The acid value (AV) has been specified as a basic feature or parameter in the specification of fats and oils.
The Acid value has been defined as the weight or amount of KOH in mg required to neutralize the organic acids present in 1g of fat and it can also be said as a measure of the free fatty acids (FFA)present in the fat or oil.
This test has analytical significance because the acid value is a measure of the amount of fatty acids, which have been released by undergoing hydrolysis from the glycerides which happens because of moisture, temperature and lipolytic enzyme lipase.
The source of the enzyme lipase could be the tissue from which the oil or fat was obtained or extracted or it can be a contaminant from other cells including microorganisms.
Besides FFA, hydrolysis of triglycerides produces glycerol.Free fatty acids (FFA) in oils and fats provides a quality wise feature for these fats. The high levels of FFA in fats are more prone to oxidative deterioration, they become rancid more hastily.
By using the potentiometric titration in Ethanol / Diethyl ether as solvent with KOH in Isopropyl alcohol as a tirant , the FFA in Oils and fats are determined.
This titration method is appropriate for edible fats and oils like butter, olive oil, palm oil or sunflower oil.
A titration method is defined as a technique which is volumetric, in this a solution of one reactant which is the titrant is poured into a solution of a second reactant which is the analyte, until the point of equivalence is achieved. In the context of definition of equivalence point it is the the point at which the titrant has been added in the accurately right quantity/amount to react stoichiometrically with the analyte.
Objective:
It is used to determine the Acid value or the amount of free fatty acids present in the given oil or fat Sample.
Principle:
Different fat samples may contain varying amounts of fatty acid. In addition to this the fat often might become rancid during storage and this rancidity is caused by chemical enzymatic hydrolysis of fats into free fatty acids that can be determined volumetrically by titrating the sample with ‘KOH’. The acidity of fats and oil is expressed as its acid value or number which is defined as an mg of KOH required to neutralize the free fatty acid present in 1mg of fats and oils. The calculated quantity or amount of FFA present or the acid value of fat acts as a parameter which indicates the age and extent of its deterioration.
The determination and and calculation of acid value is processed by directly titrating the oil/fat sample in an alcoholic medium
against standard titrant – potassium hydroxide/sodium hydroxide solution. The calculated value of % FFA is dependent on the type of titrated sample and the fatty acid to which is being examined.
Requirements for Acid Value of Fats and Oils:
Material and Reagents required –
Apparatus:
- Burette
- Burette or clamp stand
- Conical flask
Reagents:
a) Phenolphthalein indicator solution: Weigh one gram of phenolphthalein and dissolve it in 100mL of 95% Ethanol.
b) 0.1N potassium hydroxide titrant: Weigh accurately 5.6 g of potassium hydroxide and place it in a 1000-mL volumetric flask. Makeup to the mark with water ,that is dissolve the KOH in distilled water and make up the volume to 1 L.
This solution is Standardised by titrating it with a known volume of 0.1 N oxalic acid (prepared by taking 630 mg oxalic acid in 100 ml of distilled water.) Add two drops of phenolphthalein indicator. Using phenolphthalein as an indicator still a permanent pink colour appears. Calculate the actual normality( N2 )of potassium hydroxide solution (KOH) from the equation: N1V1=N2V2
Where N1 and V1 are the normality and volume of oxalic acid taken ,and V2 is the volume of KOH Solution used.
c) Ethanol-ether solution – it is the fat solvent . It is prepared by mixing 95% ethanol with diethyl ether in this volume to volume ratio(1:1, v/v). After mixing these , the Solution is Neutralized with potassium hydroxide KOH titrant and a phenolphthalein indicator is added about 1.0 mL ,adding this indicator until a pink colouration is observed. Always prepare the solution fresh.
d) Test samples: Olive oil,butter,margarine etc.
Procedure:
- Take 10g of fat sample and place it in a 250-mL conical flask.
- Then add 50 mL of ethanol-ether solution i.e. fat solvent, Shake it well. If required, the mixture is gently refluxed until the taken fat sample is completely dissolved.
- Titrate the solution with 0.1N KOH titrant until pink colouration can be observed which persists for 20-30 s.
- Measure the volume of potassium hydroxide KOH titrant used in titration.
Note: always note down the reading shown in burette after “dark pink colour OR Orangish red” has been obtained, as the end point in the titration should always be avoided because it will lead to erroneous result.
- Repeat the steps 1 to 4 with a blank (ethanol ether solution) which does not contain any fat sample.
- In this determination of acid value the weight of the oil/fat sample used and the volume/strength of the alkali (titrant) used for titration shall be about that the volume of alkali (titrant) needed for the titration doesn’t exceed the volume 10 mL.
Calculation of Acid Value of Fats and Oils:
The process to calculate the acid value according to the following obtained information and equation:
| 0.1N | KOH Solution which is used for titrating the blank | = x ml |
| 0.1N | KOH Solution which is used for titrating the sample | =y ml |
Titre value for sample ( V)= (y-x) ml
Acid value (mg KOH /g fat ) = 56.1 ×V ×N/ W
Where, V = Volume in mL of standard potassium hydroxide used or titre value for sample N = Normality of the titrant used potassium hydroxide solution ; and W = Weight in gm of the fat sample being examined.
1 ml of 0.1N KOH Contains 56.1mg of KOH. Thus ,to obtain the weight of potassium hydroxide from the volume of 0.1N KOH solution used during this titration a factor of 56.1 is incorporated in the numerator of the equation.
Result of Acid Value of Fats and Oils:
On putting the special values obtained in the calculation equation ,one can calculate the Acid value for a given fat Sample.
The calculated Acid value for the given fat sample is as ——-mg KOH / g fat.
Refernces –
