Stability Constants of Cu2+, Pb2+, and Cd2+ Complexes with Humic Acids
1976
Stevenson, F. J.
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 kⱼ = bⱼ/Kᵢ (Kᵢ = 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.
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