TY - JOUR

T1 - On the acid-base properties of humic acid in soil.

AU - Cooke, James D.

AU - Hamilton-Taylor, John

AU - Tipping, Edward

PY - 2007/1/15

Y1 - 2007/1/15

N2 - 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.

AB - 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.

U2 - 10.1021/es061424h

DO - 10.1021/es061424h

M3 - Journal article

VL - 41

SP - 465

EP - 470

JO - Environmental Science and Technology

JF - Environmental Science and Technology

SN - 0013-936X

IS - 2

ER -