Comparison of Models for Determining Stability Constants of Metal Complexes with Humic Substances

Abstract

AbstractSeveral recent approaches for determining apparent stability constants of metal complexes with humic and fulvic acids are shown to represent graphical modifications of the basic equation relating the formation function of binding (v = sites bound/macromolecule concentration) to stability constants for the system v = ΣnjKj(M)/Σ[1 + Kj (M)], where Kj is the stability constant for binding at class j, nj is the number of binding sites of class j, and (M) is the free metal ion concentration. They include the Scatchard [v/(M) vs. v], reciprocal [1/v vs. (M)], and double reciprocal [1/v vs. 1/(M)] plots. Another approach is shown to have the same form as the Hill plot, which is based on an assumption of positive cooperativity. Comparison of the various models using a common set of experimental data revealed that approaches based on an assumption of linearity yield poor estimates for stability constants. Of the various models, the Hill plot is the least reliable.