Difference spectrophotometric determination of p-hydroxybenzoic acid in presence of its esters

A difference spectrophotometric analytical method was developed for the selective determination of p-hydroxybenzoic acid in the presence of its alkyl esters without prior separation. Based on the spectral shift to a shorter wavelength accompanyint carboxyl dissociation, the procedure measures as little as 2% of this acid in mixtures with the alkyl ester preservatives and has an accuracy of 2% mean relative error over the 0.16-12.0 microgram of p-hydroxybenzoic acid/ml range.