Stability of the complexes of tricarballylic and citric acids with bivalent metal ions in aqueous solution

The complexes existing in aqueous solutions of citric and tricarballylic acids in the presence of the bivalent metal ions Cu2+, Ni2+, Co2+, Zn2+, Cd2+, Mg2+, Ca2+ and Ba2+ were investigated by the potentiometric method at the ionic strength 0·10 and at 20°C. The stability constants of these complexes were determinated: among them the equilibrium constants for the complexes that cupric ions form with citric acid with displacement of hydrogen ions from the alcoholic hydroxyl, and of the complexes of the kind Cu2HnA. The order of stability of the complexes of the different metal ions with the two acids is in agreement with the IRVING-WILLIAMS rule. The comparison of the complexes of citric and tricarballylic acids shows the stabilizing effect due to the presence of the alcoholic hydroxyl group in the ligand.