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Acid Number VS Base Number

Acid number: 
Test for acid number measures the level of acidity present in the lubricant.
  1. The acid number is an indication of oil serviceability. It is useful in monitoring acid buildup in oil due to the depletion of antioxidants. Oil oxidation causes acidic byproducts to form.
  2. High acid levels can indicate excessive oil oxidation or depletion of the oil additives and can lead to corrosion of internal components.
  3. By monitoring the acid level, the oil can be changed before damage occurs.
Operating principle:
  1. A portion of the sample is diluted and then titrated with a basic solution to a certain endpoint. The AN result is an expression of how much titrant is needed to neutralize per amount of sample. A higher number indicates a higher acidity.
  2. Total acid number means KOH in mg required to neutralize total acid component in 1 gram of lubricating oil.
Significance: As antioxidants are consumed during operation, generated acids will not be neutralized and will start to increase in concentration. By measuring AN of lubricant, its health can be monitored.


Base number:
Test for Base number measures the reserve alkalinity in a lubricant.

  1. Base number testing is very similar to acid number testing, except that the properties are reversed. The sample is titrated with an acidic solution to measure the oil’s alkaline reserve.
  2. Measuring the oil BN can help ensure that the oil is able to protect the components from corrosion due to acid. Condemning limits are based on application, normally BN should not be allowed to fall below 25% of the original BN value

Significance: Lubricating oils have alkaline additives. This test measures the amount of reserve remaining and recommends its suitability for further usage. 

Base number is expressed in terms of the equivalent number of potassium hydroxide per gram of oil sample. (mgKOH/gm)


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