Fig. 4. Model A applied to metal binding data; the bound species considered included: monodentate Zn–carboxyl complex (ZnC), bidentate Zn–carboxyl complex (ZnCC), and bidentate Zn–(carboxyl + phenolic hydroxyl) complex (ZnC). The background electrolyte was 0.1 M NaNO₃. Data are presented as the natural log of the ration of bound to free Zn concentrations ([Zn]_bound/[Zn]_free), modeled vs. measured. The concentration of Zn_bound includes all Zn–humic acid complex forms and Zn held as a counter ion; Zn_free includes all free inorganic species. Different symbols show individual data subsets. The solid line indicates a 1:1 relationship. The values of the optimized Model A parameters were: –log intrinsic stability constant for metal complexation to a monodentate carboxyl binding site = 0.968, –log intrinsic stability constant for metal complexation to a monodentate phenolic hydroxyl binding site = 4.52, proportion of carboxyl groups capable of forming bidentate chelates = 0.171, proportion of monodentate binding carboxyl groups capable of forming bidentate chelates to a phenolic hydroxyl group = 0.883, and proportionality factor F_W = 0.259; residual standard deviation = 0.326, n = 42.