Universal Acid Number (AN) Determination Using FTIR Spectroscopy

Traditionally, acid number (AN) has been measured by direct-read instruments reliant on chemometrics. However, conventional Fourier-transform infrared spectroscopy (FTIR) spectrometers used for condition monitoring can now also be used for AN determination. This can be done by adding a spectrally active base to 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. This is especially true in the 0-2 AN range typically found in the used turbine, hydraulic and compressor oils. The method is great for onsite condition monitoring (CM) labs and provides an alternative to potentiometric titrations 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 the determination of oxidation using the carbonyl C=O group, formed when the lubricant combines with oxygen from the air. These are predominantly organic acids, but also include ketones, aldehydes and esters. The measurement is considered a rough acidity screen and requires confirmatory 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 diluted-oil stoichiometric procedure used in high-volume laboratories. Both are problematic since they are not universal and are reliant on “Library Calibrations” or additional chemometric modelling 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 protocols to titrimetric AN data of in-use oils representative of a particular oil type, class or family. The results determined are not part of the sanctioned ASTM method. Users rarely validate AN results and assume that the “Library Calibrations” will perform adequately, in part relying on its linkage to the ASTM protocol (e.g., D7418), which ultimately does not involve AN as part of its specifications. However, with no practical, reliable and rapid alternative to estimate AN (or BN), there has been little choice for users but to rely on potentially flawed chemometric direct-read FTIR technology as a determinative assessment without follow-up titrimetric confirmation.

Stoichiometric AN by FTIR

The concerns noted above have led to the development of a manual stoichiometric AN method that overcomes the limitations inherent to the chemometric approaches currently in use. This was prompted by the need for both onsite CM monitoring as well as concurrent determinative AN analyses in a remote mining site in Papua New Guinea. With potentiometric titration not being an option and direct-read AN systems considered inadequately deterministic, the automated stoichiometric method was re-examined and reconfigured. The FTIR unit selected for this work was a compact Agilent 5500t with an open architecture TumblIR® accessory (Figure 1), requiring only a few drops of sample and simply wiped between analyses. The method reconfigured was once used for edible oil analysis where high analytical throughputs were not paramount, but accuracy was. This used a paired split-sample approach which involves some additional sample preparation, but its benefits are substantive, including:

• Independent of oil type
• Avoidance of chemometrics
• Facilitating acid type differentiation (strong vs. weak)
• Robust analytical rate of ~20 samples/hr.

Calibration

The method employs pure oleic acid as a primary calibrant added at various levels to a neutral hydrocarbon oil for calibration standards. Each standard is split and a reagent-free solvent-diluent is added to one part (Ion) and a reagent-diluent containing NaPhenolate added to the other (In). These are scanned as a sequential pair, the first a background scan followed by the sample scan producing a differential absorbance spectrum, -Log (In/Ion) = ΔAbsn. This process gives rise to two measurable signals (changes in NaPhenolate and oleic acid absorptions) which facilitate AN determination as well as acid differentiation (see Figure 2) in real samples.

Generic Implementation

With access to basic spectral data collected by an FTIR, there is little to differentiate instruments. The main bottleneck is data processing, calibration, prediction and reporting. These limitations have been greatly improved by SpectraGryph®, a free, generic, post-spectral data processing software package. It is powerful and easy to use in this application and when combined with Excel, facilitates calibration, prediction and reporting of results for on-site AN analysis.

Facsimile paired split-sample calibration spectra illustrating how two distinct measurable signals are generated in the different spectra while the oil matrix component is simultaneously ratioed out. The arrows indicate the signal direction as acidity increases with that of oleic acid facilitating measurement of weak (COOH) acids and NaPhenolate the total acidity.

