The formation of weak complexes by metal ions in aqueous solutions

The modern approaches to the determination of the composition and the stepwise stability constants of weak metal complexes (K_n = 0.1–5.0) with ionic and molecular ligands in aqueous solutions are discussed. The difficulties in the unambiguous separation of the effects of the medium from those of the short-range specific interaction, described by equilibrium constants, and the consequent unreliability of the lower values of the formation constants, even when the latter are selected on the basis of a critical assessment, are examined. It is recommended that the linear relations between the algebraic sums of the logarithms of the activity coefficients of the components of the equilibria investigated and the ligand concentrations, the standard isotherms, or the measurement of the properties of all the components of the complex formation equilibria be employed for the separation of the long-range and weak short-range interactions. The bibliography includes 130 references.