Iodine value

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.[1] 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

Fat Iodine number[1]
Tung oil Template:Nts – 173
Grape seed oil Template:Nts – 143
Palm oil Template:Nts – 51
Olive oil Template:Nts – 88
Coconut oil Template:Nts – 10
Palm kernel oil Template:Nts – 19
Cocoa butter Template:Nts – 40
Jojoba oil Template:Nts ~80
Poppyseed oil Template:Nts ~133
Cottonseed oil Template:Nts – 117
Corn oil Template:Nts – 133
Wheat germ oil[2] Template:Nts – 134
Sunflower oil Template:Nts – 144
Linseed oil Template:Nts – 178
Soybean oil Template:Nts – 136
Peanut oil Template:Nts – 105
Rice bran oil Template:Nts – 108

Methodology

This particular analysis is an example of iodometry. A solution of iodine ions is yellow/brown in color. When added to a complex solution however, any chemical group (usually C=C double bonds) in solution that react with iodine effectively reduce the strength, or magnitude of the colour (by taking iodine ions 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.

In a typical procedure, the fatty acid is treated with an excess of the Hanuš or Wijs solutions, 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.[3]

The chemical reaction associated with this method of analysis involves formation of the diiodo alkane (R and R' symbolize alkyl or other organic groups):

${\displaystyle {\mathrm {R{-}CH{=}CH{-}R'+I_{2}\longrightarrow R{-}CHI{-}CHI{-}R'} }}$

The precursor alkene (RCH=CHR') is colourless and so is the organoidine product (RCHI-CHIR').

References

1. {{#invoke:citation/CS1|citation |CitationClass=encyclopaedia }}
2. http://online.personalcarecouncil.org/ctfa-static/online/lists/cir-pdfs/pr248.pdf
3. {{#invoke:Citation/CS1|citation |CitationClass=journal }}