Determination of the pKa values of some selected aromatic amines and naturally occurring compounds in polar and non-polar solvents at 25oC

Abstract

Ionisation constant is a measure of the strength of acids and bases and is expressed in terms of the pKa of conjugate acids
and bases. The pKa of naturally occurring khivorin, 7-keto-khivorin and some aromatic amines were measured in chloroform,
toluene, 50% ethanol-methanol, 50% ethanol-water and 75% ethanol-methanol mixture at 25oC by potentiometric titration.
The pKa values obtained are 3.23 for khivorin in toluene and 1.20 for 7-ketokhivorin in chloroform which demonstrate their
acidic nature. The aromatic amines: indole and 4-dimethylaminopyridine have a higher pKa values in 50% ethanol-water
solvent compared to their values in ethanol-methanol solvents. 3-methylindole and para-toluidine have a higher pKa
values in 50% and 75% ethanol-methanol solvents than in ethanol-water solvent. The higher pKa values of indole and 4-
diemthylaminopyridine in 50% ethanol-water signifies the best medium at which the strength of these amines could be
determined; whereas p-toluidine is best determined in 50% ethanol-methanol and 3-methylindole in 75% ethanol-methanol
mixture.