Fig. 5. Model A applied to metal binding data; the bound species considered included: monodentate Cu–carboxyl complex (CuC), bidentate Cu–carboxyl complex (CuCC), and bidentate Cu–(carboxyl + phenolic hydroxyl) complex (CuC). The background electrolyte was 0.1 M NaNO₃. Data are presented as the natural log of the ratio of bound to free Cu concentrations ([Cu]_bound/[Cu]_free), modeled vs. measured. The concentration of Cu_bound includes all Cu–humic acid complex forms and Cu held as a counter ion; Cu_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 = 0.042, –log intrinsic stability constant for metal complexation to a monodentate phenolic hydroxyl binding site = 4.04, proportion of carboxyl groups capable of forming bidentate chelates = 0.126, proportion of monodentate binding carboxyl groups capable of forming bidentate chelates to a phenolic hydroxyl group = 0.585, and proportionality factor F_W = 0.397; residual standard deviation = 0.219, n = 26.