An automated FTIR method for the routine quantitative determination of moisture in lubricants: An alternative to Karl Fischer titration

An accurate primary Fourier transform infrared (FTIR) method for the determination of moisture in mineral and ester based lubricants has been developed based on the extraction of moisture into dry acetonitrile. FTIR evaluation of acetonitrile extracts from new and used lubricants as well as common lubricant additives and contaminants which might co-extract indicated that phenolic constituents interfered significantly with moisture measurements. By measuring moisture at 3676 cm⁻¹ on the shoulder of the asymmetric OH stretching band, spectral interferences from extracted phenolic constituents were minimized. The spectra of calibration standards (0-2100 ppm), prepared by gravimetric addition of water to dry acetonitrile, were recorded in a 1000-μm CaF₂ transmission flow cell and produced linear standard curves having an S.D. of ∼±20 ppm. Lubricant sample preparation involved the vigorous shaking (20 min) of a 1:1.5 (w/v) mixture of lubricant and dry acetonitrile, centrifugation to separate the phases, acquisition of the FTIR spectrum of the upper acetonitrile layer, and subtraction of the spectrum of the dry acetonitrile used for extraction. A Continuous Oil Analyzer and Treatment (COAT^®) FTIR system was programmed to allow the automated analysis of acetonitrile extracts, and the methodology was validated by analyzing 58 new and used oils, independently analyzed by the Karl Fischer (KF) method. Linear regression of FTIR versus KF results for these oils produced a linear plot with a between-method S.D. of ±80 ppm. As implemented on the COAT^® system, this FTIR method is capable of analyzing 72 acetonitrile extracts/h and provides a high-speed alternative to the KF titrimetric procedures for the determination of water in lubricants.