Impurity Effect of Cr³⁺ on Crystal Growth Process of Ammonium Sulphate

Abstract

Surface reaction kinetics in the crystal growth process of ammonium sulphate is investigated by estimating the interfacial concentration on the single crystal. It is clear that both NaN and BCF models can be applied to each surface reaction process of (100), (010) and (001) faces in pure solution with the same accuracy. In the presence of Cr³⁺, the growth rate remarkably decreases with Cr³⁺ concentration and finally stops at a concentration of 2ppm for (010) face, 2.5ppm for (001) face and 3ppm for (100) face. Under a constant concentration of Cr³⁺, crystal begins to grow when the supersaturation becomes larger than a critical value, after which the growth rate increases predominantly with supersaturation and attains the maximum, the value in the pure solution.
As for the mechanism of the impurity effect, adsorption of Cr³⁺ on steps of the crystal surface is considered. Bow-shaped macrosteps were confirmed by microscopic observation. Applying the model of Cabrera and Vermilya, the distance between the adsorbed Cr³⁺ ions was roughly estimated as 14 nm for (100) and (010) faces, and 18 nm for (001) face.