Conductimetric and Potentiometric Titration of Some Hydroxylated Cinnamic Acids with Tetrabutylammonium Hydroxide in Non-Aqueous Media


Abstract: In this study, four hydroxycinnamic acids, namely 3,4-dihydroxycinnamic acid, 4-hydroxycinnamic acid, 4-hydroxy --3- methoxycinnamic acid and 4-hydroxy-3,5-dimethoxycinnamic acid, were titrated conductimetrically and potentiometrically using triethylamine and tetrabutylammonium hydroxide in acetonitrile, 2-propanol, and pyridine solvents under a nitrogen atmosphere at 25 ^oC. In conductimetric titration with tetrabutylammonium hydroxide all of the compounds exhibited conventional stoichiometric titration curves in 2-propanol . As is evident from the titration curves, all these compounds gave two well-defined end points. The first end point corresponds to exactly one equivalent and the second one to the two equivalents of the base. In conductimetric titration with tetrabutylammonium hydroxide, all of the acids (except caffeic 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 tetrabutylammonium 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 potentiometric titration curves upon titration with triethylamine, which is a weaker base than tetrabutylammonium hydroxide.


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