What are Total Acid Number (TAN) and Total Base Number (TBN)?
Total Acid Number (TAN):
Total Acid Number (TAN) is a value which indicates 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 level 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, 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. MANTECH has developed a system which can be used to automate the TAN titration and final endpoint detection of a sample, either potentiometric or photometric. The system can also be automated to perform the liquid handling of the solvents used to dissolve the oil sample prior to titration analysis and the used solvent and oil samples after the analysis.
TAN methods automated by the MANTECH 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
Total Base Number (TBN):
Total Base Number (TBN) is the measure of the quantity of basic components in a petrochemical sample. TBN is determined in both new and used oils and is a value of the oil’s ability to neutralize the acids produced in the oils during normal use. The higher the TBN value of an oil sample is initially, the more acidic components can be produced during the use of the oil.
Oils used for different purposes have different ranges of TBN values, but typically all types of oils are considered to have been completely consumed when the TBN value of the oil sample drops below 2mg KOH/g of sample. An ideal TBN range for all types of equipment is set by the equipment manufacturer and for optimal performance, frequent monitoring of the oil is used to ensure it stays in the acceptable range. Automotive oils have the lowest initial TBN values, with a range of 7-10 mg KOH/g sample. Contrarily, marine oils have very high initial TBN values, around 70mg KOH/g sample. The very high initial TBN values extend the amount of time allowed between changing the oil in the ships or submarines.
Two of the most common methods used to determine the TBN value of a sample are by potentiometric titration analysis. Although both methods use the same principle, each method uses a different mixture of solvents to dissolve the samples and different titrants for the analysis. This means that although both tests produce a TBN value, the results are not directly comparable. Thus it is recommended that the same method be used to monitor the samples over time to ensure consistent results are obtained.
MANTECH has an automated system which can be used to automate the TBN titration and endpoint detection. The system can also automate the liquid handling of solvents used to dissolve the oil samples prior to titration analysis and the used solvent and oil samples after the analysis.
TBN methods automated by the MANTECH titration system include the following:
ASTM D2896 – Base Number of Petroleum Products by Potentiometric Perchloric Acid Titration
ASTM D4739 – Base Number Determination by Potentiometric Titration