Testing the Reserve Alkalinity: Base Number
Base Number (BN) testing is very similar to AN testing, except that the properties are reversed. The sample is titrated with an acidic solution to measure the oil’s alkaline reserve. ASTM test methods D-2896 or D-4739 are most commonly used to measure BN. Measuring the BN can help ensure that the oil is able to protect the component from corrosion due to acid.
Many oils (especially motor oils) are fortified with alkaline additives to neutralize acids that are formed as a result of oil oxidation. In diesel engine applications, acid is formed in the combustion chamber when moisture combines with sulfur under pressure.
The BN of an oil is highest when the oil is new and decreases with use. Once again, condemning limits (limit where the oil is condemned for use and should be replaced) are based on the application. As a rule, the BN should not drop below 25 percent of its original value. BN values for new engine oils run from 4 to 30 depending on the application.
