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The total base number (TBN) plays an important role in the analysis of petroleum products, especially for engine oils and lubricating oils. These oils contain basic additives that protect them from oxidative degradation by neutralizing acidic components formed during combustion. Without these additives, strong acids can build up in the engine, causing metal wear. This can negatively affect engine operation and life. The TBN serves as an indicator of the quantity of reserve base additive present.

Determining the right time for an oil change is critical to maximize oil usability while protecting the engine. Premature oil changes, while protecting the engine, result in increased operating costs. Therefore, TBN and other parameters such as total acid number (TAN) are essential indicators in engine oil analysis to estimate the remaining lifetime of an oil.

Countless standard procedures in use around the world illustrate the importance of this analysis. These procedures may differ in the composition of the solvents and titrants used. Perchloric acid in glacial acetic acid is used as the titrant for fresh oil samples, while hydrochloric acid is used as the titrant for in-service measurements of used oils.

This Application Bulletin describes the analysis of the base number (BN) in petroleum products using perchloric acid as the titrant. To determine the BN, the oil is dissolved in a solvent mixture. The specific solvent mixture depends on the standard procedure. The solution is then titrated with perchloric acid and the result is expressed as milligrams of potassium hydroxide per gram of sample (mg KOH/g).  

There are several methods available to detect the endpoint of the base number titration. The most commonly used method is potentiometric determination, as described in ASTM D2896 or ISO 3771. Here, a pH electrode signals the endpoint. The color of the sample does not affect this method. It is suitable for petroleum products.

ASTM D974 and ISO 6618 both describe the photometric titration which uses an optical sensor for the indication. This test method is well suited for light-colored petroleum products and lubricants.

The third endpoint detection method is thermometric titration. As the titrant and analyte react, the temperature changes. A difference in the rate of temperature change indicates the endpoint. This method is appropriate for all kinds of petroleum products.

The final method is conductometric titration, as described in IP 400. This method uses a conductivity sensor to detect the endpoint. 

Download this Application Bulletin to learn more about the four base number titration methods. The document includes detailed instructions and troubleshooting tips for all four methods.

Watch our video to learn more the automated analysis of the total acid number and total base number:


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