VOLUME CHANGES FOR IONIZATION OF FORMIC ACETIC AND N-BUTYRIC ACIDS AND GLYCINIUM ION IN AQUEOUS SOLUTION AT 25 DEGREES

Partial molar volumes of glycine hydrochloride and of molecular formic, acetic, and n-butyric acids and their sodium salts in aqueous solution at 25° have been obtained from pycnometric and dilatometric measurements. The volume changes associated with ionization of the acids are -6.80 for the glycine cation, -8.43 for formic acid, -11.50 for acetic acid, and -14.22 ml mol-1 for n-butyric acid. The packing density, defined as the ratio of van der Waals volume to partial molar volume of a solute species, is a convenient property to use in comparing solutes with different structures and volumes. Above a certain minimum size all molecules or ions of a given type have the same packing density, and these values are close to the packing densities of randomly packed spheres. The effect of chain length in the acid on the volume change for ionization follows from the existence of such plateau densities. Both the electrostriction of solvent and the dead space or void volume between solute and solvent make important contributions to the volume changes for ionization reactions.