Conductimetric and potentiometric titration of some hydroxylated cinnamic acids with tetrabutylammonium hydroxide in non-aqueous media

In this study, four hydroxycinnamic acids, namely 3,4-dihydroxycinnamic aci d, 4-hydroxycinnamic acid, 4-hydroxy -3- methoxycinnamic acid and 4-hydroxy -3,5-dimethoxycinnamic acid, were titrated conductimetrically and potentiom etrically using triethylamine and tetrabutylammonium hydroxide in acetonitr ile, 2-propanol, and pyridine solvents under a nitrogen atmosphere at 25 de greesC. In conductimetric titration with tetrabutylammonium hydroxide all of the co mpounds exhibited conventional stoichiometric titration curves in 2-propano l. As is evident from the titration curves, all these compounds gave two we ll-defined end points. The first end point corresponds to exactly one equiv alent and the second one to the two equivalents of the base. In conductimet ric titration with tetrabutylammonium hydroxide, all of the acids (except c affeic acid) show two end points in pyridine solvent. These compounds were titrated conductimetrically in acetonitrile and gave only one end point. Potentiometric titration curves of these phenolic acids with tetrabutylammo nium hydroxide in pyridine solvent look very much like the titration curves of weak diprotic acids obtained in an aqueous media with a strong base. In addition, equilibrium is attained very rapidly after each addition of the titrant. However, a second end point cannot be easily observed in the case of 2-propanol. None of the compounds cited above gave meaningful conductimetric and potent iometric titration curves upon titration with triethylamine, which is a wea ker base than tetrabutylammonium hydroxide.