| Abstract: | The effect of different bi‐ and trivalent cationic impurities on the solubility of ammonium oxalate and the composition and distribution of chemical complexes formed in saturated ammonium oxalate aqueous solutions as a function of impurity concentration are investigated. The knowledge of the composition and stability of complexes formed in saturated aqueous solutions is then employed to explain the appearance of dead zones of supersaturation for growth and the difference in the effective segregation coefficient of the impurities. Analysis of the experimental results revealed that: (1) at a constant temperature, the dependence of concentration of complex species formed in saturated solutions on the concentration of different impurities can be described by an equation similar to that of the concentration dependence of density of solutions, (2) the dominant metal‐containing species present in saturated solutions are negatively‐charged, most stable oxalato complexes like Cu(C2O4)22−, Mn(C2O4)34−, Zn(C2O4)34−, Cr(C2O4)33− and Fe(C2O4)33−, (3) in the investigated range of impurity concentration, the solubility of ammonium oxalates increases linearly with the concentration of all impurities and the increase is associated with the stability of dominant complexes, (4) appearance of dead supersaturation zones in the presence of impurities is associated with instantaneous adsorption of all growth sites by dominant oxalato complexes in relatively short adsorption time, and (5) the segregation coefficient of an impurity cation M of charge z + increases with a decrease in the solubility product constant Ksp for the hydrolysis products of reactions of the type: Mz + ↔ M1(z −1)+ + H+ (where the cation M has z + charge, and H+ is hydrogen ion). (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim) |
