VARIABILITY IN THE MEASUREMENT OF HUMIC CARBOXYL CONTENT

The acetate reaction method for estimating the operationally defined carboxyl content of humic matter was evaluated via laboratory experiments with a soil-derived humic acid and via computer simulations with hypothetical acids. The behavior of monoprotic acids, mixtures of monoprotic acids, and a model humic acid was simulated. Both the experimental data and the theoretical analyses demonstrated that the acetate reaction method yields a strong acidity index that varies inversely with the ratio of equivalents of acidity to equivalents of acetate. Unless the exact reaction conditions are known, individual carboxyl content measurements cannot be compared. The equilibrium chemistry of the acetate reaction was examined with particular attention to the influence of acids that partially dissociate in acetate solution. For acids with continuous pK distributions such as humic acids, neither the acetate method nor any other titration method can be assumed to quantitate acidic groups described by a particular pK or pK range.