Incorporating Ion Pairing in an Extended Specific Ion Theory: 2-2 Electrolytes

Abstract

The specific ion theory (SIT) is a relatively simple model for the description of activity coefficients of ions in electrolyte solutions. However, the SIT model does not account for ion pairing, a phenomenon that is pronounced in 2–2 electrolytes. In this work, a recent extension of the SIT model is further extended to account for ion pairing. The model extension is tested with reported data for BeSO2, MgSO4, MnSO4, NiSO4, CuSO4, ZnSO4, and CdSO4. The standard error of the fit is typically around 1%. To get this accuracy in model fits of the activity coefficients that are consistent with literature data of the MgSO40 ion pair concentration, it was necessary to work in a modified molality scale and to reduce the SIT parameter B from 1.5 to 1.2. The drawback of this choice is that a value of B equal to 1.2 leads to worse predictions of the activity coefficients of 1–1 electrolytes. It is concluded that not accounting for ion pairing in 2–2 electrolytes leads to overestimations of the ionic strength of up to 30% and leads to overestimations of the Debye-Huckel term for the activity coefficient of 2–2 electrolytes by up to 20%.