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The acid number (also known as the acidity, the acid value, and the neutralization number) is the mass of potassium hydroxide (KOH) in milligrams that is required to neutralize one gram of the substance. The acid number (AN) or the total acid number (TAN) of crude oil is a measure of the amount of carboxylic acid groups and other acidic species (such as phenols and naphthol derivatives) in crude oil and indicates the potential corrosion during refining.

The acid number is used to quantify the amount of acid present, for example in a sample of biodiesel. It is the quantity of base, expressed in milligrams of potassium hydroxide, that is required to neutralize the acidic constituents in 1 g of sample.

In this equation, V_eq is the amount of titrant (ml) consumed by the crude oil sample and 1 ml spiking solution at the equivalent point, b_eq is the amount of titrant (ml) consumed by 1 ml spiking solution at the equivalent point, and 56.1 is the molecular weight of potassium hydroxide.

The molarity concentration of titrant (N) is calculated as such:

In the equation, W_KHP is the amount (g) of KHP in 50 ml of KHP standard solution, V_eq is the amount of titrant (ml) consumed by 50 ml KHP standard solution at the equivalent point, and 204.23 is the molecular weight of KHP.

There are standard methods for determining the acid number, such as ASTM D974 and (for mineral oils, biodiesel), or specifically for biodiesel (ASTM D664). The acid number (mg KOH/g oil) for biodiesel should be lower than 0.50 mg KOH/g standard fuels.

See also: Acidity and Alkalinity, Neutralization Number.