Volumetric properties of, and ion-pairing in, aqueous solutions of alkali-metal sulfates under superambient conditions

Abstract

Apparent molar volumes for Na₂SO₄(aq) and K₂SO₄(aq) have been obtained from our recent densitometric measurements and selected experimental data from literature at temperatures up to 573 K and pressures up to 30 MPa. The Pitzer ion-interaction model was used to correlate the data in order to obtain recommended values as a function of temperature, pressure and concentration and to yield partial molar volumes at infinite dilution. By comparison with the volumetric data of other strong electrolytes, and consistent with their reported ion-association constants, the present data indicate that ion-pairing is significant for Na₂SO₄(aq) and K₂SO₄(aq) at higher temperatures. A semiquantitative treatment is presented which enables the partial molar volumes of 1 : 2 (and 2 : 1) electrolytes to be corrected for the effects of ion-pairing.