Interaction of Alginic Acid with Organic Diacidic Base Piperazine

抄録

The molar binding ratio (x_pip) of piperazine (i.e., piperazinium ion) to a carboxylate group of alginate (Alg) was studied by using the titration/dialyzing method in a salt-free system. The value of x_pip at the equivalence point of piperazine to alginic acid was far less than untiy, because the dominant ionic species of piperazine at the equivalence point (pH 5.0) was bivalent piperazinium cation. The ratio increased further with the amount of added piperazine, leveling off at unity after adding a sufficient quantity of piperazine, where the dominant species was univalent cation. On the other hand, competition of alkali metal ion with piperazinium ion for Alg was observed when MCl (M=Li⁺, Na⁺, or K⁺) was added. The effect was in the order of Li⁺>Na⁺>K⁺. The sum of x_pip and x_M (binding ratio of M) was almost unity even at low pH where dominant species of piperazine was bivalent piperazinium cation. This fact suggests that a bivalent piperazinium cation is bound to a carboxylate group of Alg through one of two positively charged sites while another site captures chloride anion as another counter ion by electrostatic force.