Thermodynamic quantities of some biochemically important organic acids in aqueous buffered solutions at 25°C

Abstract

Osmotic and activity coefficients for 1-malic acid, citric acid, succinic acid, alpha-ketoglutaric acid, and glycine in water, 0.25 and 0.50 molal THAM[dot]1/2HCl, and 0.25 and 0 .5 0 molal NaH₂PO₄ were measured by the isopiestic vapor pressure method and the varor pressure osmometer method at 25°C over the concentration range of 0.1 molal to saturation of acid in the aqueous and aqueous buffered solvents. These data were fit to equations and comparison between observed and calculated values were tabulated. The activity coefficients of the acids in the aqueous solvents along with the experimentally determined solubilities of each acid in each solvent were used to calculate the Gibbs energy of solution for each system. In addition, the heat of solution of each acid in each aqueous solvent was measured at 25°C, using a reaction-solution calorimeter. These data were used to obtain the heats of solution at infinite dilution for each system but the water system. A simple extrapolation method was used to obtain the heats of solution at infinite dilution for each acid in each aqueous buffer. This method was not applicable to the water solutions for the acids completely dissociate at infinite dilution in this solvent, and not enough data exist for the calculation of the heats of solution at infinite dilution for each acid in water.