SYMPOSIA PAPER Published: 01 December 2022
STP163420200066

Generic Method for the Rapid and Accurate Determination of Acid Number in Lubricants Using FTIR Spectroscopy

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A manual, stoichiometric Fourier-transform infrared (FTIR) acid number (AN) method was developed for lubricants that can substitute for ASTM D664, is oil matrix-independent, and does not rely on correlational chemometrics to estimate results. The method parallels titration in principle, using oleic acid as the calibrant and sodium phenolate as the basic reactant and measuring the residual base spectrally. Split-paired sample spectra of reacted and unreacted oil are ratioed against each other to obtain differential absorbance spectra that respond proportionately to total acidity (AN) as well as its carboxylic acid (COOH) content per Beer's law. The method is generic and can be implemented on any FTIR spectrometer equipped with a conventional flow cell or open-architecture sample handling accessory at rates of up to 20 samples/h. Using macros available in two readily available software packages it automates postanalytical spectral data processing, calibrates or predicts AN and COOH content, or both, and differentiates strong from weak acids by difference via the AN and COOH measures. Predictive performance was validated using a high-AN ester-based hydraulic oil and a used biogas engine crankcase mineral oil, each of which was serially diluted with its opposing acid-free counterpart, a mineral- and ester-based oil, respectively. For these dilutions, the AN and COOH predictions were shown to be linearly proportionate, accurate (less than ± 0.10 AN), and reproducible (less than ± 0.05 AN). Quantification was unaffected by the spectral perturbations induced by diluting the validation oils with a spectrally incompatible oil, and quantitative differentiation between strong and weak acids was demonstrated. Aside from being more rapid, accurate, reproducible, and environmentally friendly than titration this FTIR approach provides for a reliable, stoichiometric, and instrument manufacturer-independent means of determining AN, reducing reliance on problematic FTIR/IR PLS-based chemometric methods that are of questionable accuracy.

Author Information

van de Voort, F., R.
McGill IR Group, Dept. of Food Science and Agricultural Chemistry, Macdonald Campus, McGill University, Sainte-Anne-de-Bellevue, CA
Viset, Michael
6 Sentinel Ave., Kellyville, AU
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Pages: 279–294
DOI: 10.1520/STP163420200066
ISBN-EB: 978-0-8031-7715-4
ISBN-13: 978-0-8031-7714-7