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Concept
Indice d'iode
Résumé
In chemistry, the iodine value (IV; also iodine absorption value, iodine number or iodine index) is the mass of iodine in grams that is consumed by 100 grams of a chemical substance. Iodine numbers are often used to determine the degree of unsaturation in fats, oils and waxes. In fatty acids, unsaturation occurs mainly as double bonds which are very reactive towards halogens, the iodine in this case. Thus, the higher the iodine value, the more unsaturations are present in the fat. It can be seen from the table that coconut oil is very saturated, which means it is good for making soap. On the other hand, linseed oil is highly unsaturated, which makes it a drying oil, well suited for making oil paints.
The determination of iodine value is a particular 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 (usually in this test double bonds) that react with iodine effectively reduce the strength, or magnitude of the color (by taking out of solution). Thus the amount of iodine required to make a solution retain the characteristic yellow/brown color 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 (R and R' symbolize alkyl or other organic groups):
R-CH=CH-R' + I2 -> R-CH(I)-CH(I)-R'
The precursor alkene () is colorless 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 (IBr) and iodine monochloride (ICl) in glacial acetic acid. Unreacted iodine monobromide (or monochloride) is then allowed to react with potassium iodide, converting it to iodine , whose concentration can be determined by back-titration with sodium thiosulfate () standard solution.
The basic principle of iodine value was originally introduced in 1884 by A. V. Hübl as “Jodzahl”.
Source officielle
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