Dissolution rates of ionic solids. Part 2.—Calcium bis(dihydrogen phosphate) monohydrate + anhydrite (calcium sulphate)

Abstract

The dissolution rate of commercial superphosphate fertiliser was studied at 25, 35 and 45°C using a rotating disc of the solid material. The kinetics were followed by means of a calcium ion-selective electrode and spectrophotometric analyses. The initial process was observed to consist predominately of calcium bis(dihydrogen phosphate) monohydrate dissolution and was transport controlled with an apparent activation energy of 13 ± 3 kJ mol^–1. Thereafter diffusion of dissolved monocalcium bis(dihydrogen phosphate) monohydrate through a porous layer of increasing thickness determined the rate of dissolution.