Complex formation equilibria in the binary Zn2+ -oxalate and In3+ -oxalate systems

The complex formation equilibria between oxalate and Zn2+ as well as between oxalate and In3+ have been investigated through a potentiometric methodology at 25 °C in 1, 2 and 3 M NaClO4, by using both glass and metal amalgam electrodes. Owing to the low solubility of the Zn(II) and In(III) oxalates, the analytical concentration of the metal ions was forced to be kept below the 5 mM level: constant current coulometry has been used in order to prepare solutions containing the metal ion at very low but accurately known total concentration. At each ionic strength, the formation constants of the complexes ZnC2O4(aq), Zn(C2O4)22-, InC2O4+, In(C2O4)2and In(C2O4)33- have been determined. By using the specific ion interaction theory the constants have been extrapolated at zero ionic strength. Thus relevant interaction coefficients have been obtained, which appear in the mathematical development of the theory. Solutions, containing InC2O4+ or In(C2O4)2- as the prevailing metal-containing species, were prepared on the basis of the thermodynamic parameters determined, and were analysed through electrospray mass spectrometry. The fragmentation of the In(C2O4) (H2O)2+ and In(C2O4)2- complexes detected produced respectively the InH2O+ and InC2O4- species of In(I).