One application of the iodine number is the determination of the amount of unsaturation contained in fatty acids. This unsaturation is in the form of double bonds which 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.
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 deionized water is added. This is then titrated against 0.1M sodium thiosulfate (VI) solution. 1cm3 of 0.1M sodium thiosulfate 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.