Acid Value of Oil/Fat
The acid value of oils is defined as the number of milligrams of Potassium hydroxide required to neutralize the free fatty acids present in one gram of fat. Acid value of oils is a relative measure of rancidity as free fatty acids are normally formed during decomposition of triglycerides.
Acid value of oils is also expressed as per cent of free fatty acids calculated as oleic acid, lauric, ricinoleic and palmitic acids. Acid Value is a measure of the amount of fatty acids, which have been liberated by hydrolysis from the glycerides due to the action of moisture, temperature and/or lipolytic enzyme lipase.
Acid Value Principle
The acid value of oils is determined by directly titrating the oil/fat in an alcoholic medium against standard potassium hydroxide/sodium hydroxide solution.
1. 250 mL conical flasks
1. Ethyl alcohol (C2H5OH): Ninety five percent alcohol or rectified spirit neutral to phenolphthalein indicator.
2. Phenolphthalein indicator: Dissolve one gram of phenolphthalein in 100 mL of ethyl alcohol.
3. Aqueous KOH 0.5 N: The solution should be colourless and stored in a brown glass bottle. For refined oils, the strength of the alkali should be fixed to 0.1 N.
Acid Value Procedure
1. Weigh accurately appropriate amount of the cooled oil sample in a 250 mL conical flask and add 50 mL to 100 mL of freshly neutralised hot ethyl alcohol and about one ml of phenolphthalein indicator solution.
2. Boil the mixture for about five minutes and titrate while hot against standard alkali solution shaking vigorously during the titration.
3. The weight of the oil/fat taken for the estimation and the strength of the alkali used for titration shall be such that the volume of alkali required for the titration does not exceed 10 mL.
4. Mix the oil or melted fat thoroughly before weighing. The mass of the test sample shall be taken based on the colour and expected acid value.
Acid Value Calculation
Acid Number Formula = VxN/W
V = Volume in mL of standard potassium hydroxide or sodium hydroxide used
N = Normality of the potassium hydroxide solution or Sodium hydroxide solution
W = Weight in gm of the sample