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 number of carboxylic acid groups in a chemical compound, such as a fatty acid, or in a mixture of compounds. In a typical procedure, a known amount of sample dissolved in an organic solvent (often isopropanol) and titrated with a solution of alcoholic potassium hydroxide (KOH) of known concentration using phenolphthalein as a colour indicator.
The acid number is used to quantify the acidity of a substance e.g. 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.
Veq is the volume of titrant (ml) consumed by the crude oil sample and 1 ml of spiking solution at the equivalent point, beq is the volume of titrant (ml) consumed by 1 ml of spiking solution at the equivalent point, and 56.1 g/mol is the molecular weight of KOH. WOil is the mass of the sample in grams.
The molar concentration of titrant (N) is calculated as such:
In which WKHP is the mass (g) of KHP in 50 ml of KHP standard solution, Veq is the volume of titrant (ml) consumed by 50 ml KHP standard solution at the equivalent point, and 204.23 g/mol is the molecular weight of KHP.
There are standard methods for determining the acid number, such as ASTM D 974 and DIN 51558 (for mineral oils, biodiesel), or specifically for biodiesel using the European Standard EN 14104 and ASTM D664 are both widely used worldwide. Acid number (mg KOH/g oil) for biodiesel should to be lower than 0.50 mg KOH/g in both EN 14214 and ASTM D6751 standard fuels. This is since the FFA produced may corrode automotive parts and these limits protect vehicle engines and fuel tanks.
As oil-fats rancidify, triglycerides are converted into fatty acids and glycerol, causing an increase in acid number. A similar observation is observed with biodiesel aging through analogous oxidation processes and when subjected to prolonged high temperatures (ester thermolysis) or through exposure to acids or bases (acid/base ester hydrolysis). Low Acid value indicates good cleansing by soap.