Estimation of oil iodine value
Objective:
To determine the iodine value of fats and oils and thus estimate the unsaturation of the
fats and oils.
Theory:
Fats and oils are a mixture of triglycerids. Triglycerides are made up of three fatty acids
linked to glycerol by fatty acyl esters. Fatty acids are long chain hydrocarbons with
carboxyl groups (COOH groups). These fatty acids can be classified into saturated or
unsaturated based on the number of double bonds present in the fatty acid. Saturated fatty
acids contain only single bond between the carbon atoms and are tend to be solids at
room temperature. Unsaturated fatty acids contain double bonds between the carbon atom
in addition to the single bonds present in the fatty acid chain. They are likely to exists as
liquids at room temperature. The double bonds present in the naturally occurring
unsaturated fats are in the Cis form. Trans fatty acids are associated with health problems
and cardiovascular diseases.
unsaturated fatty acids
Unsaturated fatty acids can be converted into saturated by the process of
hydrogenation. Depending upon the degree of unsaturation, the fatty acids can
combine with oxygen or halogens to form saturated fatty acids. So it is important to know
the extend to which a fatty acid is unsaturated. There are different methods for checking
the unsaturation level in fatty acids, one among them is by determining the iodine value
of fats. Iodine value or number is the number of grams of iodine consumed by 100g of
fat. A higher iodine value indicates a higher degree of unsaturation.
Principle:
Fatty acids react with a halogen [iodine] resulting in the addition of the halogen at the
C=C double bond site. In this reaction, iodine monochloride reacts with the unsaturated
bonds to produce a di-halogenated single bond, of which one carbon has bound an atom
of iodine.
After the reaction is complete, the amount of iodine that has reacted is determined by
adding a solution of potassium iodide to the reaction product.
ICl + KI ---------------> KCl + I2
This causes the remaining unreacted ICl to form molecular iodine. The liberated I2 is
then titrated with a standard solution of 0.1N sodium thiosulfate.
I2 + 2 Na2S2O3 -----------------> 2 NaI + Na2S2O4
Saturated fatty acids will not give the halogenation reaction. If the iodine number is
between 0-70, it will be a fat and if the value exceeds 70 it is an oil. Starch is used as the
indicator for this reaction so that the liberated iodine will react with starch to give purple
coloured product and thus the endpoint can be observed.
Procedure
Weigh out 1 gm of the oil into a 500 ml glass stoppered iodine flask. Dissolve it in 65 ml
CCl4 and then accurately add 25 ml of Wijs solution ( Iodine monochloride) with the help
of measuring jar. Stopper the flask and allow it to stand in dark for 2 hours. Add 20 ml of
KI solution, mix well and add 100 ml of water while shaking. Titrate the excess of iodine
with 0.1 N sodium thiosulphate solution using starch as indicator. Perform a blank
experiment simultaneously by repeating the entire procedure without using the oil. Note
the 0.1 N sodium thiosulphate solution required for the blank experiment also.
Wijs solution (Iodine monochloride)
Calculation
Volume of Sodium thiosulphate used = [Blank- Test] ml
Equivalent Weight of Iodine = 127
Normality of sodium thiosulphate ( Na2S203) = 0.1
