Acid Value Test as Quality Indicator for Essential Oil
Essential oils are complex mixture. Many of the essential oils have active component(s) that can have medicinal benefit. It is not uncommon for essential oils to have 10 (mono-) or 15 (sesqui-) carbon atom that bind to an alcohol, aldehyde, acid, or ketone molecular group containing oxygen atom. In addition, they also may have one or more carbon bonds which is one of the characteristics of unsaturated fatty acids .
The double bond carbon is unstable, therefore some essential oils can easily oxidized when exposed to air and temperature . One of the examples is geraniol which can be found in the essential oil of geranium (Pelagornium graveolens)  or in the essential oil of citronella (Cymbopogon winterianus). On the other hand, the active compound of linalool contained in lavender (Lavandura angustifolia) essential oil is generally more stable . The oxidation reaction can cause unpleasant odor in the essential oil .
One way to determine the decrease in the quality of the essential oils caused by oxidation is by measuring the acid value. The acid value is used to calculate the amount of free fatty acids that are formed due to oxidation. The oxidation reaction is characterized by hydrolysis triglycerides by water in the air and the possibility of the presence of bacteria, as well as decomposition by high temperature and light . The higher the acid value in the essential oil means the oil has been oxidized, thus reducing its quality .
How to measure the acid value.
Acid value is defined as how much KOH in mg is required to neutralize 1 gram free fatty acids in sample (essential oil) .
Simple titration conducted by adding phenolphthalein as indicator and KOH solution (the concentration has been known before) to the essential oil sample. The addition of KOH is stopped when the solution turns to slightly purple. The volume of KOH is entered to the formula below to determine the acid value in the essential oil sample.
V = Titration volume of sample (mL)
W = Weight of fat in the volume of extract (g)
56.1 = Molecular weight of KOH
Nb = Concentration of KOH
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 Sharma, S. and Jain, V.K., 2015. Acid Value of Various Domestic Uses Oil. Research Journal of Science and Technology, 7(2), p.109.