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Experimental results of an investigation of aqueous ammonium oxalate solutions containing Cu(II) impurity by ultraviolet-visible spectroscopy are described and discussed from the standpoint of speciation of complexes. The results show that absorption of light by aqueous ammonium oxalate solutions containing Cu(II) impurity in the range 5 < ln(ci/c) < 2.5 of the ratio of concentrations ci and c of impurity and solute, respectively, leads to decrease or increase in the intensity of bands of the ultraviolet-visible spectral regions, and these changes may be expressed by full width at half maximum, molar extinction coefficient, peak wavelength and oscillator strength. The changes are caused by the coordination of C2O42 ligand with Cu(H2O)62+ aquocomplex, and are related with the impurity solute concentration ratio ci/c. The coordination of C2O42 ligand with Cu(H2O)62+ aquocomplex in the range 0 < ln(ci/c) < 2.5 leads to the formation of Cu(C2O4) complex, but the coordination of the C2O42 ligand with Cu(C2O4) complex in the concentration ratio range 5 < ln(ci/c) < 0 results in the formation of predominantly Cu(C2O4)22 complex. The effect of successive coordination of the C2O42 ligand is well-defined in the ultraviolet spectral region but poor in the visible region.
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