The acid number is a measurement of acidity that is determined by the amount of KOH in milligrams that is needed to neutralize the acids in one gram of oil. It is an important quality measurement of oil.
Useful application of acid number is to the crude oil refining process where the potential of corrosion problems exist. This number can be used to determine acidic species that pause corrosion. It is usually the naphthenic acids in the crude oil that cause corrosion problems.