Talanta 228 (2021) 122250 Available online 26 February 2021 0039-9140/© 2021 Elsevier B.V. All rights reserved. Measurement of water concentration in oils using CaO powder and infrared spectroscopy Sfoog H. Saleh a, b , Carl P. Tripp a, b, * a Department of Chemistry, University of Maine, Orono, 04469, Maine, USA b Frontier Institute for Research in Sensor Technologies (FIRST), University of Maine, Orono, 04469, Maine, USA A R T I C L E INFO Keywords: Water concentration in oil CaO powder Transparent membranes Infrared spectroscopy ABSTRACT A simple method for measuring water concentration from 1 to 10000 mg L 1 is described. The approach involves adding CaO powder to an oil sample and measuring the amount of Ca(OH) 2 produced by the reaction of CaO with water. Collection of the powder occurs by passing a fxed volume of the oil through an infrared transparent membrane and the amount of water is determined from the intensity of the OH stretching mode of Ca(OH) 2 at 3645 cm 1 . The approach is demonstrated with transmission, vegetable, and extreme pressure oils. These oils represent classes of oils that are problematic for measurement by transmission infrared spectroscopy, using a fxed pathlength cell, as described in ASTM method E2412. Values for the water levels equivalent to those measured by Karl Fischer titration are obtained with a linearity R 2 value of > 0.996 and %RSD of 6.7% over a wide detection range of 1 mg L 1 to 10000 mg L 1 . No calibration is required, as the amount of water is determined using the extinction coeffcient for the band at 3645 cm 1 . 1. Introduction The most widely used method to measure water concentration in oil is Karl Fischer titration (KFT). This lab-based method requires toxic and expensive chemicals, which has prompted efforts to develop alternative methods [1–7]. One of the more promising approaches has centered on the use of infrared spectroscopy [8–12]. The method, as described in ASTM E2412, simply involves placing an oil sample into a transmission liquid cell and measuring the water content from the broad peak at 3400 cm 1 due to liquid water. However, distortions in the water band arising from interactions with the oil or components in the oil, along with scattering that occurs with water droplets in the oil, mean that matrix specifc calibrations are required [13]. Several approaches have been used to overcome these problems which include the use of surfactants to stabilize water droplet size [1] and involve extraction of the water into a solvent [14,15] or the addition of a chemical that reacts with water [16, 17]. These approaches still require matrix specifc calibrations. Recently, we reported a method for detection of water in oil samples at concentrations of 1 to 5000 mg L 1 [18]. The method involves passing an oil sample through an infrared transparent membrane and then quantifcation of the amount of water by infrared spectroscopy. The water dissolved in the oil or existing as emulsifed water droplets ad- sorbs as a layer on the membrane surface and thus, the infrared spectrum obtained is that of liquid water, devoid of any light scattering or dis- tortions of the water spectrum due to interaction with oil components. The method required no reagents and provided linear detection over a wide range from 1 to 5000 mg L 1 . No calibration was required, as values equal to those obtained with KFT were obtained using literature values for the extinction coeffcients of the various infrared bands of water. The wide detection range results from varying the volume of oil passed through the membrane and the choice of the water band for quantifcation. While simple and easy to use, the system requires a cooled sample holder to avoid evaporation of the water. There is also the need to draw a representative sample into a syringe to pass the sample through the membrane. It is the inability to obtain representative sam- ples above 5000 mg L 1 that defned the upper range value for detection for both KFT and our membrane-based methods. Furthermore, detection of water levels at 5000 mg L 1 and above by KFT required a minimum of 3 h to perform the measurement. The approach described here does not involve collecting water on a membrane but rather is based upon the reaction of water with a solid particle. In this method, CaO powder is added to a stirred beaker of the Abbreviations: 1) ASTM, American Society for Testing and Materials; 2) EP Extreme Pressure, 3) KFT, Karl Fischer Titration. * Corresponding author. E-mail address: ctripp@maine.edu (C.P. Tripp). Contents lists available at ScienceDirect Talanta journal homepage: www.elsevier.com/locate/talanta https://doi.org/10.1016/j.talanta.2021.122250 Received 15 January 2021; Received in revised form 22 February 2021; Accepted 23 February 2021