Total Acid Number (TAN):
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.
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.
The TAN value itself can not 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 have 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.
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. The system can also be used for TBN (Total base number) analysis
TAN methods automated by titration system include the following:
ASTM D664 – Acid Number of Petroleum Products by Potentiometric Titration
ASTM D974 – Acid and Base Number by Color Indicator Titration
ASTM D3339 – Acid Number of Petroleum Products by Semi-Micro Color Indicator Titration
The full range of titration analysers are available from Camlab in the UK including analysers for Total acid number and Total Base Number