STABILITY-CONSTANTS OF CU2+, PB2+, AND CD2+ COMPLEXES WITH HUMIC ACIDS

Application was made of the potentiometric titration method for determining apparent stability constants for the complexes between Cu2+, Pb2+ and Cd2+ and 3 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 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 3 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 2 major sites were involved in the binding of metal ions.