The most important analytical determination of an oil or fat is the measurement of its unsaturation. The most commonly confirmed parameter for measuring the degree of carbon to carbon unsaturation of fat oil or their derivatives is Iodine value.
The iodine value (or iodine adsorption value or iodine number or iodine index, commonly abbreviated as IV) in chemistry is the mass/amount of iodine in grams that is consumed by 100 grams of a fat or oil sample.
Fats and oils are actually called the mixture of triglycerides. Triglycerides are composed of three fatty acids which are linked to the glycerol by fatty acyl esters. These fatty acids in triglycerides are classified into saturated or unsaturated ,which is based on the number of double bonds present in the fatty acid.
Saturated fatty acids contain only a single bond between the carbon atoms and tend to be solids at room temperature. On the other hand double bonds are present in the Unsaturated fatty acids between the carbon atom in addition to the single bonds present in the fatty acid chain. They exist readily as liquids at room temperature.
By the process of hydrogenation the unsaturated fatty acids can be converted into saturated fat
- The fatty acids can easily undergo and combine with the O2 or Chlorine , iodine , fluorine to form the saturated fatty acids because the fatty acids are dependent upon the degree So to know the extent to which a fatty acid is unsaturated ,is of great importance.
Principle of Iodine Value Determination:
Iodine number of the fat sample is determined which is based on allowing the treatment or reaction of a familiar amount/weight of fat or oil sample with a familiar/known volume of standard solution of Iodine monochloride ICl and then from iodine released on addition of excess of potassium iodide KI the the amount of unused iodine monochloride is determined.The released iodine is titrated against 0.1 N sodium thiosulfate solution using starch as an indicator.
In the mechanism of chemical reaction of this experiment the Fatty acids undergo to react with a halogen [ iodine] resulting in the addition of the halogen at the C=C double bond site or position. In this reaction,di-halogenated single bonds (of which one carbon has bound an atom of iodine) are produced when iodine monochloride reacts with the unsaturated bonds.
After the completion of reaction, by adding a solution of potassium iodide to the reaction product the amount of iodine that has reacted is determined.
- ICl + KI  —————>   KCl + I2
This leads to the formation of the molecular iodine from the remaining unreacted ICl. The I2 which is released is then titrated with a standard solution of 0.1 N sodium thiosulfate.
- I2 + 2Na2S2O3 —————–>  2 NaI + Na2S2O4
If the value of iodine is Ranging between 0-70, it will be a fat and if the value is exceeding 70 ,then it is an oil. The liberated iodine will react with starch to give a purple coloured product and thus the endpoint can be observed because the starch is used as an indicator.
Requirements for Iodine Value Determination:
Material required:
- Stoppered bottles
- Burette (25ml)
Test compounds:
2% Solution of corn oil, olive oil, and butter in Chloroform(fat solvent)
Reagents required:
Wij’s solution: weigh 8.5 gram and 7.8g of iodine trichloride separately ,then mix them separately in 450 ml of acetic acid each. Mix both the solutions and make the volume of the one litre .
0.1 N Sodium thiosulphate: dissolve 28.42 g of Na2S2O3.5H2O in 1 L water. To check its normality, take 20 ml of 0.1N potassium dichromate, add 10 mL of 15% KI and then 5 ml of HCL. Dilute to 100ml with water and titrated with thiosulfate solution till the yellow colour appears. After that add a few drops of starch solution and continue the titration till the blue colour disappears.
Note the volume of thiosulfate solution and calculate its exact normality by N1V1 =N2V2 where N1is the normality, V1 is the volume of dichromate solution taken for titration and V2 is the volume of the sulfate solution used.
10% Potassium Iodide solution: dissolve 10 gram of KI crystals in water and make up the volume to 100 ml.
1% Starch indicator: take one gram of starch and dissolved in 100ml of, boil for a minute ,cool and centrifuge to get a clear solution.
Procedure:
- Take 10 ml of fat solutions in stoppered bottles and add 25 ml of Wij’s solution. Shake the solution thoroughly and allow it to stand in the dark for 1 hour.
- In the same way , a blank solution in which fat solution is replaced by chloroform is prepared , which will be a control for this experiment.
- After the reaction time of 1 hour in the dark , rinse the stopper and neck of the bottle with 50 ml of water and add 10 ml of potassium iodide solution.
- Now ,with standard sodium thiosulphate solution the liberated iodine is titrated till the content or material of the conical flask becomes pale yellow in colour.
- Add a few drops of starch solution and continue to titrate it further with sodium thiosulphate solution will the blue colour disappear.
Calculations:
The difference between the blank and the test reading gives the amount of 0.1 N Sodium thiosulphate required to react with an equivalent volume of Iodine. In 1 L of 0.1 N Iodine solution contains 12.7 g of Iodine. The Iodine number or value can thus be calculated as follows:
Volume of 0.1 N sodium thiosulphate used for blank. = x ml Volume of 0.1 sodium thiosulphate used for test sample = y ml
Iodine number= (x -y )×12.7g/1000 × 100 /
weight of sample in g
Results of Iodine Value Determination:
On performing the experiment , certain values will be obtained and then putting these values in the calculation equation given above ,the iodine value of the fat sample examined can be calculated.
The obtained and calculated Iodine value of a fat sample is ——- gl/100g.
Precautions:
- Iodine monochloride is a caustic agent . So the reagent must be handled with gloves.
- For getting better results in the experiment, keep in mind to perform the experiment in the absence of any time gap while the addition of reagents as the released iodine is prone to oxidation by light.
- The stoppered bottles must be shaken thoroughly ,while performing the titration to make sure that all the Iodine is expelled from the Chloroform layer.
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