A generalised treatment of the appearance of supersaturation barriers during the growth of single crystals is outlined from the standpoint of well-defined critical values of relative step velocities on a face. The final theoretical expressions are based on the premise that: (1) there are critical values of the relative step velocities associated with different average distances between adsorbed impurity particles during instantaneous, time-dependent and time-independent adsorption of the impurity on the growing surface, (2) the growth rate of a face is proporptional to velocity of steps on the growing face, and (3) Freundlich and Langmuir adsorption isotherms apply for different impurities. The theoretical expressions are then used to critically analyse the experimental data on supersaturation barriers observed during the growth of ammonium oxalate monohydrate and potassium dihydrogen phosphate single crystals from aqueous solutions containing different impurities. It was found that: (1) Langmuir adsorption isotherm is more practical for the analysis of the experimental data of the dependence of supersaturation barriers on the concentration of an impurity, and (2) the ratios of successive supersaturation barriers for an impurity either increases or remains constant with an increase in impurity concentration ci, and may be explained in terms of the mechanism of adsorption of impurity particles.

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