Machinery Lubrication magazine published by Noria Corporation
Issue link: http://www.e-digitaleditions.com/i/520842
22 | May- June 2015 | www.machinerylubrication.com BY CRAIG WINTERFIELD, FLUID LIFE, FREDERIK VAN DE VOORT, MCGILL UNIVERSIT Y The acid number and base number of in-service oils are considered key indicators of oil quality and are used to monitor the accumulation of acid and the depletion of the base additive package. A signifi cant rise in acid number or decrease in base number may refl ect a deterioration in oil quality either due to chemical reactions, oxidation, incorrect oils, additive depletion and contamination. Tables 1 and 2 summarize common acid number and base number methods. ASTM methods, including D664, D974, D2896 and D4739, are the current industry standard methods for measuring the acid and base number. These titration-based methods are slow and expen- sive to execute, require signifi cant volumes of sample and solvent, and are prone to interferences. As a result, these methods have relatively wide repeatability and reproducibility limits, accounting for their signifi cant inter-laboratory variability. The high cost and poor accuracy of these methods limit their usefulness and applica- tion to routine oil monitoring. The acid number and base number are also sometimes reported using a variety of partial least squares (PLS) direct-read Fourier transform infrared (FTIR) methods, which estimate the acid and base number of used oils by directly measuring the spectrum of undiluted oil. Quantitative FTIR Spectroscopy FTIR spectroscopy has been touted as a potential alternative means of obtaining quantitative acid number and base number data. However, there previously had been no solid evidence that FTIR was viable or reliable in commercial practice. FTIR has predominantly been utilized as an automated fi ngerprint-based survey technique, as per ASTM D7418-07 or the Joint Oil Analysis Program, which is typically used for screening and trending changes in lubricant parameters such as moisture, glycol, soot, oxidation, antioxidants and wear additives. As such, FTIR condition moni- toring analysis provides a rapid, automated means of screening a OIL ANALYSIS Determining Used Oils the Acid and Base Number of A New Approach for ACID NUMBER METHOD TYPE REAGENT CALIBRATION NOTES ASTM D664 Titration Potassium hydroxide Stoichiometric Slow, high uncertainty ASTM D974 Titration Potassium hydroxide Stoichiometric Dark oils interfere with measurement PLS-FTIR FTIR direct read None PLS calibration Subject to interferences Stoichiometric-FTIR FTIR acid/base reaction based IR active base PLS and stoichiometric Improved precision over ASTM D664 Table 1. Common Acid Number Test Methods BASE NUMBER METHOD TYPE REAGENT CALIBRATION NOTES ASTM D2896 Titration Perchloric acid in glacial acetic acid Stoichiometric Slow, labor-intensive, used for new oil quality control ASTM D4739 Titration Hydrochloric acid Stoichiometric Slow, high uncertainty PLS-FTIR FTIR direct read None PLS calibration Subject to interferences Stoichiometric-FTIR FTIR acid/base reaction based IR active acid PLS and stoichiometric Improved precision over ASTM D4739 Table 2. Common Base Number Test Methods