Equilibria of weak acids and organic Al complexes explain activity of H+ and Al3+ in a salt extract of exchangeable cations

Two sequential extractions with unbuffered 0.1 m BaCl2 were done to study t he release of salt-exchangeable H+ and Al from mineral horizons of five Pod zols and a Cambisol. Released Al was found to have a charge close to 3+ in all horizons and in both extractions. This finding was supported by the nea r-equality of the titrated exchangeable acidity (EA(T)) and the sum of exch angeable acids (EA = H-e + 3Al(e), calculated from the pH and Al concentrat ion of the extract). The ratio between EA of the second and the first extra ction was over 0.50 in the Bs2 and C horizons and smaller in the other hori zons. H+ was assumed to be in equilibrium with weak acid groups, and the mo dified Henderson-Hasselbach equation, pK(HH) = pH - n log (alpha/(1 - alpha )), was used to explain pH of the extract. The degree of dissociation (alph a) was calculated as the ratio between effective and potential cation excha nge capacity. Value of the empirical constant n was found to be near unity in most horizons. When the monoprotic acid dissociation was assumed in all horizons, pK(HH) had the same value in both extractions. For Al3+, two equi librium models were evaluated, describing (i) complexation reactions of Al3 + with soil organic matter, and (ii) equilibrium with Al(OH)(3). Apparent e quilibrium constants were written as (i) pK(o) = xpH - pAl(3+), and (ii) lo g Q(gibbs) = log Al3+ - 3log H+. The two extractions gave an average reacti on stoichiometry x close to 2 in all horizons. Results suggest that an equi librium with organic Al complexes can be used to express dissolved Al3+, al uminium being apparently bound to bidentate sites. The value of log Q(gibbs ) was below the solubility of gibbsite (log K-gibbs = 8.04) in many horizon s. In addition, log Q(gibbs) of the second extraction was greater than that of the first extraction in all horizons except the C horizon. This indicat es that equilibrium with Al(OH)(3) cannot explain dissolved Al3+ in the soi ls. We propose that the models of pK(HH) and pK(o) can be used to simulate exchangeable H+ and Al3+ in soil acidification models.