Determination of Complexing Capacities and Conditional Stability Constants Using Ion Exchange and Ligand Competition Techniques

Abstract

After the early reports on the effects of dissolved organic material on the bioavailability of metal ions to aquatic organisms (e.g. Goldberg, 1952, den Dooren, 1965, Neilands, 1967) we have come a long way towards the development of sensitive chemical techniques which provide a measure of the organic and inorganic metal fractions in natural waters. A number of these techniques have been collected in Table 1. Most techniques are based on the physical separation of the organically complexed and free (inorganic) metal fractions. Some methods achieve this separation by kinetic means, by adsorbing the free metal onto a cation exchanger (chelex resin) or by adsorbing the complexed metal fraction onto an anion exchanger (XAD2) or onto a surface containing high molecular weight C-18 molecules (SEP-PAK). It is then assumed that equilibria are not shifted when the sample is passed through a column containing one of those adsorbents with a contact time of only a few minutes. Usually these methods only determine the complexed metal concentration as a fraction of the total metal concentration, which is a measure of K_ML’C_L, the product of the conditional stability constant, of the complex ML, with the ligand concentration (see the section on theory).