Ion Association versus Ion Interaction Models in Examining Electrolyte Solutions: Application to Calcium Hydroxide Solubility Equilibrium

Abstract

The heterogeneous equilibrium of the solubility of calcium hydroxide in water is used to predict both its solubility product from solubility and solubility values from solubility product when inert salts, in any concentration, are present. Accepting the necessity of including activity coefficients to treat the saturated solution of calcium hydroxide (and by extension, all electrolyte solutions) and the inadequacy of any general or nonspecific equation (such as the Debye–Hückel limiting law or Davies equations) to calculate activity coefficients of 1:2 electrolytes, our results uncover (i) how the inclusion of ion pairs in the last mentioned equations makes them adequate for low ionic strength solutions and (ii) the need of sophisticated models, such as the ion-interaction Pitzer equations, for calculating the activity coefficients when ionic strength is high. In addition, we have developed a set of MATLAB scripts and propose the use of the free code PHREEQC version 3 to perform all the calculations described in the article. The tasks proposed here can be complemented with the experimental determination of the solubility of calcium hydroxide in water along with the experimental checking of its solubility in aqueous salt solutions during several lab sessions. Experimental guidance is provided as Supporting Information.