Study of copper complexes in saturated and unsaturated aqueous ammonium oxalate solutions containing Cu(II) impurity

Abstract

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 c i 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: Cu 2+, Cu(OH) +, Cu(OH) 2, CuC 2O 4 0 and Cu(C 2O 4) 2 2− in addition to C 2O 4 2−, HC 2O 4 −, H 2C 2O 4 and (NH 4) 2C 2O 4 0 species, and their concentration depends on concentrations c i of Cu(II) impurity and c of ammonium oxalate. The dependences of solution pH and of absorbance A and the corresponding wavelength λ for unsaturated aqueous solutions on ammonium oxalate concentration c containing different concentrations c i of Cu(II) ions showed three well-defined regions characterised by transition values of solution pH or solute concentration c. Speciation analysis revealed that Cu 2+ and CuC 2O 4 0, CuC 2O 4 0 and Cu(C 2O 4 )2 2−, and Cu(C 2O 4) 2 2− complexes are predominantly present in the solute concentration intervals c ≤ 0.01 mol/dm 3, 0.01 mol/dm 3 < c < 0.03 mol/dm 3 and c ≥ 0.03 mol/dm 3, respectively. The concentration interval range 0.01 mol/dm 3 < c < 0.03 mol/dm 3 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(C 2O 4) 2 2−-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 CuC 2O 4 0 and Cu(C 2O 4) 2 2− species.