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Equilibrium approaches to natural water systems—6. Acid-base properties of a concentrated bog-water and its complexation reactions with aluminium(III)
Authors:Lars Lvgren  Tomas Hedlund  Lars-Olof hman  Staffan Sjberg
Affiliation:Lars Lövgren, Tomas Hedlund, Lars-Olof Öhman,Staffan Sjöberg
Abstract:A filtered bog-water, concentrated by means of a freezing technique, has been studied with respect to acid-base properties and aluminium(III) complexation reactions. Sampling was performed during autumn and winter periods with a resulting acidity or alkalinity due to oxic (autumn) or anoxic (winter) conditions. The measurements were performed as potentiometric titrations in constant ionic media (0.02, 0.1 and 0.6 M NaCl) with the use of a glass electrode. The samples show buffer ranges at 3 pH 5 and pH 7.5. The first is ascribed to the presence of carboxylate groups and is characterized by fast equilibria. The second is due to phenolic OH groups and precipitation reactions with resulting sluggish equilibria. For one sampling period comprehensive measurements were undertaken to study the possible polyelectrolytic character of the organic acids. Due to the small increase in apparent carboxylate pKa-values with the degree of dissociation at low (0.02 M) as well as at high (0.6 M) ionic strength, the possible polyelectrolytic feature of the acids was neglected. Instead, a good fit to data was obtained by introducing a di-protic acid (H2L) as a model compound. Furthermore the medium dependence of the two acidity constants could satisfactorily be fitted to the expression:
, where K is the constant at infinite dilution, a and b parameters of which b has been adjusted to present data. The following K values were obtained: pK1 = 3.65 and pK2 = 4.30. The complexation with Al(III) could be described by the formation of AlL+, AlL2 and the ternary species AlLH−1. The stability constants (log k1 = 4.4 (winter), 4.2 (autumn); log k2 = 4.2, 4.7; pKa (AlL+) = 4.2, 4.2) show no significant trend with sampling period but indicate a stability of the complexes greater than for phtalic acid but lower than for oxalic acid. Finally, the theoretical solubility of the clay mineral kaolinite in the presence of bog-water was computer modelled. The calculations show up on a 10-fold increase with respect to soluble aluminium at pH = 5.
Keywords:natural water  bog-water  acid-base properties  complexation  aluminium  potentiometric titrations  equilibria  speciation
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