Effect of Fe(III) ions on the growth kinetics of ammonium oxalate monohydrate crystals from aqueous solutions

The effect of concentration of Fe(III) ions and solution supersaturation on growth rates of different faces of ammonium oxalate monohydrate (AO) single crystals is studied at a constant temperature of 30 degreesC and predefined supersaturations up to 8%. It was observed that, in contrast to the usually observed decrease in face growth rates of crystals with an increase in the concentration of an impurity, Fe(III) ions lead to an increase in the growth rates of different faces of AO crystals. The experimental data of the dependence of face growth rates R on supersaturation a at different concentrations ci of Fe(III) were analysed from the standpoint of BCF surface diffusion theory and its modified version in the power-law form, spiral growth theory involving cooperating dislocation source and multiple nucleation model. It was found that the cooperating screw dislocation and multiple nucleation models represent the experimental data on growth kinetics from pure and impure solutions most satisfactorily. Analysis of the data according to these models revealed that the free energy of a growing face decreases with an increase in the concentration of Fe(III) ions, suggesting that the impurity facilitates the formation of kinks in growth steps. It was also found that the values of the differential heat of adsorption of Fe(III) impurity, obtained in the present study, are similar to those observed in the case of growth systems in which impurities lead to a decrease in the growth rate. (C) 2001 Elsevier Science B.V. All rights reserved.