Infrared Spectroscopy of Aqueous Carboxylic Acids: Malic Acid

Abstract

Quantitative infrared (IR) titration of 1.80 M malic acid is presented where factor analysis (FA) was used to obtain the principal species' spectra and their abundances. Three malic species and three water species were obtained. The distribution of the species as a function of pH was made from which their pKa values were determined. The experimental points of the distribution curves correspond to the values calculated from the thermodynamic equilibrium equations. A precise determination of aqueous malic IR bands was obtained from the real spectra of the malic species. The hydrates' hydration numbers were determined to be 2.0 ± 1.0, 3.0 ± 2.0, and 4.0 ± 0.5 for malic acid, mono-, and disodium malate, respectively. The hydrates are stable throughout the pH range where the species are present. The double and single CO stretch bands of malic acid are situated at 1719 and 1272 cm-1, respectively. The antisymmetric and symmetric CO stretch bands of mono- and disodium malate are situated at 1580, 1400 cm-1 and 1563, 1395 cm-1, respectively. Malic acid shows four very broad bands in the 3800−1800 cm-1 region as a continuous absorption assigned to the OH stretch of hydrogen bonded water and alcoholic groups to carboxylic groups. The 2930 and 2580 cm-1 bands, which are far from the 3500 cm-1 band, indicate strong hydrogen bonds. Disodium malate shows one large band at 3320 cm-1 assigned to the OH stretch of solvated water and alcoholic groups. Monosodium malate has the bands of both malic and disodium malate slightly displaced, but with half their intensities.