A PHYSICO-CHEMICAL STUDY OF THE PYRIDINE MONOCARBOXYLIC ACIDS

Abstract

1. The thermodynamic dissociation constants of the pyridine monocarboxylic acids and their esters and of trigonelline have been determined by potentiometric titration. 2. The absorption spectra of the acids, their methyl esters and II-methylbetaines have been determined over a wide pH range. From these the absorption spectra of the pure anionic, cationic and isoelectric forms of the acids have been deduced, and it has been shown that the complete absorption curve at any pH can be predicted from them. 3. The betaines were prepared by a new method involving the use of an ion exchange resin in place of the usual silver oxide. 4. The structure of the isoelectric form is discussed. It is shown from potentiometric and spectroscopic measurement that at least 90% of the isoelectric form is present as the dipolar ion. 5. Observations have been made on the distribution coefficient of picolinic and nicotinic acids between chloroform and water and it is shown that only isoelectric form dissolves in the chloroform layer. 6. Isopiestic vapour pressure measurements of solutions of picolinic acid from 0.1m to 8.0 m have been made at 25? and the osmotic coefficient is recorded.