QUANTITATIVE AQUEOUS ATTENUATED TOTAL REFLECTANCE FOURIER-TRANSFORM INFRARED-SPECTROSCOPY .2. INTEGRATED MOLAR ABSORPTIVITIES OF ALKYL CARBOXYLATES

A quantitative attenuated total reflectance Fourier transform infrared (ATR-FT-IR) spectroscopic method is developed for the analysis of total carboxylate concentration, [COO-], in aqueous solution. The short (12-13 mum) and highly reproducible pathlength of the ATR cell permits quantitative subtraction of the water peak at 1640 cm-1. Carboxylate quantitation is based on the area of the asymmetric stretching peak, which is nearly independent of compound structure. The molar absorptivity of alkyl carboxylates in water is 438 +/- 58 l mol-1 cm-1, and the integrated molar absorptivity is 2.95 +/- 0.08 X 10(4) l mol-1 cm-2 (n = 15 compounds, 0.1 M less-than-or-equal-to [COO-] less-than-or-equal-to 1.5 M). The [COO-] in solutions of mixed carboxylates is measured with a root mean square error of 2.4% and a small (+ 1.5) positive bias. The accuracy of the method is limited by the assumption that integrated absorbance is constant for all COO- groups.