Effect of the Electrolyte Concentration on the pH of Sodium Chondroitin Sulfate in Aqueous Solution

Abstract

Sodium chondroitin sulfate, as a mucopolysaccharide, shows the effects of ionizable sulfate and carboxyl groups of the polyelectrolyte on selective counter ion binding. When the electrolytes were added to its aqueous solution, pH decreased due to counter ion binding to the chondroitin sulfate ion. The decrease of pH with the addition of the electrolyte was in the sequence of Li⁺<Na⁺<K⁺<NH₄⁺ for 1 : 1 electrolytes and Sr²⁺<Co²⁺<Mg²⁺<Ni²⁺<Cd²⁺ for 2 : 1 electrolytes. When sodium chloride was added to the chondroitin sulfate of various degrees of neutralization, the values of the apparent dissociation constants for the carboxyl group of chondroitin sulfate increased with the increase of sodium chloride concentration. It is concluded that metal ion and hydrogen ion bound competitively to the carboxyl group of chondroitin sulfate ion, and that the difference in the decrease of pH, which depends on the kind of metal ion, is due to the difference in the binding affinity of the metal ions to the carboxyl group.