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Acid value (or neutralization number or acid number or acidity, abbreviated as AV or AN) is the mass of potassium hydroxide (KOH) in milligrams that is required to neutralize one gram of fats or oils. The Acid value is a measure of the amount of free (or non-esterified) fatty acids, which generally indicates the freshness and the degree of rancidity of a given fat or oil. AV is also considered as an indicator for oils edibility, or their suitability for use in soaps or paints manufacturing. According to FAO reccomandations, the AV of edible refined vegetable oils should be than to 0.6 (mgKOH/ g oil). AV for B100 biodiesel should to be lower than 0.5 (mgKOH/ g) in both EN 14214 and ASTM D6751 fuel standards.

Determination
In a typical procedure, a known amount of sample dissolved in neutralized 95% ethanol (or isopropanol) and titrated with a solution of potassium hydroxide (KOH), of known concentration, using phenolphthalein as a colour indicator. The acidity value is calculated according to the formula : Where :


 * 56.1 is the molar mass of KOH, in g · mol-1 ;
 * $N$ is the normality of KOH standard solution, in mol · L-1 ;
 * $V$ is the volume of KOH standard solution used to the endpoint, in mL ;
 * $m$ is the mass of the tested sample, in g.

The potentiometric method is recommended for heavily coloured oils and fats.

The acidity or the percentage of free fatty acids (% FFA) is calculated from the results obtained from the previous methods by the formula :

With the same parameters of equation (1), $M$ is the molecular weight of the representative fatty acid adopted to express acidity value accordingly : lauric acid (200) for coconut, palm kernel and similar oils ; palmitic acid (256) for palm oil or oleic acid (282) for all other of oils.

There are several standrad methods for AV determination such as ISO 660, AOCS Cd 3d-63, ASTM D664.

Interpretation
The measure of fat acidity provides the amount of free fatty acids, produced by the hydrolysis of triglycerides, sequentially, into diglycerides, monoglycrides until total breakdown due to the action of specific enzymes (lipases) during fats/ oils storage or processing.