Complex Formation of Copper(II) and Cadmium(II) with Pectin and Polygalacturonic Acid in Aqueous Solution. An ISE-H+ and ISE-Me2+ Electrochemical Study

The acid-base properties of pectin and its ability to complex with copper(II) or cadmium(II) were quantitatively determined in order to define the ability of this natural polyelectrolyte to sequester metal ions. Potentiometric measurements carried out at different ionic strengths (0.1 - 0.7 mol L-1, in Na+ ionic medium) allowed us to define the acid-base properties of pectin together with the dependence of the protonation constants on ionic strength and to calculate the protonation constant at infinite dilution. The formation of complex species between pectin and copper(II) or cadmium(II) was investigated potentiometrically (using H+, Cu2+ and Cd2+ Ion Selective Electrodes, ISEs) in NaNO3 at 0.1 mol L-1 at T = 25°C. The results thus obtained showed the formation of ML and MLH species for both the pectin-Cu2+ / Cd2+ systems. The acid-base properties and the complexation capacity of a commercial polygalacturonic acid with copper(II) and cadmium(II) ions were also investigated, polygalacturonic acid being the most abundant component of pectin. On the basis of the proposed complexation model, the abilities of pectin and polygalacturonic acid to sequester cadmium(II) and copper(II) ions were evaluated at different pH values. The data obtained on the sequestration capacity of the two ligands used here were compared with previous data for other polycarboxylic polymer systems, including alginate, polyacrylate and polymethacrylates, under the same experimental conditions.