Measuring Moisture in Lubricants with FTIR: An Alternative to Karl Fischer Titration

A rapid and 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. 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 SD of ±80 ppm. Introduction Fourier transform infrared (FTIR) spectroscopy is a rapid instrumental technique that is widely used for lubricant condition monitoring in accordance with ASTM Standard Practice E2412 (van de Voort et al., 2006). Over the past five years, research has been undertaken by the McGill IR Group in collaboration with Thermal-Lube Inc. to facilitate FTIR condition monitoring and to broaden the scope of FTIR lubricant analysis. This research initiative has involved the design of both an automated, high-sample-throughput FTIR oil analyzer (Figure 1) and quantitative FTIR methods for the determination of acid number (AN) and base number (BN) (van de Voort et al., 2003). The development of these methods has provided alternatives to standard ASTM titrimetric procedures that offer significant advantages in terms of speed of analysis and reduced solvent and reagent use. An FTIR method that accurately detected moisture levels as low as 50 ppm was also developed (van de Voort et al., 2004), but was restricted in scope. This paper describes a new and broadly applicable FTIR method that we have developed and implemented on Thermal-Lube’s Continuous Oil Analysis and Treatment (COAT) FTIR system. It also presents the results of a comparison of the analytical performance of this method by comparison with that of the coulometric Karl Fischer (KF) method (ASTM D6304), which may be considered the gold standard for the quantitative determination of water in lubricants and petroleum products, including lubricant additives, base oils, fully formulated lubricants, and automatic transmission fluids. Principle of the Method FTIR lubricant condition monitoring is a simple reagent-free methodology that involves the trending of a wide range of parameters, including oxidation, nitration, sulfate by-products, soot, glycol contamination, diesel fuel dilution, antiwear depletion and water contamination by measurement of various peak areas or heights in the FTIR spectra of in-service oils (or the baseline tilt of these spectra in the case of soot). However, although water contamination appears in the list of condition monitoring parameters, the adaptation of this methodology as the basis for a quantitative FTIR analytical procedure would provide limited sensitivity and accuracy owing to matrix effects and other complicating factors. To overcome these problems, we designed an