Cryoscopic and volumetric studies in the maleic acid + water system

Freezing points of aqueous solutions of maleic acid were determined. They served to obtain water activities and osmotic and activity coefficients at freezing temperatures. The integration of the Gibbs - Duhem equation was performed by taking into account the dissociation of maleic acid. Densities of (0.09831, 0.4902, and 0.9790) mol·kg^-1 maleic acid solutions were measured at 5 K intervals from T = (288.15 to 333.15) K, and they are compared with the literature results. From the determined densities, the apparent molar volumes, the cubic expansion coefficients, and the second derivatives of volume with respect to temperature which are interrelated with the derivatives of isobaric heat capacities with respect to pressure were evaluated. These derivatives were qualitatively correlated with the changes in the structure of water when maleic acid is dissolved in it.