theoretical considerations. The results have been used to define limiting ionic Despretz constants for various ions.

The individual limiting ionic Despretz constants computed using the additivity principle are found to be in the order F^– < Cl^– < Br^–⩽ HSO₃^– < I^– < ClO₄^– < SO₄^2– for anions and H⁺ < Li⁺ < NH₄⁺ < K⁺ < Na⁺ < Rb⁺ < Cs⁺ for cations.

Limiting ionic K_m values correlate better with the effective ionic radii of ions computed by Leyendekkers using the Tammann–Tait–Gibson (TTG) model of aqueous solution properties, than Pauling ionic radii. From a plot of K_m against effective ionic radius it has been possible for the first time to rationalize the θ behaviour of cations of tetraalkylammonium salts, which are known to be structure stabilizers and whose θ behaviour could not be explained properly by earlier workers.