This particular analysis is an example of iodometry. A solution of iodine is yellow/brown in color. When this is added to a solution to be tested, however, any chemical group that react with iodine effectively reduce the strength, or magnitude of the colour. Thus the amount of iodine required to make a solution retain the characteristic yellow/brown colour can effectively be used to determine the amount of iodine sensitive groups present in the solution. The chemical reaction associated with this method of analysis involves formation of the diiodo alkane : The precursor alkene is colourless and so is the organoiodine product. In a typical procedure, the fatty acid is treated with an excess of the Hanuš or Wijs solution, which are, respectively, solutions of iodine monobromide and iodine monochloride in glacial acetic acid. Unreacted iodine monobromide is then allowed to react with potassium iodide, converting it to iodine, whose concentration can be determined by titration with sodium thiosulfate.
Methods for the determination of iodine value
Huebl's iodine
Introduced the iodine value was Hübl which titrated fats in the presence of mercuric chloride with iodine, but with the actual reagent is formed in situ from mercuric chloride and iodine. In huebl's method, fatty oil is dissolved in carbon tetrachloride and treated with exess iodine solution and then it is titrated against sodium thiosulfate using starch as indicator and thus the free iodine concentration is determined
Wijs iodine value after
Addition of iodine chloride and back-titration with sodium thiosulphate by DIN 53241-1:1995-05 In the Wijs method fat/oil is dissolved in chloroform and treated with excess iodine chloride and glacial Acetic acid solution. Then to this mixture potassium iodide is added which reacts with remaining free iodine chloride to form potassium chloride. Now this solution is titrated against sodium thiosulfate to determine the concentration of free iodine.
Iodine by H. P. Kaufmann
The fat is mixed with an excess of bromine. This bromine is added to the double bonds in the unsaturated fats. This reaction must be carried out in the dark, since the formation of bromine radicals is stimulated by light. This would lead to undesirable side reactions, and thus falsifying a result consumption of bromine. Then the unused bromine is reduced to bromide with iodide. Now, the amount of iodine formed is determined by titration with sodium thiosulfate solution.
