REAL IONIC-SOLUTIONS IN THE MEAN SPHERICAL APPROXIMATION - 3 - OSMOTIC AND ACTIVITY-COEFFICIENTS FOR ASSOCIATING ELECTROLYTES IN THE PRIMITIVE MODEL

Osmotic and activity coefficients for aqueous solutions of pure associ ating electrolytes are described by a MSA-like theory that includes th e following effects: association (ion pairing or chemical) and solvati on. The association of ions of opposite charges is described using the BIMSA, based on the Wertheim formalism. As for nonassociating electro lytes, the hydration is taken into account by introducing a cation siz e and permittivity that are allowed to vary with concentration. New ex pressions for the osmotic and activity coefficient are given that acco unt for this variation. These equations have been applied to fit data for a variety of aqueous solutions of pure salts at 25 degrees C. Abou t 80 new salts have been treated including perchlorates, nitrates, hyd roxides, and sulfates. For some solutions, good fits could be obtained up to very high concentrations, such as 25 mor kg(-1) for ammonium ni trate and 34 mol kg(-1) for potassium nitrite. Solutions of NaOH and n itric acid could be described to 10 mol kg(-1). A description for sulf uric acid has been obtained at low concentration (below 0.1 mol kg(-1) ). Another description has been made for high concentrations (between 6 and 28 mol kg(-1)) by using a chemically plausible model. In most ca ses the association constant is of the same order of magnitude as the literature value when available.