The SIT model parameters for interactions of phosphate ions with Na+ and K+ ions accounting for dimerization of the H2PO4‐ anion

Abstract

The SIT model is a simple yet physically sound model for activity coefficients in aqueous solutions, which has only one binary interaction parameter for each pair of cations and anions. It has long been known that the SIT model performs poorly for the thermodynamic activity data for aqueous solutions of $N{\text{aH}}_{2}P{O}_4$ and $K{H}_{2}P{O}_4$. However, the previous treatment ignored the dimerization of the $H_{2}P{O}_4^{‐}$ anion with the formation of $H_4{P}_2{O}_8^{2‐}$, which has been convincingly demonstrated, mainly in Raman experiments. Here we show that explicitly taking into account the dimerization of the $H_{2}P{O}_4^{‐}$ anion allows to reproduce accurately the experimental data on the stoichiometric mean ionic activity coefficients for aqueous $N{\text{aH}}_{2}P{O}_4$ and $K{H}_{2}P{O}_4$. The data were processed using relations from the thermodynamics of partially associated electrolytes. An additional source of information were the values of the dissociation constants of phosphoric acid in $\text{NaCl}$ and $\text{KCl}$ solutions. In total, we determined 8 SIT parameters for the interactions of ${\text{Na}}^{+}$ and $K^{+}$ with different phosphate ions at 298.15 K and 0.1 MPa.