Unsteady-state impurity effect of chromium (III) on the growth rate of potassium sulfate crystal in aqueous solution

Growth length of a single potassium sulfate crystal was measured at a constant supercooling of 7 degrees C in a flow cell in the presence of a trace of chromium (III) (up to 5 ppm) under natural and controlled pH conditions. The impurity action of chromium (III! exhibited unsteady-state behavior. The growth rate decreased gradually, reaching zero in about 100 min at an impurity concentration of 2 ppm under the condition of pH = 5.8. At higher impurity concentrations (3 and 5 ppm), the growth was completely stopped at an early stage of the run nearly at the same pH level. No impurity effect was observed at higher pHs (= 7 and 10), while, at lower pHs(= 3 and 5), the effect was very strong even at an impurity concentration of 2 ppm. The unsteady-stale growth behavior is reasonably explained by a model assuming that the impurity species, which acts as a stopper for the step advancement, adsorbs very slowly on the step lines. The effect of pH on the impurity action is explained by assuming that the first hydrolysis product, [Cr(H2O)(5)(OH)](2+), Of the aqua complex compound of chromium (III) acts as the active species for growth inhibition.