On the acid-base properties of humic acid in soil

Cooke, J. D.; Hamilton-Taylor, J.; Tipping, E. ORCID: 2007 On the acid-base properties of humic acid in soil. Environmental Science & Technology, 41 (2). 465-470.

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Humic acid was isolated from three contrasting organic-rich soils and acid-base titrations performed over a range of ionic strengths. Results obtained were unlike most humic acid data sets; they showed a greater ionic strength dependency at low pH than at high pH. Forward- and back-titrations with the base and acid revealed hysteresis, particularly at low pH. Previous authors attributed this type of hysteresis to humic acid aggregates-created during the isolation procedure-being redissolved during titration as the pH increased and regarded the results as artificial. However, forward- and back-titrations with organic-rich soils also demonstrated a similar hysteretic behavior. These observations indicate (i) that titrations of humic acid in aggregated form (as opposed to the more usual dissolved form) are more representative of the acid-base properties of humic acid in soil and (ii) that the ionic strength dependency of proton binding in humic acid is related to its degree of aggregation. Thus, the current use of models based on data from dissolved humic substances to predict the acid-base properties of humic acid in soil under environmental conditions may be flawed and could substantially overestimate their acid buffering capacity.

Item Type: Publication - Article
Programmes: CEH Programmes pre-2009 publications > Biogeochemistry
UKCEH and CEH Sections/Science Areas: Shore
ISSN: 0013-936X
Format Availability: Electronic, Print
Additional Keywords: Natural organic matter, binding model VI, proton-binding, ion-binding, electrostatic model, fulvic-acid, metal-ions, substances, equilibria, pH
NORA Subject Terms: Ecology and Environment
Earth Sciences
Date made live: 19 Jul 2007 15:57 +0 (UTC)

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