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

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 cici 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 cici of Cu(II) impurity and c of ammonium oxalate. The dependences of solution pH and of absorbance A   and the corresponding wavelength λλ for unsaturated aqueous solutions on ammonium oxalate concentration c   containing different concentrations cici 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 c≤0.01c≤0.01 mol/dm3, 0.01 mol/dm3