Competition of copper and/or calcium in nickel-pectin interaction by potentiometric and voltammetric techniques

The interaction between nickel and pectin extracted from citrus fruit was studied in 0.10 M KNO(3), at pH 5.5 and 25 degrees C. Differential pulse and/or square wave polarography were used to determine free nickel. For a high coverage degree (theta) of the pectin by the metal ion a good fitting was observed between experimental results and the model that includes both complex species, ML and ML(2) (M for the metal ion and L for the ligand). In the ML(2) species, Ni(II) interacts with two carboxylate groups of different chains, resulting in an inter-chain association. For low theta values, the formation of ML(2) is hindered due to the repulsion between the negative charges of carboxylic groups in two independent segments of pectin. The influence of calcium or copper ions on the free nickel concentration, in the presence of pectin, may lead to a decrease in free nickel concentration, contrary to what would be expected from direct competition between Ca(II) or Cu(II) and Ni(II) for the pectin binding sites. This is due to the partial neutralisation of the negative carboxylic charges by the positive charges of the divalent cations, which favours NiL(2) formation through the association of independent chains.