Pure and Applied Chemistry

Abstract: This paper reviews progress made over the last decade with the shrinking object approach to the kinetics of batch dissolution. It demonstrates how the O’Connor–Greenberg equation leads to three dissolutions: those that remain well undersaturated, those that saturate with a great excess of solid left-over, and those in the middle ground where saturation is approached or attained, but where much of the solid originally added dissolves. The equations that describe these conditions are discussed, alongside sample results that validate their use with test substances, for example, salts, sucrose, silica gel, and gypsum. The equations are then shown to be consistent with the hydrodynamic approach to dissolution. Finally, further work with middle-ground dissolutions of gypsum lead to a mechanism for the back-reaction, which involves the CaSO₄⁰ ion-pair. After comparison with existing studies of calcium carbonate dissolution, it is argued that this is a universal mechanism for salt dissolutions. The work improves batch dissolution as a technique to the point where it can be used synergistically with chemo-stat and rotating disc approaches. Suggestions are made for greater standardization in dissolution conditions, especially in environmental work where the data collected has to have global consistency.