Determining the Acidity of Machinery Lubrication Oils Via Infrared Spectroscopy

Machine lubricants deteriorate over time, thus forming acidic by-products that can corrode machine components. Hence, it is necessary to monitor the acidity level in the oil as it ages by measuring either the acid in the oil or the remaining alkalinity from additives in the oil. The deterioration of machine lubricants can be measured through Total Acid Number (TAN) and Total Base Number (TBN) values in order to prevent problems such as machine corrosion, wear, varnish and clogged filters. TAN and TBN values are specified based on the quantity of potassium hydroxide or equivalent required for neutralizing one gram of the sample (mgKOH/g). These values can be measured by electrochemical titration technique, which is, however, complex and expensive. This article discusses the application of IR spectroscopy as an alternative technique to measure TAN and TBN values.

TAN and TBN Measurement by Infrared Spectroscopy

Infrared spectroscopy, both through directly investigating the lubricant ‘as-is’ by correlating the IR spectrum to titrated TAN or TBN values utilizing multivariate techniques, and through the measurement of the response of the lubricant to a chemical reaction, has been employed for determining the TAN and TBN values of lubricant samples. The experimental setup involves the measurement of the absorbance of the lubricant through a 100-200 µm transmission cell utilizing an empty cell background. The IR method for TBN measurement for reciprocating engine oils, owing to their comparatively homogenous chemical composition, has been most commonly reported and is employed as a TBN screening technique by some analytical laboratories. However, the lack of standard techniques in addition to the overall chemical complexity of the lubricants makes a ‘one size fits all’ direct infrared technique difficult. To date, lubricant-specific calibrations for specific end-user applications are generally considered as quantitative.

Spectro FluidScan Q1000 Infrared Spectrometers

Infrared spectroscopy can provide TAN and TBN measurements through various methods of obtaining the spectrum, ranging from the FTIR technique to grating equipment to emerging technologies such as tunable quantum cascade lasers. However, the only prerequisite is to obtain spectra with sufficient resolution between roughly 900-1900 cm^-1 and 2500-4000 cm^-1, which are the regions that allow the calibrations of TAN and TBN.

Figure 1. Increase of TAN in a gear oil as reflected in the infrared spectrum of the fluid. This data is taken using a Spectro FluidScan.

FluidScan, a handheld instrument from Spectro is engineered to obtain quality TAN and TBN results. The Q1000 is a user-friendly but precise and reliable on-site device. The capabilities of the FluidScan Q1000 infrared spectrometer are listed in Table 1.

Table 1 . FluidScan capabilities

Direct Infrared Approach

First step of the three-step approach developed by Spectro is the collection and storage of hundreds of different types of new and used lubricants and their level of deterioration into a sample library. Then a standard ASTM titration technique (D4739 for TBN and D664 for TAN) is used to record the infrared spectrum as well as the TAN and TBN values of the lubricants. The second step is the classification of each oil type according to its infrared spectrum into unique chemical ‘families’ through the use of Soft Independent Method of Class Analogy (SIMCA), a standard multivariate classification technique. The final step is to correlate the known TAN or TBN to the infrared spectrum for each lubricant within a given chemical family using either the Principle Component Regression (PCR) or Partial Least Squares (PLS) multivariate regression technique.

This creates a set of family-specific TAN- or TBN-to-IR calibration curves that exhibit quantitative correlation across a wide variety of lubricants, including industrial, gear, turbine, marine diesel, and reciprocating engine fluids. The end result is to obtain quantitative readings using infrared spectroscopy through the careful classification of the chemical composition of the lubricant prior to be processed for TAN or TBN.

Benefits of Direct Infrared Approach

The following are the key benefits of the direct infrared approach developed by Spectro:

• No sample preparation, no solvents and no reagents
• Cleaning requires only a towel or shop rag
• Unlike time-intensive titration methods, the new approach completes measurement within a minute
• Significant cost savings through reduced materials, labor and hazardous waste
• Provides quantitative TAN and TBN readings for the in-service or used lubricant
• After classification, continuous analysis of samples is possible without referring back to the new sample

However, not all lubricants are classified into a chemical family because that will result in the continuous expansion of the library. For these cases, it is necessary to use established ASTM infrared methods to evaluate oil degradation. Nevertheless, at this point, it is possible to classify more than 90% of the lubricants with the existing library.

Performance of Infrared TAN/TBN Approach

There are many different ASTM standard titration methods available to measure TAN and TBN for lubricants. The repeatability and reproducibility of these standard titration techniques for used oils are well documented. For an in-service sample with a TAN of 2.0 mgKOH/g, the TAN reproducibility ranges between 1.12 and 2.88mgKOH/g, while the repeatability range is between 1.77 and 2.23 mgKOH/g.

For comparison, the IR techniques exhibit typical TAN reproducibility in relation to an ASTM titration measurement of 0.49 mgKOH/g in the normal operating range of TAN less than 4 mgKOH/g at a 99% confidence interval. Hence, for a nominal sample with a TAN of 2.0 mgKOH/g, the expected results can range between 1.51 and 2.49 mgKOH/g, which are comparable to the ASTM method.

Repeatability is computed at the mid-range of the measurement window (0-10 mgKOH/g) at 6.8% RSD at the 99% confidence interval, which is in line with the ASTM method. A typical calibration curve for turbine oils is illustrated in Figure 2, which demonstrates various brands and states of degradation of lubricants.

Figure 2. Relationship between ASTM D664 and infrared TAN values.

For an in-service sample with a TBN of 10 mgKOH/g, titration results can fall between 5.5 and 14.5 mgKOH/g within the specification, with a repeatability range of between 9.6 and 10.4 mgKOH/g. For TBN, both IR techniques exhibit typical reproducibility corresponding to an ASTM titration of 3 mgKOH/g at a 99% confidence interval over a range of 0-16 mgKOH/g. Hence, for a sample with a nominal TBN value of 10 mgKOH/g, the results can range between 7 and 13 13 mgKOH/g, which is line with the ASTM method at this typical level of TBN for new engine oils. However, repeatability is 0.37% RSD at a 99% confidence interval, which is superior when compared to the ASTM D4739 method.

A calibration curve for reciprocating engine oils is depicted in Figure 3, which shows many different oil brands and states of breakdown of lubricants.

Figure 3. Relationship between ASTM D4739 and infrared TBN values. The usefulness of TAN and TBN measurements for monitoring oil degradation lies in consistent, frequent sampling and measurement. This practice produces many data points that show the trend in decrease in base reserve and increase in acidity over the lifetime of the oil. IR TAN and TBN measurements are quick and easy to perform and cost effective. Therefore, samples can be measured at short intervals and a close trend of the values can be followed effectively.

Conclusion

The results demonstrate the ability of the FluidScan to get reliable TAN or TBN readings for many different lubricating fluids, using its software and the infrared spectrum of a lubricating fluid sample. The software is built upon a three-step process of library building, sample classification, and multivariate regression techniques. The expanding library and potential results in line with conventional titration techniques suggest that this approach can be used for both off-site laboratory and on-site oil analysis programs.

This information has been sourced, reviewed and adapted from materials provided by AMETEK Spectro Scientific.

For more information on this source, please visit AMETEK Spectro Scientific.