Total Acid Number (TAN) is a measure of the quantity of acidic compounds present in a petrochemical sample. The TAN value is determined for both new and used oil samples. New oils are monitored to ensure the initial quality of the oil meets the specific TAN range for the sample type. Used oils are tested to determine when the acidity of the oil has increased to a level at which negative effects of the increased acid will be seen. The TAN value of used samples is monitored over time and it is the change in the TAN value of a sample, rather than the absolute TAN value, which is the result of most interest to those testing the oil. The standard way of determining the TAN value of a sample is by titration analysis, with the endpoint determined by either potentiometric or photometric titration
Many different types of acidic compounds can contribute to the increase the TAN value of an oil sample, with oxidation products from normal use the largest contributor in used oil. Organic and inorganic acids, phenols, esters, detergents and inhibitors are common additive components and can also contribute to the total TAN value of an oil sample.
Acid in the engine oil is a major driver of corrosion, oxidation, nitration and increased viscosity, all of which are downsides that affect performance. Ultimately, it’s a root cause of equipment harm—and where TAN measurements more clearly indicate the actual operating conditions within your engine environment, TBN is limited in that it indicates only how much additive remains to protect against those performance downsides. As the industry continues to evolve, this will continue to result in even lower TBN results in used oil analysis. Over time, it will become more and more important for the industry to find better and more specific ways to monitor oil and engine condition. Good performance in your engine oil isn’t about having a high-detergent TBN measurement alone: It’s about having the right balance of oxidative stability, soot dispersancy, deposit control, wear performance, detergency and TBN retention.
Ultimately, a more comprehensive mix of indicators is needed to provide a full picture of oil health and performance, including:
• Wear metals
The TAN value itself cannot be used to predict the corrosive nature of an oil, as the test only measures the amount of acid in a sample, not the specific quantities of different acidic compounds in the sample. Two samples might have the same TAN value, but one has high levels of corrosive acids while the other much lower levels of the same corrosive acids. An increase in viscosity, which can cause physical damage to metallic parts, and the formation of gums and resins, which can attach and cause parts to not wear correctly, are two other negative effects which can be attributed to an increased TAN value.