Quantitative moisture measurements in lubricating oils by FTIR spectroscopy combined with solvent extraction approach

Quantitative measurements of moisture in mineral and synthetic polyol ester based lubricating oils using FTIR spectroscopy combined with solvent extraction approach are described. The samples are prepared by mixing dry dimethyl sulfoxide (DMSO) and lubricating oils, and after phase separation, the extracted bottom DMSO layers are analyzed with FTIR. The results from the solvent extraction show that the DMSO is an excellent aprotic solvent for water removal from lubricating oils. The spectroscopic data reveal that near IR region (5400–4800 cm^{− 1}, molecular water) gives the best results for water determination in both mineral and synthetic lubricants, followed by mid-IR region (3800–3200 cm^{− 1}, O―H stretching). However, the water content estimated from the IR region (1800–1550 cm^{− 1}, O―H bending) has the lowest accuracy due to the interference from aminic, phenolic additives and other oxidation products present in the lubricants. The accuracy of the FTIR spectroscopy combined with solvent extraction approach is exemplified by monitoring the water content in mineral oil during oxidation process at 150 °C for 30 days. The quantitative determination of the moisture in the fresh and oxidized oils by the developed approach is shown to be an alternative technique to Karl Fischer titration.