Impurity Effect of Chromium(III) on the Growth Rate of Potassium Hydrogen Phthalate Crystals

Abstract

Growth rates of potassium hydrogen phthalate (KAP) crystals were measured at 28°C in a flow cell under a microscope in the presence of chromium(III) added in the form of CrCl₃·6H₂O. Chromium(III) concentration was changed up to 5.7×10² ppm (mg Cr(III)/dm³ solution). Measurements were done at three levels of supersaturation of KAP solution (relative supersaturation σ: 0.0204, 0.0417 and 0.0878). The growth rate decreased with increasing impurity concentration and it eventually stopped. The impurity effect was stronger at lower supersaturations. The concentration effect of the impurity (chromium(III)) on the crystal growth rate and the supersaturation effect on the impurity action was explained by the model proposed previously by Kubota and Mullin (Journal of Crystal Growth, 152, 203–208 (1995)). The impurity effects are shown theoretically to be divided into three parts: effect of growth environments (chemical potential difference of the crystallizing species between in the solution and in the crystal, or, in other words, temperature and supersaturation), effect of characteristic properties of the crystal (edge free energy and size of the crystal growth unit) and effect of mutual interaction between the impurity and the crystal (average distance between the active sites and coverage of the active sites by adsorbed impurities). The effect of the impurity concentration is related to the coverage of the active sites through the Langmuir adsorption isotherm. Anomalous dissolution of KAP crystals was observed even under superaturated conditions in the presence of chromium(III) at higher concentrations.