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Stability Constants of Cu²⁺, Pb²⁺, and Cd²⁺ Complexes with Humic Acids¹

ABSTRACT

Application was made of the potentiometric titration method for determining apparent stability constants for the complexes between Cu²⁺, Pb²⁺, and Cd²⁺ and three humic acids from diverse sources. Complexes of Cu²⁺ and Pb²⁺ were considerably more stable than those for Cd²⁺. Log K₂ values, obtained from the relationship k_j = b_j/K_i (K_i = ionization constant) increased rather dramatically with decreasing salt concentration and were of the order of those reported in the literature for metal complexes with known biochemical compounds. Average log K₂ values for the three humic acids (absence of neutral salts) were 8.9 for Cu²⁺, 8.7 for Pb²⁺, and 6.9 for Cd²⁺. Differences between humic acids in their ability to bind metal ions were slight. At least two major sites were involved in the binding of metal ions.

¹ Contribution from the Dep. of Agronomy, Univ. of Illinois, Urbana. Financial support was provided by the Illinois Agric. Exp. Stn. and the National Science Foundation (grant RANN GI-31605). The technical assistance of F.J. Drone, M. Thompson, and Linda Himmelrick is gratefully acknowledged.

² Professor of Soil Chemistry.

Received for publication December 30, 1975. Accepted for publication June 1, 1976.

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