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Acid value

In chemistry, acid value (or "neutralization number" or "acid number" or "acidity") is the mass of potassium hydroxide (KOH) in milligrams that is required to neutralize one gram of chemical substance. The acid number is a measure of the amount of carboxylic acid groups in a chemical compound such as a fatty acid. In a typical procedure, a known amount of sample dissolved in organic solvent is titrated with a solution of potassium hydroxide with known concentration and with phenolphthalein as a color indicator.

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.

$AN=(V_{eq}-b_{eq})N\frac{56.1}{W_{oil}}$

Veq is the amount of titrant (ml) consumed by the crude oil sample and 1ml spiking solution at the equivalent point, beq is the amount of titrant (ml) consumed by 1 ml spiking solution at the equivalent point, and 56.1 is the molecular weight of KOH.

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

$N=\frac{1000W_{KHP}}{204.23V_{eq}}$

In which WKHP is the amount (g) of KHP in 50 ml of KHP standard solution, Veq 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.

Acid number (mg KOH/g oil) for biodiesel is preferred to be lower than 3.

There are standard methods for determining the acid number, such as ASTM D 974 andDIN 51558 (for mineral oils, biodiesel).

As fats rancidify, triglycerides are converted into fatty acids and glycerol, causing an increase in acid number.