Determination of Iodine Value (Wijs) in vegetable oil

 Definition of Iodine Value (Wijs) :

 The number of grams of iodine, absorbed per 100 grams of the oil or fat, when determined by using Wijs solution

Principle: 

The sample is dissolved in a non- polar solvent like carbon tetra chloride and after adding with a known excess of iodine monochloride solution in glacial acetic acid  (Wijs solution), it is kept in dark for specified time. Potassium Iodide is then added to it and liberated Iodine is titrated against Standard Sodium thiosulphate solution using starch as indicator.

Apparatus: 

a. Iodine value flask- cap. 500 ml with stopper, 

b. Pipettes, 

c. Burette, 

d. Measuring cylinders

Reagents:

a. WIJ’S solution (Solution of Iodine monochloride in glacial acetic acid): Iodine Mono chloride (ICI) -98  percent  chemically  pure.  Iodine mono chloride Solution-is prepared by dissolving10 ml of Iodine mono chloride in about 1800ml of glacial acetic acid (chemically pure) and shake vigorously. Pipette 5ml of this, add 10 ml of potassium iodide solution and  titrate  with  0·lN standard sodium thiosulphate solution, using starch solution as indicator. Adjust the volume of the solution till it is approximately 0·2N.

b. 10 % KI solution: Prepare a fresh solution by dissolving 10g of potassium iodide free from potassium iodate, in 90 ml of water

c. Starch solution: 5 g of starch with 30ml of cold water and slowly pour it with stirring in to one liter of boiling water. Boil for three minutes. Allow to cool and decant off the supernatant clear liquid.

d. Chloroform or CTC 

e. Std. Sodium Thiosulphate solution: approximately 0·l N.

Dissolve approximately 24.8 g of sodium thiosulphate crystals (Na2S203.5H20) in water which has been well boiled to free it from carbon dioxide and make up to 1000 ml. Store the solution in a cool place in a dark coloured stock bottle. Standardize against  0.1N  Potassium dichromate  solution. Calculate  the normality.

0.1N Potassium Dichromate : Weigh accurately 1.2259g  dried ( at 120°c for 1hr. ) potassium dichromate in beaker. Dissolve in distilled water and transfer into 250ml vol.flask and  make  the  volume  upto mark with distilled water.

Procedure:-

Weigh accurately quantity in g (as per table 1 mentioned below) of the sample into a clean and dry iodine flask. Add 25 ml of Chloroform solution or CTC  to it and agitate to dissolve the content. Add 25 ml of Wij’s solution to it and replace the glass stopper after wetting with 10% KI and swirl for mixing. Allow it to stand in dark for one hour. Carry out the blank test simultaneously under similar condition. After standing for one hour, add 15 ml 10% KI and 100 ml distilled water, rinsing in the stopper and titrate the liberated iodine against the 0.1N Std. Sodium thiosulphate solution, with continuous swirling the flask until the colour of  the  contents  is straw yellow. Add 1 ml of freshly prepared starch indicator and continue the titration until the disappearance of the blue colour after thorough shaking with the stopper on.

Calculation:-

Iodine Value= 12.69 x ( B- S ) x N/W

Where,

B= Volume in ml of std. sodium sulphate solution required for blank 

S= Volume in ml of std. sodium sulphate solution required for sample

 N= Normality of std. sodium thiosulphate solution

W= Weight in grams of the sample taken for the test