Gauging Acidity: Acid Number
Acid Number (AN) is an indicator of oil serviceability. It is useful in monitoring acid build-up in oils due to the depletion of antioxidants. Oil oxidation causes acidic byproducts to form. High acid levels can indicate excessive oil oxidation or depletion of the oil additives and can lead to corrosion of the internal components. By monitoring the acid level, the oil can be changed before any damage occurs.
An oil analyst is looking for a sudden increase. When your oil is flagged for high acid levels, it indicates accelerated oil oxidation, and you should change the oil as soon as possible. If any of the remaining highly acidic oil is left, it will quickly deplete the antioxidants in the new oil.
AN is measured by titration using ASTM D-664 or D-974. Both methods involve diluting the oil sample and adding incremental amounts of an alkaline solution until a neutral endpoint is achieved.
The AN of new oil will vary based on the base oil additive package. An R&O oil will usually have a very low AN, around 0.03. An AW or EP oil will have a slightly higher value, typically around 0.5. Engine oils commonly have a higher AN, in the neighborhood of 1.5.
