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

Generalized two-dimensional (2D) correlation analysis has been applied to t he 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 cor relation maps were obtained. From the 2D analysis based on changes in band intensity and band shifts due to deprotonation of the two carboxylic acid m oieties in each molecule, the influence of each deprotonation step on the s pectra of the whole molecule could be obtained despite the proximity of the two pK(a), values in each molecule. In the case of succinic acid, the depr otonation of the first acid induces a shift of the vC = O of the second car boxylic acid moiety. In the case of malic acid, it was found that the depro tonation of the more acidic carboxylic acid moiety exerts a shift of the ab sorption band of the ol-hydroxy group whereas deprotonation of the second c arboxylic acid moiety does not affect the C-O stretch vibration of the hydr oxyl group. The additional information obtained with 2D correlation analysi s allows frequencies of carboxylate groups from different species (monoanio n and diahion) to be identified. (C) 2000 Elsevier Science B.V. All rights reserved.