VOLUMETRIC PROPERTIES OF SOME METHYL-SUBSTITUTED BENZOIC-ACIDS AND THEIR SODIUM-SALTS IN WATER AS A FUNCTION OF TEMPERATURE

Density data are presented for benzoic, the toluic and 2,6-dimethylben zoic acids and their sodium salts at temperatures from 5 to 65-degrees -C. Apparent and partial molar volumes have been calculated from the e xperimental data. From the partial molar volume data the volume change on substitution of a methyl group has been calculated as has the volu me change on ionization of the acids. It is found that the volume cont ribution of a methyl group is somewhat dependent on the substituent po sition and on the temperature. The relation between volume change on i onization and the corresponding entropy change is presented for a rang e of temperature. The data support the idea that electrostriction is a n important feature of these systems and that its importance increases with rising temperature.