Analysis of Acidity in Oil-Based Matrices by Infrared Spectroscopy.

摘要

The feasibility of employing a portable variable filter array (VFA) infrared spectrometer equipped with a transmission flow cell to quantitatively analyze edible oils for free fatty acids (FFA) was evaluated. The approach to FFA determination was based on a previously reported FTIR method that involves the extraction of FFAs into methanol containing the base sodium hydrogen cyanamide (NaHNCN), which converts the FFAs to their salts, followed by measurement of the carboxylate absorbance at 1571 cm-1 in the spectrum of the methanol phase. The VFA IR spectrometer, which had a relatively weak pulsed IR source, was found to provide insufficient energy for accurate measurement of the carboxylate absorption superimposed on the strong methanol absorption at 1450 cm-1. By changing the extraction solvent to ethanol, good spectra and calibrations having an overall SD of +/-0.07% FFA could be obtained. This work in turn led to the investigation of an ethanol solution of NaHNCN as a signal transduction reagent for Acid Number (AN) analysis in mineral-based lubricants, whereby total acidity would be measured by monitoring the decrease of the nuC≡N absorption of NaHNCN at 2109 cm-1 as a result of the acid/base reaction. The equivalent response of the nuC≡N band to strong inorganic acids and oleic acid demonstrated that NaHNCN, a somewhat weaker base than KOH, fully ionizes organic acids. Calibration standards were prepared by direct addition of oleic acid to the NaHNCN/ethanol solution, and a calibration equation for the determination of AN was obtained by a quadratic fit of the concentration data to the FTIR nuC≡N absorbance data. The AN values obtained for ethanolic NaHNCN extracts of used oils by the FTIR method correlated well with those produced by titration of these extracts. Comparison between FTIR and titrimetric AN values (obtained by ASTM Standard Method D664-89) for a set of used oils spanning an AN range of 0.3-5 mg KOH/g showed a reasonably good linear relationship (R = 0.985), with the FTIR method generally producing lower values. This tendency was attributed to the presence of weakly acidic species, which would be less extensively ionized by NaHNCN than by KOH. Implementation of the FTIR AN method on an autosampler-equipped spectrometer allowed for the automated analysis of up to 120 pre-prepared samples/h, representing a significant increase in analytical throughput relative to traditional titrimetric procedures. Thus, this research work has led to the development of two practical IR spectroscopic methods, one an economical instrumental means for at-line monitoring of FFA levels in crude and refined edible oils, and the second, an automated instrumental procedure for determining AN in mineral-based lubricants.