NATURE OF DIVALENT TRANSITION METAL COMPLEXES OF HUMIC ACIDS AS REVEALED BY A MODIFIED POTENTIOMETRIC TITRATION METHOD

Abstract

The nature of the binding of divalent metal ions by humic acid was examined by a modified potentiometric titration procedure involving sequential additions of metal ion to solutions of the humic acid at constant pH. At pH values below the point of oxide hydrate formation, the drop in pH resulted from release of protons from participating reactive groups; at higher pH values and at high M2+/humic acid ratios, additional protons were released from hydration water of the metal held in 1:1 complexes. At least two major sites were involved in complex formation. Stability constants were in excellent agreement with those obtained by conventional titration and followed the order Cu>PbCd>Zn. Log K2 values varied only slightly over a wide pH range and increased greatly with decreasing concentration of supporting electrolyte. Differences between humic acids in their ability to bind divalent metal ions were negligible. © Williams & Wilkins 1977. All Rights Reserved.