SOLUBILITY OF GYPSUM IN AQUEOUS SOLUTIONS OF SALTS

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Abstract
The system CaSO4-NaCI-H2O-CO2, part of the system CaCO3-CaSO4-NaCl-H2O-CO2, was investigated through a range of NaCl concentrations. Calcite and gypsum solubilities were studied under conditions approximating those of the natural environment as closely as possible, for the particular purpose of determining the behavior of slightly soluble salts in the presence of readily soluble salts. In that even their saturated solutions remain dilute, concentration of calcite and gypsum is insignificant compared to that of NaCl. From a moderate amount of experimental data, then, it was possible to compute the solubility diagram for a complex system through application of the equations of Debye and Hückel. The Debye-Hückel approximate equation was found to be best suited to the purposes of this experiment. Solubility of gypsum in aqueous NaCl solutions (up to saturation) was calculated according to these equations. If a system goes to equilibrium very slowly, which is the case here, investigation is much simplified when it is possible to find the solubility by calculation. Mean effective ion diameters were assumed to be a function of concentration. A linear equation was obtained for the system CaSO4-NaCl-H2O, to describe the relation between effective ion diameters and NaCl content. A method of calculation to determine gypsum solubility was verified for several similar systems according to experimental data from various authors. An approximate form is given for experimental linear equations for correlation of effective ion diameters with concentrations. It is possible that crystallographic dimensions of ions as well as solubility of added salts, constitute bases for an attempt to explain the complexity of dependence of the mean effective ion diameters upon concentration of systems that are composed of gypsum and aqueous solutions of readily soluble salts with no ions common to gypsum or ability to react chemically with it.

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