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The nature of the binding of divalent metal ions to humic acid can be examined by a modified potentiometric titration method involving the sequential addition of metal ions to the humic acid solution at a constant pH. When the pH is below the oxide hydrate formation point, the decrease in pH is due to the release of protons from the participating reaction groups; at higher pH and higher M2+/ humic acid ratios, additional protons are released from the metal hydrated water in the 1:1 complex.