Machinery Lubrication magazine published by Noria Corporation
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24 | May- June 2015 | www.machinerylubrication.com large number of oil samples, in part to determine if additional quantitative analyses, such as acid or base number determinations, are required. Types of FTIR Calibrations Currently, there are three distinct quantitative approaches avail- able for acid number or base number analysis by FTIR. One is based on direct neat-oil analysis solely using partial least squares (PLS) chemometrics. Another is based on the use of ASTM-like acid/base stoichiometric reactions. A third approach, called a mixed-mode calibration, combines the advantages of both the PLS and stoichiometric calibrations. PLS-only Calibrations A PLS-only calibration is created by comparing ASTM D664 and D4739 data to the undiluted F TIR spectra of a representative set of in-service oils. Partial least squares is then used to correlate variations in these spectra to the acid number and base number results measured by titration. The spectrum of unknown samples can then be used to predict or estimate the acid number or base number result. PLS-based spectral methods rely solely on non-specifi c spec- tral changes that can become problematic when dealing with used oil samples because they have signifi cant and tricky interfer- ences that can be diffi cult to model adequately. These older direct-read methods have trouble accounting for real-life variables such as soot, water, incorrect lubricant or mixtures of lube and oil degradation products. Stoichiometric Calibrations The stoichiometric calibration relies on acid/base reactions that are directly measured using the absorbance of infrared light. This is made possible through the use of weaker but "IR active" organic acids and bases rather than stronger inorganic potassium hydroxide (KOH) or hydrochloric acid (HCl), as per ASTM methods. With this approach, the analytical principles are similar to the ASTM methods, e.g., acid/base reactions, but instead of titrating to deter- mine the end point, the acid/base reaction is monitored and measured spectrally. One of the consequences of using weaker organic acids and bases is that the predicted analytical FTIR acid number/base number values obtained will be signifi cantly lower than those obtained using the titration methods. Until recently, this divergence of FTIR acid number and base number values from those obtained using titration procedures was a major impediment to laboratories wishing to make use of the stoichiometric approach. Oil analysis and reliability clients are naturally reluctant to change their familiar analytical frame of reference, and a shift in expected acid number and base number values could lead to confusion. This problem was solved by the development of a "mixed-mode" stoichiometric cali- bration with the concurrent implementation of PLS. Mixed-mode Calibrations The mixed-mode method combines the advantages of both types of FTIR calibration. This calibration is anchored using gravimetri- cally prepared ideal standards to defi ne the fundamental acid/base relationship. Hundreds of in-service oil samples are then used to FTIR CALIBRATION TYPE CALIBRATION SAMPLES NOTES PLS (partial least squares) In-service oil samples with correspond- ing ASTM acid number or base number data • Fast analysis • Easy-to-set-up calibration • Subject to interferences • Estimate ASTM D664 and D4739 Stoichiometric – linear fit Gravimetrically prepared standards of organic acid/base diluted in mineral oil • Accurate and precise • Calibration is anchored to stoichiometric chemical reactions • Results cannot be directly compared to ASTM D664 or D4739 Mixed mode = PLS + stoichiometric Gravimetrically prepared standards and samples of used oil with corresponding ASTM D664 or D4739 data • Results are a direct match to ASTM D664 and D4739/ D2896 • Accurate and precise • High initial development cost Table 3. Summary of Calibration Types for FTIR Acid Number and Base Number Methods Figure 1. An example of a system used for the FTIR determina- tion of ASTM-identical acid number and base number results for in-service mineral oils