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Fats are the mixture of triglycerides that are made up of three fatty acids that is linked to glycerol by fatty acyl esters. Fatty acids are long chain hydrocarbons along with carboxyl groups. Fatty acids are divided into saturated and unsaturated depending on double bonds present in the fatty acid. In saturated fatty acid there is only one bond between carbon atoms and remains in solid state at room temperature. In unsaturated fatty acids there are two bonds between carbon atoms along with the single bond. Unsaturated fatty acids through hydrogenation is converted into saturated fatty acids. Based on degree of unsaturation fatty acids is combined with oxygen to form saturated fatty acids. Therefore, it is vital to know the extent to which the fatty acid is unsaturated. There are several methods for knowing the fatty acids unsaturation level and among them is calculating the iodine value of fats and is the number grams of iodine that is consumed by 100 g of fat. High the iodine value high is unsaturation. So, the basic objective of this test is to determine the iodine number of fat.1
Principle: Fatty acids react with halogen that is iodine that results in addition of halogen at C=C double bond site. In this reaction, iodine monochloride and unsaturated bonds react to produce a di-halogenated single bond, in which one carbon has bound one atom of iodine. When the reaction is over, the iodine amount that has reacted is estimated by adding a solution of potassium iodide to the reaction product. This make the unreacted ICl to make molecular iodine. Then the iodine that is liberated s titrated with standard solution of 0.1N sodium thiosulphate. Starch is used as indictor so that the liberated iodine reacts with starch to create purple colour product and then the endpoint can be observed.
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Chemicals: Iodine monocloride
0.1N Sodium thiosulphate standardized.
1% starch indicator solution.
Fat sample in chloroform
Others: Iodination flask
Burette and burette stand with magnetic stirrer
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At first arrange all the reagent solutions that you have prepared and all the requirements of the experiment in the table. Then take an iodination flask and label it as “TEST”. With the help of pipette take out 10 ml of fat sample dissolved in chloroform. In the flask as about 20 ml of iodine monocloride reagent. Now mix everything finely. Allow the flask to stand for about half an hour for incubation. In another iodination flask set up BLANK by addition of 10 ml chloroform in the flask. To the blank add 20 ml of iodine monocloride reagent and mix it properly. Keep the Blank for incubation for about 30 min. Now bring out the TEST flask from incubator if 30 min is over and add 10 ml of potassium iodide solution. Wash the stopper and the flask side by using 50 ml of distilled water. Now put the “Test” for titration against standardized sodium thiosulphate solution till pale straw colour is seen.1 Then add 1ml starch indicator to the flask where a purple colour is seen.
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The iodine number is measurement of the degree of unsaturation in an oil. It remains constant for a oil. It is an important parameter for knowing the oxidative rancidity of oils and high the unsaturation high is possibility that the oil will go rancid.
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- William Horowitz (ed). Official Methods of Analysis of AOAC Association of Official Analytical Chemists Washington (12th Ed) 11975: 488.