first you have to determine acid value as such of oil. first you have to take approx. 2 gm sample + IPA + mix well+ add HpH indicator and titrate with 0.1 or 0.5 N KOH solution till end point. by using following formula you will get acid value:
V*N*56.1/ wt of oil taken. this will be acid value in mgm KOH/gm.
now you can convert acid value in % by using formula:
Acid value/56.1 * mol wt of oil/10. this will give result.
You can determine the Free Fatty Acid (FFA) Percentage in non-edible oils by using HPLC instrument which separates the components according to polarity.Area under the peak will give the percentage of the each component.
I agree with Rawat for calculation of FFA% from Acid Value AV, but the important step is the calculation of the Mwt of the non-edible oil or any unknown oil.
To calculate the Mwt of unknown oil, you need to measure Saponification Value SV, and follow the below equation:
Mwt= 56.1*1000*3/(SV-AV).
I have attached two references, where one of them talking about the SV and the other one about Mwt.
Try using a simple method called "Isopropyl Alchol Method".
You simply add 1ml of oil with 10ml of IP Alcohol and few drops of phenolphthalein indicator. Titrate this mixture against 0.1N KOH until pale pink color last longing for 10 sec appears. Note down that as your end point and That's your acid value.
Its very simple and more reliable. This method helps in giving you your oil's acid value directly and FFA through a simple relation.
You can determine the free fatty acid content of a vegetable oil by direct ly by using the Sodium metaperiodate method or by calculating from the Acid value
First, it was a mistake I made to recommend the sodium metaperiodate method. The actual method I use in my laboratory is the titration of the oil sample in 95% Ethanol using 0.1N or 0.25N KOH solution and 3 drops of phenolphthalein as indicator (AOCS Method Ca 5a - 40)