Calcium D-Saccharate: Aqueous Solubility, Complex Formation, and Stabilization of Supersaturation

摘要

Molar conductivity of saturated aqueous solutions of calcium D-saccharate, used as a stabilizer of beverages fortified with calcium D-gluconate, increases strongly upon dilution, indicating complex formation between calcium and Dsaccharate ions, for which, at 25 degrees C, K-assoc = 1032 +/- 80, Delta H degrees(assoc) = -34 +/- 6 kJ mol(-1), and Delta S degrees(assoc) = -55 +/- 9 J mol(-1) K-1, were determined electrochemically. Calcium D-saccharate is sparingly soluble, with a solubility product, K-sp, of (6.17 +/- 0.32) X 10(-7) at 25 degrees C, only moderately increasing with the temperature: Delta H degrees(sol) = 48 +/- 2 kJ mol(-1), and Delta S degrees(assoc) = 42 +/- 7 J mol(-1) K-1. Equilibria in supersaturated solutions of calcium D-saccharate seem only to adjust slowly, as seen from calcium activity measurements in calcium D-saccharate solutions made supersaturated by cooling. Solutions formed by isothermal dissolution of calcium Dgluconate in aqueous potassium D-saccharate becomes spontaneously supersaturated with both D-gluconate and D-saccharate calcium salts, from which only calcium D-saccharate slowly precipitates. Calcium D-saccharate is suggested to act as a stabilizer of supersaturated solutions of other calcium hydroxycarboxylates with endothermic complex formation through a heat-induced shift in calcium complex distribution with slow equilibration upon cooling.