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Lube-Tech


Summary This article previews a generic, manual quantitative Fourier-transform infrared spectroscopy (FTIR) method for determining Acid Number (AN) capable of delivering deterministic ASTM titrimetric-equivalent data for new and in-use lubricants. The method is universally applicable to any lubricant type, differentiates between weak and strong acids, does not involve the use of chemometrics and is capable of analysing ~20 samples/hour using any FTIR spectrometer equipped with a transmission flow cell or open architecture accessory. The generic nature of the AN method could provide a mechanism for standards bodies (ISO/ASTM) to consider it as well as similar approaches used to determine Base Number (BN) and moisture (H2


O) by FTIR spectroscopy. Introduction


In 2015 LUBE Magazine reviewed a commercial automated FTIR system configured for lubricant condition monitoring (CM) analysis with additional capability to determine AN (or BN), both using pre-diluted samples to achieve higher analytical speeds (1). Recently, a generic, manual AN version has been developed, readily implemented on any standard FTIR without recourse to instrument-specific or proprietary software to obtain ASTM titrimetric equivalent results (2).


24 LUBE MAGAZINE NO.163 JUNE 2021


PUBLISHED BY LUBE: THE EUROPEAN LUBRICANTS INDUSTRY MAGAZINE


No.134 page 1


Titrimetric-Equivalent Acid Number (AN) Determination of Lubricants by FTIR Spectroscopy


Frederik van de Voort (Emeritus Professor, McGill University, Montreal, Canada) Michael Viset (Hydrocarbon Consultant, Kellyville, Australia)


Almost every lubricant CM laboratory has either a dedicated autosampler – based or stand-alone FTIR to carry out lubricant CM screening by ASTM D7418, E2412 or its founding methodology, the Joint Oil Analysis Program (JOAP) protocol (3). These trending methods are relied on to screen lubricants to either accept their continued use or follow up with more determinative methods; typically, titrimetric AN (D664) or BN (D2896/D4739) analyses if an oil is deemed questionable. These methods are effectively opposite sides of the same analytical coin, with AN rising over time/use as acids are produced by heat, pressure and oxidation in non-combustion applications commonly used in turbines and compressors, while for combustion engine applications, the added base present expressed as BN, drops as it is consumed by both strong combustion byproduct acids and weaker acids produced by oxidation. Currently, proprietary neat-oil chemometric (4) and diluted-oil stoichiometric FTIR approaches/methods (5) exist to determine AN, however both are problematic in not having universal applicability to provide accurate and reliable deterministic results for any oil type. A generic FTIR-based method has recently been developed that overcomes these limitations; its principles, benefits, limitations and performance are compared here to methods currently in use.


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