The SIT model parameters for interactions of sulfate ion with alkali metal ions accounting for complex formation

The goal of this study is to determine the interaction coefficients between alkali metal ions M⁺ (M = Li, Na, K, Rb, Cs, NH₄), the sulfate ion $S{O}_4^{2-}$, and ion pairs $\text{MS}{O}_4^{-}$ for the use in the SIT model for activity coefficients, which is popular among solution chemists and adopted by OECD Nuclear Energy Agency (NEA) to develop a thermodynamic database relevant to the safety of radioactive waste repositories. As alkali metal sulfates are partially associated electrolytes, relations from the thermodynamics of associated solutions (“equilibrium model”) are employed for the correct treatment of tabulated activity and osmotic coefficients. As a part of this work, a compilation of stability constants of the ion pairs $\text{MS}{O}_4^{-}$ is made together with selection of the recommended values. In addition, solubility values in ternary $M_{2}S{O}_4-N_{2}S{O}_4-H_{2}O$ are used to expand the list of relevant interactions. In total, the numerical values of the SIT coefficients are determined for 38 ionic interactions among indicated species. All values refer to 298.15 K and 0.1 MPa.