Machinery Lubrication magazine published by Noria Corporation
Issue link: https://www.e-digitaleditions.com/i/1356397
36 | March - April 2021 | www . machinerylubrication.com 36 | March - April 2021 | www . machinerylubrication.com Current direct- read chemometric instrumentation serves as a "better than nothing" approach with limitations which are rarely grasped by users." " Universal Acid Number (AN) Determination Using FTIR Spectroscopy Michael Viset, Hydrocarbon Consultant | Frederik vand de Voort, McGill University CONDITION MONITORING Traditionally, acid number (AN) has been measured by direct-read instruments reliant on chemometrics. However, conventional Fourier-transform infrared spectros- copy (FTIR) spectrometers used for condition monitoring can now also be used for AN determination. is can be done by adding a spectrally active base to an oil to neutralize the acid and determining the unconsumed base spectroscopically. Unlike direct read instruments, the FTIR method is independent of oil type, does not require the use of chemometrics and can differentiate between weak and strong acids. With FTIR, AN testing is faster, uses much less reagent and is more precise than ASTM titration. is is especially true in the 0-2 AN range typically found in used turbine, hydraulic and compressor oils. e method is great for onsite condition monitoring (CM) labs and provides an alternative to potentiometric titra- tions while still reporting accurate ASTM-equivalent AN data. FTIR in Condition Monitoring (CM) Using FTIR based on the ASTM E2412 CM practice, the ASTM D7214-20 is the FTIR method for determination of oxidation using the carbonyl C=O group, formed when the lubricant combines with oxygen from the air. These are predomi- nantly organic acids, but also include ketones, aldehydes and esters. e measurement is considered a rough acidity screen and requires confir- matory titration (ASTM D664 or D974), particularly in combustion applications where inorganic acids are also formed. Currently, the only FTIR AN determinations are chemometric-based methods, the most common using direct, neat-oil instruments. Another is the dilut- ed-oil stoichiometric procedure used in high-volume laboratories. Both are problematic since they are not universal and are reliant on "Library Calibrations" or additional chemom- etric modeling to obtain results. Due to shortcomings like this, all have diminishing veracity, particularly if the wrong oil type is assigned. Direct-read FTIR instruments target users with extensive machinery infrastructure requiring on-site monitoring of in-use oils rather than sending samples to a commercial CM lab. Aside from conventional ASTM FTIR, the instruments relate the spectra collected using ASTM proto- cols to titrimetric AN data of in-use oils representative of a particular oil-type, class or family. e results determined are not part of the sanc- tioned ASTM method. Users rarely validate AN results and assume that the "Library Calibra- tions" will perform adequately, in part relying on its linkage to the ASTM protocol (e.g., D7418), which ulti- mately does not involve AN as part of its specifications. However, with no practical, reliable and rapid alter- native to estimate AN (or BN), there has been little choice for users but to rely on potentially flawed chemom- etric direct-read FTIR technology as a determinative assessment without follow-up titrimetric confirmation. Stoichiometric AN by FTIR e concerns noted above have led to the development of a manual Learn More: noria.com/ascend/