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Three ultrafiltration methods for solutlon phase complexlng measurements have been reported In the literature. Continuous flow wash out, contlnuous flow wash In, and batch dllutlon curve methods have all been descrlbed operationally In the Ilterature. The batch dilution curve method Is compared to two contlnuous flow methods by mathematical analysls, for the partlcular case of atrazine complexlng equlllbrla in fulvlc acid solutlons. The potential advantages and disadvantages of the three methods have been identlfled for subsequent experimental checklng.

Laurentide humic acid has been titrated with paraquat and with selected divalent metal ions. In some cases, the humic acid was spiked with one cation before titration with another one. Several equivalence points were determined for the binding of the divalent cations, including paraquat. Three equivalence points agreed well 620 S0J0 ET AL with values predicted from the acidic properties of the humic acid. Eight independent experiments gave nine replicate values for an equivalence point that corresponds to 79.2 mole % of the total pairs of carboxyl groups. It is postulated that this represents a large molecular weight or structural fraction of the humic acid. The remaining 20.8 mole % can either bind one divalent cation to two carboxylate anions or bind one divalent cation to one carboxylate anion, with another anion providing for charge balance. Aggregation -disaggregation and dissolutionprecipitation phenomena may determine which case prevails, and they in turn are sensitive to the chemical compositions of samples. It should be possible to develop analytical chemical methods for characterizing the stoichiometric properties of humic acid samples.