The Effect of Divalent Metals on the Properties of Alginate Solutions. II. Comparison of Different Metal Ions.

Abstract

Addition of different divalent metal ions to solutions of sodium alginate, prepared from Laminaria digitata and the stipes of Laminaria hyperborea, revealed in each case that the concentration of the metallic ion required to bring about gel-formation and precipitation increased in the order: Ba<Pb<Cu<Sr<Cd<Ca<Zn<Ni<Co<Mn,Fe<Mg This order is not identical with that in which the ions are preferentially bound by the alginate. However, the efficacy of a divalent ion as a precipitant for alginate depends not only upon its affinity for the alginate, but also upon the amount of the ion which must be bound to the alginate for gel-formation to occur; it is shown that this last quantity is not independent of the nature of the ion. Added sodium ions markedly affect the concentration of divalent metal ion required for precipitation. When the divalent ion is strongly bound by the alginate, the effect of sodium ions is predominantly one of salting-out, whereas with weakly-bound cations the solubility of the alginate increases with increasing sodium-ion concentration, probably due to displacement of the ion-exchange equilibrium.