Studies were conducted to investigate the interaction between certain metallic cations and the humic acids isolated from one Black Chernozem and two Gray Wooded soils of Alberta. Potentiometric titrations of the humic acids in the presence of added cations showed the formation of metal‐humic acid complexes. The metallic cations released protons from functional groups of the humic acids and formed complexes. The order of magnitude of pH drop of the humic acids on addition of metallic cations was found to be Mn 2+ < Co 2+ < Ni 2+ < Zn 2+ < Cu 2+ < Al 3+ < Fe 3+ . The effectiveness of added cations in coagulating the humic acids followed the sequence Mn 2+ < Co 2+ < Ni 2+ < Zn 2+ < Cu 2+ < Fe 3+ < Al 3+ .
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S. U. Khan (1969) studied this question.