The iodine value (or "iodine adsorption value" or "iodine number") in chemistry is the mass of iodine in grams that is consumed by 100 grams of a chemical substance. An iodine solution is yellow/brown in color and any chemical group in the substance that reacts with iodine will make the color disappear at a precise concentration. The amount of iodine solution thus required to keep the solution yellow/brown is a measure of the amount of iodine sensitive reactive groups.
One application of the iodine number is the determination of saturation of fatty acids as double bonds in fatty acids also react with iodine compounds. The higher the iodine number, the more unsaturated fatty acid bonds are present in a fat.  In a typical procedure the acid is treated with an excess of the Hanus solution which is a solution of iodobromine (BrI). Unreacted iodobromine is reacted with potassium iodide which converts it to iodine. The iodine concentration is then determined by titration with sodium thiosulfate.
Standard methods for analysis are for example ASTM D1959-97 and DIN 53241.
For a simple analysis, 0.2 grams of the fat is mixed with 20cm3 Wij's solution and 10cm3 1,1,1-trichloroethane. It is then left in the dark for 30mins. Next, 15cm3 of 10% potassium iodide solution and 10cm3 of deionised water is added. This is then titrated against 0.1M sodium thiosulphate (VI) solution. 1cm3 of 0.1M sodium thiosulphate solution = 0.01269g of iodine. The difference between a control titration and the titration with the fat present multiplied by this factor gives the mass of iodine absorbed by the oil.
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