Scanning electron microscopic and kinetic studies of the crystallization and dissolution of barium sulfate crystals

Abstract

The kinetics of crystallization and of dissolution of well-characterized seed crystals of barium sulfate in stable supersaturated and undersaturated solutions, respectively, of this salt, have been studied at 25°C using a highly reproducible conductance technique. Seed crystals of three distinctly different morphologies have been used and the crystal growth has also been followed by parallel scanning electron microscopic experiments. Following an initial growth surge reflecting some secondary nucleation, the rate of crystallization is proportional to the square of the supersaturation indicating a surface controlled reaction. The rate of dissolution is also controlled by a surface reaction being proportional to the square of the undersaturation and independent of fluid dynamics. In spontaneous precipitation experiments crystals have been obtained of widely differing morphologies depending upon the nature and concentration of the reactants and the fluid dynamics of mixing. Different shaped crystals have different densities of active growth sites, monitored kinetically, on their surfaces. The rate of growth of all the crystals, however, varies linearly with the square of the barium sulfate supersaturation.