An ion-exchange technique for free-metal ion measurements (Cd2+, Zn2+):: Applications to complex aqueous media

A cation ion-exchange technique was calibrated and evaluated in terms of its selectivity for the free-metal ion species in the presence of typical monomeric ligands: chloride, citrate and various amino acids. The method was shown to be highly selective for the free-metal ion in the presence of anionic metal-ligand complexes. For cationic metal-ligand complexes, however, results were more variable: e.g., the Zn-lysine(+) complex interfered with the technique whereas CdCl+ did not; this contrasting behavior is attributed to the different loci of positive charge in the two complexes. The ion exchange technique was also successfully evaluated for its applicability in complex aqueous media containing natural organic matter (fulvic and humic acids). Adsorption of fulvic or humic acid onto the resin was undetectable and the ion-exchange properties of the resin are unaffected by prolonged pre-treatment with fulvic acid.