Study of segregation coefficient of cationic impurities in ammonium oxalate monohydrate crystals during growth from aqueous solutions
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AbstractThe segregation coefficient keff of Cu(II), Fe(III) and Cr(III) ions in ammonium oxalate monohydrate single crystals during growth from aqueous solutions at a constant temperature was investigated as a function of solution supersaturation σ and impurity concentration ci. It was observed that: (1) irrespective of the impurity concentration, there is a threshold supersaturation of about 0.03 above which Cr(III) is captured in the crystals but there is no threshold supersaturation in the case of Cu(II) and Fe(III) ions, (2) above the threshold supersaturation, keff increases with increasing σ and decreasing ci, and (3) at a given σeff and ci,keff decreases in the sequence: keff[Cu(II)] ⪢keff[Fe(III)] >keff[Cr(III)]. The dependence of effective segregation coefficient keff on supersaturation and impurity concentration is in agreement with the predictions of the model involving surface coverage due to impurity adsorption and dependence of accumulation and depletion of solvated host molecules at kinks of steps on the F faces of crystals on supersaturation [J. Crystal Growth 212 (2000) 522]. Analysis of the effective segregation coefficient keff of different impurities suggests that the dehydration energies of cations mainly determine the capability of capture of impurity species by the growing crystal.

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