The iodine value (or "iodine adsorption value" or "iodine number" or "iodine index") in chemistry 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 amount of unsaturation in fatty acids. This unsaturation is in the form of double bonds, which react with iodine compounds. The higher the iodine number, the more C=C bonds 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.
Table of iodine values
|Tung oil||163 – 173|
|Grape seed oil||124 – 143|
|Palm oil||44 – 51|
|Olive oil||80 – 88|
|Coconut oil||7 – 12|
|Palm kernel oil||16 – 19|
|Cocoa butter||35 – 40|
|Jojoba oil||80 ~80|
|Poppyseed oil||133 ~133|
|Cottonseed oil||100 – 117|
|Corn oil||109 – 133|
|Wheat germ oil||115 – 134|
|Sunflower oil||125 – 144|
|Linseed oil||136 – 178|
|Soybean oil||120 – 136|
|Peanut oil||84 – 105|
|Rice bran oil||95 – 108|
|Walnut oil||120 – 140|
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 (usually in this test C=C double bonds) that react with iodine effectively reduce the strength, or magnitude of the colour (by taking iodine out of solution). 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 (R and R' symbolize alkyl or other organic groups):
The precursor alkene (RCH=CHR') is colourless and so is the organoiodine product (RCHI-CHIR').
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 titration with sodium thiosulfate. 
Methods for the determination of iodine value
Introduced the iodine value was Hübl which titrated fats in the presence of mercuric chloride with iodine, but with the actual reagent (probably iodine chloride) is formed in situ from mercuric chloride and iodine. Pure iodine accumulates concerned not to alkenes, which is why the still valid definition of iodine is only a formal one.
Wijs iodine value after
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 suppressed 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.
Related methods of analysis
- Thomas, Alfred (2002). "Fats and Fatty Oils". Ullmann's Encyclopedia of Industrial Chemistry. Weinheim: Wiley-VCH. doi:10.1002/14356007.a10_173.
- Firestone D (May–Jun 1994). "Determination of the iodine value of oils and fats: summary of collaborative study". J AOAC Int. 77 (3): 674–6. PMID 8012219.
- Obtaining the Iodine Value of Various Oils via Bromination with Pyridinium Tribromide Michael Simurdiak, Olushola Olukoga, and Kirk Hedberg Journal of Chemical Education Article ASAP doi:10.1021/acs.jchemed.5b00283