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Comparison of Models for Determining Stability Constants of Metal Complexes with Humic Substances¹

ABSTRACT

Several 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 = n_jK_j(M)/[1 + K_j (M)], where K_j is the stability constant for binding at class j, n_j 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.

¹ Contribution from the Dep. of Agronomy, Univ. of Illinois, Urbana, and the Illinois Agric. Exp. Stn. (Project 15-339).

² Research Assistant and Professor of Soil Chemistry, respectively. The senior author is now Research Associate, Dep. of Chemistry, Univ. of Wisconsin, Madison.

Received for publication June 20, 1983. Accepted for publication May 7, 1984.