Potentiometric titrations in acetonitrile–water mixtures: evaluation of aqueous ionisation constant of ketoprofen

Non ideality of acetonitrile–water mixtures was studied from data on the excess of molar volumes and viscosities. pH and autoprotolisis constants were evaluated at the standard state of the mixed solvent from titrations of a strong acid with a strong base. In order to illustrate the evaluation of the aqueous ionisation constant of water insoluble compounds from pH titrations in ACN–water mixtures, a typical insoluble arylpropionic acid, ketoprofen, was chosen. Ketoprofen was titrated in mixtures from 10 to 70% w/w of acetonitrile against a strong base. From the titration data, the ionisation constant of ketoprofen was evaluated at the standard state of the solvent mixture (pKa*). Aqueous pKa was determined by extrapolation, as the intercept of the plot of pKa* versus ACN mole fraction.