The kinetics of the precipitation of gypsum, CaSO₄·2H₂O, over a wide range of reactant concentrations

Abstract

In this work, the precipitation kinetics of gypsum was studied over a wide range of degree of supersaturation at 25 °C in the reaction ${\text{Na}}_{2} {\text{SO}}_{{4\left( {{\text{aq}}} \right)}} + {\text{CaCl}}_{{2\left( {{\text{aq}}} \right)}} + 2{\text{ H}}_{2} {\text{O}} \to 2{\text{ NaCl}}_{{\left( {{\text{aq}}} \right)}} + {\text{CaSO}}_{4} \cdot 2{\text{ H}}_{2} {\text{O}}_{{\left( {\text{S}} \right)}}$ with the aim of constructing a comprehensive kinetic model for CaSO₄·2H₂O_(s) formation that is valid from the lowest (0.04 M) to the highest (0.20 M) feasible initial reactant concentration. To monitor the variation of reactant concentrations during the precipitation reaction, conductometry was employed. For reasonably slow reactions (where the establishment of the equilibrium potential on the indicator electrode was possible), the measurements were supplemented by a Ca-ion-selective electrode. The structure and morphology of the precipitating solids was characterized by XRD and SEM. The induction period was found to decrease about two orders of magnitude with the increasing reactant concentration. It was experimentally established that the influence of the so-called wall effect is of secondary importance. Using the data collected, a kinetic model have been suggested that can describe the entire precipitation process of gypsum simultaneously, incorporating nucleation and crystal growth, in a wide concentration range. Our calculations strongly suggest that the inclusion of the CaSO_4(aq) ion pair is necessary for the appropriate kinetic description of gypsum precipitation.