When lubricating oils are utilized in various machinery equipment, they are often subject to the harsh conditions including elevated temperatures and varying oxidation levels. As a result of these environments, lubricant properties, including their viscosity, carbon residue, sludge content, acidity and alkalinity can be altered and ultimately affect the different components of the machinery and degrade the lubricant; a process that has been recorded in various industrial settings for numerous decades1.
Furthermore, the acid concentration of a lubricant can also increase as a result of acidic contaminants entering the machinery, the use of incorrect oils, a depletion of the alkaline reserve contents and the generation of oxidation by-products.
What is pH?
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The pH level of any given sample is a useful property to determine the number of hydrogen ions present within the solution. Regarding lubricant samples, the pH level is a valuable tool to assess the corrosiveness of a sample; however, this property is limited in its ability to accurately represent the concentration of acidic and alkaline chemicals present within a lubricant sample. Although pH test methods are useful when a researcher is interested in evaluating the potential of a lubricant to cause corrosive damage to equipment, both AN and BN analysis techniques are often preferred methods in the industrial setting2.
Determining Acid Number (AN)
The Acid Number (AN) of a lubricant represents its specific acid concentration, which is typically determined by measuring the amount of potassium hydroxide (KOH) that is required to neutralize a lubricant sample.
Determining Base Number (BN)
The Base Number (BN) of a lubricant represents the concentration of bases, or alkaline chemicals, present within a given oil sample3. The determination of the BN of an oil sample is a useful tool to determine the amount of alkaline reserve present within an oil sample, as this reserve quantity is vital for neutralizing lubricants if acidic chemicals contaminate the machinery.
Methods of Determining Acidity and Alkalinity Levels in Lubricants
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The American Society for Testing and Materials (ASTM) has several standard test methods available to determine AN values of lubricants in the industrial setting. The following table provides details on each of these methods:
Figure 1: Widely used ASTM test methods for determining AN values of lubricant samples.
ASTM Test Method
Measures the concentration of acidic chemicals found within a lubricant sample by utilizing a potentiometer.
- This technique involves a mixture of toluene, isopropyl alcohol, and water that is dissolved into the sample, followed by titration of KOH.
- The potentiometer is then used to determine the amount of KOH that is titrated into the sample to achieve a neutral pH level (~7).
Measures the concentration of acidic chemicals found within a lubricant sample by analyzing the change in color followed by a chemical reaction.
- This technique involves a mixture of toluene, p-naptholbenzene, isopropyl alcohol and water that is mixed with the lubricant sample.
- KOH is titrated into the solution, and any change in color is monitored2.
While the aforementioned standard methods are useful for determining both AN and BN values of a lubricant sample, these techniques can often become complicated and increasingly expensive as they are repeated. Various industries, therefore, employ their analytical techniques to evaluate the acidity and alkalinity of their lubricants. For example, ExxonMobil offers the Mobil ServTM Lubricant Analysis, which utilizes either Fourier Transform Infrared Spectroscopy (FTIR) or Total Acid Number (TAN) tests, or a combination of both techniques, to evaluate the oxidation levels present within a lubricant sample4. In general, infrared spectroscopy (IR) is a commonly employed technique for determining both TAN and Total Base Number (TBN) values in various analytical laboratories.
References and Further Reading
- Rescorla, A. R., Carnahan, F. L., & Fenske, M. R. (1937). Development of Acidity in Certain Lubricating Oils on Use or Oxidation. Industrial & Engineering Chemistry Analytical Edition 9(12), 574-576. DOI: 10.1021/ac50116a011.
- “A Comprehensive Look at the Acid Number Test” – Machinery Lubrication
- “Measuring Reserve Alkalinity” – Machinery Lubrication
- “Understanding Total Acid Number (TAN)” – ExxonMobil