Apparent molar volumes of aqueous solutions of sodium acetate and sodium benzoate at temperatures from 323 K to 573 K and pressure 10 MPa

Abstract

Densities of aqueous solutions of sodium acetate and sodium benzoate have been measured at 50K intervals in the temperature range 323.15–573.15K at constant pressure p=10.0MPa by vibrating tube densimetry. The solute molalities ranged from 0.05 to 4.0mol·kg−1 (for sodium acetate) or 1.0mol·kg−1 (for sodium benzoate). The observed densities were used to calculate apparent molar volumes, which were fitted with an extended Redlich-Rosenfeld-Meyer-type equation to yield standard partial molar volumes of the solutes at infinite dilution. No direct comparisons with literature data were possible but the present results are broadly consistent with previous studies. This work greatly expands the database for the volumetric behaviour of both solutes at high temperatures. Despite the probable presence of anion hydrolysis the volumes of both salts show the steep decrease at high temperatures typical of simple 1:1 electrolytes. As expected, the acetate ion has a smaller volume than benzoate, with the difference increasing markedly with temperature, possibly reflecting the greater hydrophobicity of the benzoate ion. Combination of the present results with appropriate literature data enabled calculation of the volume change for the neutralization reaction: HX(aq)+NaOH(aq)→NaX(aq)+H2O. The values were similar for the two solutes and show a complex dependence on temperature.