Complexation properties of natural and synthetic polymers of environmental and biological interest in presence of copper(II) at neutral pH

Abstract Scatchard and differential equilibrium function (DEF) models were applied in parallel (for comparison) to potentiometric titration data (25 °C, pH 6.5 and ionic strength 0.1 mol l −1 ) of natural or synthetic polymers: starch, cellulose, amylopectin, chitin, Chelex-100, Chelamine and Bio-Rex-70, and humic acids (the last for comparison). Copper(II) was used as titrant. To demonstrate exactly the meaning and quality of the results, identical studies were carried out on: (i) real and well known systems with simple ligands (succinate and proline); and (ii) simulated systems including ligands with one, two, three or 10 different types of coordination sites (simulated titration curves). The aim of the work was the determination of: (a) conditional stability constants of copper complexes; and (b) copper(II) complexation capacity of the polymers. The relative affinity for copper(II) of the different polymers (given by the differential equilibrium function, k DEF ), decreased in the following order at pH 6.5: Chelamine > Chelex-100 ∼- humic acids > Bio-Rex-70 ∼- cellulose > amylopectin ∼- chitin ⩾ starch. In the range of copper(II)-to-polymer concentrations embraced, the binding strength of the Chelex-100 was ca one order of magnitude lower than that of Chelamine, and similar to that found for the humic acids. The complexation capacity of the polymers decreased in the following order: Bio-Rex-70 > Chelex-100 > Chelamine > chitin ⪢ starch > cellulose > amylopectin. All polymers, including the synthetic resins whose sorption sites are chemically homogeneous, behaved as heterogeneous ligands, like humic acids (i.e. the magnitude of the microscopic stability constants decreased when the fraction of all sites occupied by metal ions increased) probably due to both electrostatic and steric effects, and limitations of the methods of analysis and calculation.