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The influence of Cu(II) impurity on chemical equilibria in unsaturated and saturated ammonium oxalate (AO) aqueous solutions was investigated as a function of concentration of impurity. Using the computer programme “Hyss” the species present in the solutions were analysed. It was found that in the aqueous solutions of ammonium oxalate containing Cu(II) ions the following species are formed: Cu2+, Cu(OH)+, Cu(OH)2, CuC2O40 and Cu(C2O4)22− in addition to C2O42−, HC2O4−, H2C2O4 and (NH4)2C2O40 species, and their concentration depends on concentrations of Cu(II) impurity and c of ammonium oxalate. The dependences of solution pH and of absorbance A and the corresponding wavelength for unsaturated aqueous solutions on ammonium oxalate concentration c containing different concentrations of Cu(II) ions showed three well-defined regions characterised by transition values of solution pH or solute concentration c. Speciation analysis revealed that Cu2+ and CuC2O40, CuC2O40 and Cu(C2O4)22−, and Cu(C2O4)22− complexes are predominantly present in the solute concentration intervals mol/dm3, 0.01 mol/dm3 mol/dm3 and mol/dm3, respectively. The concentration interval range 0.01 mol/dm3 mol/dm3 corresponds to the pH interval where Cu(OH)2 is precipitated. It was found that the solubility of ammonium oxalate at 30 C increases practically linearly with an increase in the concentration of Cu(II) impurity. Speciation analysis of saturated aqueous solutions of ammonium oxalate revealed that Cu(II) ions contained in AO saturated solutions exist mainly as Cu(C2O4)22−-type complexes, and the increase in the solubility of AO in the presence of Cu(II) impurity is essentially due to an increase in the ratio of the concentrations of CuC2O40 and Cu(C2O4)22− species.
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