Impurity effect of iron(III) on the growth of potassium sulfate crystal in aqueous solution

Growth rates of the {1 1 0} faces of a potassium sulfate crystal were measured In a now cell In the presence of traces of impurity Fe(III) (up to 2 ppm) over the range of pH =2.5-6.0. The growth rate was significantly suppressed by the impurity. The effect became stronger as the impurity concentration was increased and at pH < 5, It became weaker with increasing supersaturation. It also became weaker as the pH was increased and at pH > 5 it finally disappeared completely. The concentration and supersaturation effects on the impurity action were reasonably explained with a model proposed by Kubota and Mullin [J. Crystal Growth, 152 (1995) 203], The surface coverage of the active sites by Fe(III) is estimated to increase linearly on increasing its concentration in solution in the range examined by growth experiments. The impurity effectiveness factor is confirmed to increase inversely proportional to the supersaturation as predicted by the model. Apart from the discussion based on the model, the pH effect on the impurity action is qualitatively explained by assuming that the first hydrolysis product of aqua Fe(III) complex compound, [Fe(H2O)(5)(OH)](2+), is both growth suppression and adsorption active, but the second hydrolysis product, [Fe(H2O)(4)(OH)(2)](+), is only adsorption active. (C) 1999 Elsevier Science B.V. All rights reserved.