Aqueous infrared carboxylate absorbances: Aliphatic di-acids

Aqueous attenuated total reflectance Fourier transform infrared (ATR-FTIR) spectra of 18 aliphatic di-carboxylic acids are reported as a function of pH. The spectra show isosbestic points and intensity changes which indicate that Beer's law is obeyed, and peak frequencies lie within previously reported ranges for aqueous carboxylates and pure carboxylic acids. Intensity sharing from the symmetric carboxylate stretch is evident in many cases, so that bands which are nominally due to alkyl groups show increased intensity at higher pH. The asymmetric stretch of the HA⁻ species is linearly related to the microscopic acidity constantof the H₂A species, with σ_pK2 intervening atoms). The results suggest that aqueous ATR-FTIR may be able to estimate `intrinsic' pK_a values of carboxylic acids, in addition to providing quantitative estimates of ionization.