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Equilibria of weak acids and organic Al complexes explain activity of H + and Al 3+ in a salt extract of exchangeable cations
Author(s) -
Nissinen A.,
Ilvesniemi H.,
Tanskanen N.
Publication year - 1999
Publication title -
european journal of soil science
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.244
H-Index - 111
eISSN - 1365-2389
pISSN - 1351-0754
DOI - 10.1046/j.1365-2389.1999.00265.x
Subject(s) - chemistry , equilibrium constant , stoichiometry , cambisol , dissociation constant , dissociation (chemistry) , podzol , analytical chemistry (journal) , inorganic chemistry , soil water , chromatography , biochemistry , environmental science , receptor , soil science
Summary Two sequential extractions with unbuffered 0.1 m BaCl 2 were done to study the release of salt‐exchangeable H + and Al from mineral horizons of five Podzols 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 near‐equality of the titrated exchangeable acidity (EA T ) and the sum of exchangeable acids (EA = H e + 3Al e , calculated from the pH and Al concentration of the extract). The ratio between EA of the second and the first extraction was over 0.50 in the Bs2 and C horizons and smaller in the other horizons. H + was assumed to be in equilibrium with weak acid groups, and the modified Henderson–Hasselbach equation, p K HH = pH − n log (α/(1 − α)), was used to explain pH of the extract. The degree of dissociation (α) was calculated as the ratio between effective and potential cation exchange 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, p K HH had the same value in both extractions. For Al 3+ , two equilibrium models were evaluated, describing (i) complexation reactions of Al 3+ with soil organic matter, and (ii) equilibrium with Al(OH) 3 . Apparent equilibrium constants were written as (i) p K o = x pH − pAl 3+ , and (ii) log Q gibbs = log Al 3+ − 3log H +. The two extractions gave an average reaction stoichiometry x close to 2 in all horizons. Results suggest that an equilibrium with organic Al complexes can be used to express dissolved Al 3+ , aluminium 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 horizons. 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 indicates that equilibrium with Al(OH) 3 cannot explain dissolved Al 3+ in the soils. We propose that the models of p K HH and p K o can be used to simulate exchangeable H + and Al 3+ in soil acidification models.