The precipitation of calcium sulphate dihydrate from aqueous solution: Induction periods, crystal numbers and final size

Abstract

The precipitation of calcium sulphate dihydrate was studied from solutions of concentration ( C) from 0.02 to 0.24 M, supersaturation ( S) from 1.5 to 16, by optical microscopy and chemical analysis; induction periods, final crystal lengths and numbers were measured. The induction periods ( t̄) decreased from 12000 to 3 sec over this range; t̄ varied with C according to the relation t ̄ = K t/▿C 4 . The crystal numbers ( N) varied with C according to the relation N = K N C 7. The final crystal lengths decreased from 3 to 0.15 cm over this range. The precipitations occured through a heterogeneous nucleation process. Heterogeneous nuclei were formed immediately after mixing and then grew, by a slow growth process, to small crystallites during the induction periods: these latter then grew by a more rapid growth process to the large crystals. The “reaction order” for the nucleation was 9–10; the “order” of the slow growth process was 4.