Predicting Solubility of Gypsum in Aqueous Salt Solutions

The solubility of gypsum was determined experimentally in aqueous salt solutions of NaCI, MgCl₂ Na₂SO₄, MgSO₄ and CaCl₂ at different concentrations. A computer programme based on the calculation of ion pairs, CaSO_4^o MgSO_4^o and NaSO_4⁻, and single ion activity coefficient using four different equations, was used to predict the solubility of gypsum. The experimental and predicted values of gypsum solubility increased with the increase in ionic strength of the solutions devoid of common ion, Ca or SO₄, and the reverse was true for solutions containing common ion. The predicted solubility of gypsum was considerably influenced by the method of calculation of single ion activity coefficient. A grading procedure based on the intercept, slope, regression coefficient and variance ratio of the linear regression between the experimental and predicted solubility of gypsum showed that the single ion activity coefficient calculated using Davies equation with 0.2 as coefficient of the ionic strength gave the best agreement between the pooled data of the experimental and predicted solubility of gypsum in diffirent salt solutions.