A reagentless and rapid method to measure water content in oils

A simple method for measuring water levels from 1 mg L^-1 to 5000 mg L^-1 in oils that uses no chemical reagents or matrix specific calibrations is described. The approach involves capturing the water from the oil on an infrared transparent membrane and then recording an IR spectrum in transmission mode through the membrane. A classic spectrum of liquid water is obtained for all oil types, as both dissolved water and emulsion-based water are extracted to form an adsorbed layer of water on the membrane surface. The method was tested with three types of oils that are identified as difficult in the ASTM method E2412, as these oils, exhibit distortions in the water bands due to interactions of the water with the oil, or have significant levels of scattering due to the presence of water droplets. In comparing water concentrations measured by Karl Fischer titration, a high level of linearity (R² > 0.995) is obtained over the range of 1 mg L^-1 to 5000 mg L^-1 with slope values of 0.99 for power steering fluid, 0.98 for vegetable oils, and 0.95 for extreme pressure fluid. This shows that water concentration obtained by the membrane method were comparable to those obtained by Karl Fischer. No matrix calibrations were needed as the concentration of water in oil was determined using literature values for the extinction coefficients of water. The wide detection range is obtained by varying the volume of oil (30 μL-30 mL) passed through the membrane and by choosing the water band for quantification.