Acid Number

Method Potentiometric Autotitration or Manual Titration
Standards ASTM D

Acid Number by potentiometric auto-titration, or by manual colorimetric titration. It is defined as the mass of potassium hydroxide (KOH) in milligrams that is required to neutralize one gram of chemical substance, in our case, oil.


What is it used for?

Test can be performed by potentiometric auto-titration or by manual titration.


How does it work?

Through the addition of potassium hydroxide to a known quantity of sample.  The more hydroxide required, the more acidic the sample.


What do the results tell us?

Results can be a good indicator of oxidation of base stock, additive depletion and acidic contamination.


The acid number value reflects acid concentration, not the strength of the acid present. Elevated or rapidly increasing AN, especially when paired with an increased water presence may indicate the potential for severe component damage.


An elevated acid number, where water is not excessively high, can be remedied by oil replacement, oil top-up (by definition, reducing the proportion of acid present) or the addition of additives that chemically reduce the content of acidic compounds in the oil.


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