Effect of trace impurities on calcium sulphate precipitation

Induction periods for the precipitation of CaSO4.2H2O in the concentration range 0.05‐lM have been determined by the stopped‐flow method utilising changes in the solution conductivity after nucleation and growth. The measured induction periods were insensitive to calcium sulphate concentration, suggesting that the nucleation process was heterogeneous. At first additions of Al3+ increasingly prolonged the induction period, but above a certain Al3+ concentration further additions caused the induction period to reduce again. Tentative explanations based on the poisoning of growth sites by [Al(H2O)6]3+ complexes for the first effect and the deaquation of aluminium complexes followed by formation of polymeric species for the second are given. It is suggested that the small crystallites which appeared on the surfaces of the gypsum crystals precipitated in the presence of relatively high fluoride concentrations are CaF2.