ASTM D2896 - 07a Standard Test Method for Base Number of Petroleum Products by Potentiometric Perchloric Acid Titration

Total base number (TBN) is a measure of a reserve alkalinity of a lubricant. The test is relevant to internal combustion engines due to the acidic byproducts of combustion generated when gasoline and diesel fuel are burned. These byproducts, including SOx, NOx, and others enter the crankcase via blow-by gases getting past the piston rings.

In addition to acids entering the engine crankcase from blowby, acids are normally generated in other areas of the engine due to heat, oxidation, and other chemical processes.

In an effort to counter the corrosive effect of acids on engine parts, constituents are added to the oil (oil additives) that have basic properties. As bases, they act to neutralize the acids in the engine. The additive commonly used is calcium sulfonate. Others, however, including magnesium sulfonate, phenates, and salicylates are also used.  In addition to contributing to the TBN of a lubricant, these additives are multifuctional in that they are also highly valued as dispersant additives.

Whereas the process of generating acids will continue for the life of the engine, the ability of engine oil to neutralize the acids is finite. This is one reason engine oil needs to be changed. The basic properties of engine oil are eventually overcome by the constant generation of acids and the oil must be changed before the lubricant loses its ability to neutralize the acids.

In simple terms, many describe the Total Base Number (TBN) of an engine oil to be something akin to a "Tums" used to neutralize the acids in ones stomach. And to this end, the higher the base number, the greater is the lubricants ability to neutralize acids. At the same time, it should be noted that like "Tums: too much of anything is not necessarily good. In fact, whereas an overtreat of calcium sulfonate may be beneficial in neutralizing the acidic by-product of combustion, too much can generate higher levels of ash which can be damaging to an engine.

In an effort to strike a balance, the total base number of a new oil is typically in the range of  7 to 10 for gas engines and 10 to 14 for diesel engines. When the TBN in a used oil drops below 3, it typically indicates the need for an oil change.

It should be noted that in addition to ASTM D2896, the TBN of a lubricant can be tested by the procedure detailed in ASTM D4739. There are subtle, yet important differences to consider when looking at TBN data from each. Technically, the primary differences between D4739 and D2896 are that whereas D4739 uses hydrochloric acid as the titrant, perchloric acid is used in D2896. In addition, they use different titration solvents.

Because hydrochloric acid is weaker than perchloric, D4739 is less effective than D2896 in titrating weak bases. This can result in what ASTM refers to as a “falsely exaggerated” or sometimes even “falsely understated” TBN’s for test method D4739. For these reasons, ASTM says “When the base number of the new oil is required as an expression of its manufactured quality, Test Method D 2896 is preferred, since it is known to titrate weak bases that this test method may or may not titrate reliably.” (See paragraph 5.3.1 in test method ASTM D4739.

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b- ASTM D2896

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