Define Principle for Iodine Number Estimation - Lipids?

Iodine number is defined as the number of grams of iodine absorbed by 100 grams of fat, it gives an idea of the relative degree of un-saturation of fats or the amount of double bonds present in the fats. Iodine number is directly proportional to the degree of un-saturation.

In actual practice, Iodine monobromide (IBr) (Hanus method) or Iodine monochloride (ICI) (Wij's method) is used in preference to iodine solution, as these are more reactive and give more reliable and reproducible results. The method is based on the absorption of halogen by the unsaturated fatty acids present in fats. The method employs dissolution of fat in chloroform (CHCI₃) and treatment with a known excess of halogen solution. After allowing for absorption of the halogen, the excess halogen is liberated as I, and is back titrated against standard sodium thiosulphate solution using starch as indicator. By using an excess of sodium thiosulphite to react with each double bond in the fatty acid chains and measuring the amount used by back titrating, we can determine the amount of sodium thiosulphate used and thus the relative number of double bonds.