Implementation

Basically, any FTIR equipped with a standard flow cell loaded by aspiration or an open architecture accessory can be used. Preparing only two 2.5 ml samples (sample-reagent-diluent and sample-blank-diluent, prepared with 1:4 oil: diluent ratio) was more than sufficient using a 100 µm “wipeable” TumblIR® accessory integral to the Agilent 5500 used and handled as per the protocol presented in Figure 3. For both calibration and sample analysis, the 2nd derivative differential spectra were used and measured for the NaPhenolate absorptions at 1589 cm-1 (Figure 4a) and related by linear regression to the mg Oleic Acid/ml (Figure 4b) which represents total acidity (AcidityTOTAL). Similarly, the COOH signal available (~1710 cm-1), representative of weak acids (AcidityCOOH) present in unknown samples was calibrated. Both acidity values are converted to mg KOH/g (AN). In AN terms, the calibration precision (Figure 4b) is < ±0.10 over a range of 0-4 AN. With these two AN measures, the strong acids can be determined by difference: ANSTRONG = ANTOTAL – ANCOOH This additional acid differentiation capability may be useful in gauging the relative corrosiveness of oils, especially if combined with moisture information.

Performance

The methods’ performance is illustrated in Figure 5, where an oxidized mineral oil containing both weak and strong acids has been serially diluted with acid-free, ester-based oil. The ANTOTAL and ANCOOH values for blends were determined and plotted as a function of dilution. Both measures track linearly even though the spectral signature of the oil varies continuously by mixing the two dissimilar oils in differing proportions. Even so, the method succeeds in tracking AN because each sample analyzed serves as its own reference. The oil matrix changes are accounted for and ratioed out accordingly, leaving only the spectral changes associated with the acid-base reaction to be measured. All competing chemometric-based FTIR methods would fail this test as the spectral changes induced cannot be modelled or anticipated. As such, it addresses their key limitation: the need to know the oil type and to have modelled it in advance in order to have any hope of estimating AN. Even in the best of circumstances, the predicted AN is an approximation without even considering common confounding issues.

Conclusion

Conventional titrimetric AN and BN methods are slow, expensive and environmentally problematic. They also have no determinative ASTM-sanctioned FTIR AN methods available. Current direct-read chemometric instrumentation serves as a “better than nothing” approach with limitations that are rarely grasped by users. In contrast, this new stoichiometric AN method provides a robust alternative and uses a readily understood Beer’s Law cause and effect calibration, oil-type universality and ready validation as well as acid type differentiation. Generic implementation via SpectraGryph® is available, and more substantive details of the methodology are to be presented elsewhere. Its lower cost, higher speed, better accuracy and minimal waste-stream are significant benefits over ASTM titrimetric procedures. Early adopters should consider involving ASTM or ISO to further assess, validate and standardize this methodology to fully define and characterize its benefits, especially its acid differentiation feature.

References

1. In-Service Oil Analysis Handbook 3rd Edition. Spectro Scientific 2017: https://web.archive.org/web/20200923163612 https://www.spectrosci.com%2Fdefault%2Fassets%2FFile%2FSpectroSci_OilAnalysisHandbook_FINAL_2014-08.pdf
2. C. Winterfield and F.R. van de Voort. A new approach to determining the Acid and Base Number AN and BN of used oils. Machinery Lubrication. May-June 2015.
3. C. Winterfield and F.R. van de Voort, Quantitative condition monitoring of in-use oils by FTIR spectroscopy. LUBE Magazine No 127 pp. 24-29 2015.
4. C. Winterfield, and F.R. van de Voort. Automated acid and base number determination of mineral-based lubricants by Fourier transform infrared spectroscopy: Commercial laboratory implementation. Journal of Laboratory Automation 1-10 2014 DOI: 10.1177/221 10682 14551825
5. M. Meng, L. Lei, Q. Ye and F.R. van de Voort. Fourier transform infrared (FTIR) spectroscopy as a utilitarian tool for the routine determination of acidity in ester-based oils. Journal of Agricultural and Food Chemistry. 63, 37, 8333-8338. 2015 DOI: 10.1021/acs.jafc.5b02738
6. Spectroscopy Ninja: https://web.archive.org/web/20200923165652/https%3A%2F%2Fwww.effemm2.de%2Fspectragryph%2Fdown.html 7. van de Voort, F.R. and Viset, M. Generic method for the rapid and accurate determination of Acid Number (AN) in lubricants by FTIR Spectroscopy. Proceedings of the ASTM Symposium Highlighting Standard Guides and Practices that Support the Lubricant Condition Monitoring Industry. (Postponed, December 2020; In Press).