Application was made of the potentiometric titration method for determining apparent stability constants for the complexes between Cu 2+ , Pb 2+ , and Cd 2+ and three humic acids from diverse sources. Complexes of Cu 2+ and Pb 2+ were considerably more stable than those for Cd 2+ . Log K 2 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 K 2 values for the three humic acids (absence of neutral salts) were 8.9 for Cu 2+ , 8.7 for Pb 2+ , and 6.9 for Cd 2+ . 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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F. J. Stevenson (1976) studied this question.