Complexation of citric and tartaric acids with Na and K ions in aqueous solution

The formation of MTar^–, MCit^2– and M₂Cit^– complexes (M is Na⁺ or K⁺) was established in reactions of aqueous solutions of citric and tartaric acids with sodium or potassium chloride solutions; their stability constants were determined by potentiometric titration in aqueous solution at 298.15 K and ionic strength 0.1 and 0.3 mol/l with tetraethylammonium chloride (TEACl) as a supporting electrolyte. Heat effects of reactions between citric acid solutions and sodium or potassium chlorides were measured by calorimetric method at 288.15, 298.15, and 308.15 K and at the ionic strength 0.1, 0.2, and 0.3 M TEACl. The increasing ionic strength was found to decrease exothermic effect of complex formation processes, while the temperature produced the opposite effect. Extrapolation to zero ionic strength was used to find thermodynamic stability constants and standard heat effects of complex formation reactions in solutions of oxyacids. The changes in entropy and heat capacity were calculated as well as standard enthalpies of formation of Na and K complexes of the indicated oxyacids in aqueous solution at 298.15 K.__________Translated from Koordinatsionnaya Khimiya, Vol. 31, No. 4, 2005, pp. 253–261.Original Russian Text Copyright © 2005 by Zelenina, Zelenin.