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

Abstract

The effect of concentration of Cr(III) ions on the growth rates of different faces of ammonium oxalate monohydrate (AO) single crystals was studied at a constant temperature of 30°C and predefined supersaturations up to 8%. It was observed that: (1) at a given supersaturation Cr(III) ions lead to a decrease in the growth rates of different faces of AO crystals, (2) the growth of a particular face of the crystals occurs above a critical supersaturation σ c, and (3) the value of σ c increases with an increase in the concentration of the impurity. The experimental R( σ) data for different Cr(III) concentrations c i were analysed according to the model involving complex source of cooperating screw dislocations. Analysis of the R( σ) data showed that, in general, the parameters related with the kinetic coefficient for step motion and interfacial energy initially decrease and then increase with an increase in c i, exhibiting a minimum at certain c i. This observation is associated with the changing activity of source of steps. The same data were then analysed from the standpoint of Cabrera–Vermilyea and Kubota–Mullin models using Langmuir and Freundlich adsorption isotherms. It was found that: (1) Freundlich isotherm describes the R( c i) dependences at given σ more reliably than Langmuir isotherm, and (2) the appearance of supersaturation barrier in the presence of Cr(III) ions during the growth of different faces of AO crystals is associated with the transition from Cabrera–Vermilyea to Kubota–Mullin mechanism.