Study of acid–base titration of succinic and malic acid in aqueous solution by two-dimensional FTIR correlation spectroscopy

Abstract

Generalized two-dimensional (2D) correlation analysis has been applied to the study of pH-dependent spectral changes in the Fourier transform infrared (FTIR) spectra of succinic and malic acid. pH modulation from pH 2.0 to 12.0 was used to generate a dynamic set of spectra from which 2D infrared correlation maps were obtained. From the 2D analysis based on changes in band intensity and band shifts due to deprotonation of the two carboxylic acid moieties in each molecule, the influence of each deprotonation step on the spectra of the whole molecule could be obtained despite the proximity of the two p K a values in each molecule. In the case of succinic acid, the deprotonation of the first acid induces a shift of the νCO of the second carboxylic acid moiety. In the case of malic acid, it was found that the deprotonation of the more acidic carboxylic acid moiety exerts a shift of the absorption band of the α-hydroxy group whereas deprotonation of the second carboxylic acid moiety does not affect the CO stretch vibration of the hydroxyl group. The additional information obtained with 2D correlation analysis allows frequencies of carboxylate groups from different species (monoanion and dianion) to be identified.