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Mass of iodine absorbed by 100 grams of a given substance

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 basic principle of iodine value was originally introduced in 1884 by A. V. Hübl as “Jodzahl”. He used iodine alcoholic solution in presence of mercuric chloride (|HgCl2) and carbon tetrachloride (|CCl4) as fat solubilizer. The residual iodine is titrated against sodium thiosulfate solution with starch used as endpoint indicator. This method is now considered as obsolete.

Measurement of iodine value with the official method is time consuming (incubation time of 30 min with Wijs solution) and uses hazardous reagents and solvents. Accordingly, the IVs of oleic, linoleic, and linolenic acids are respectively 90, 181, and 273. Therefore, the IV of the mixture can be approximated by the following equation :

IV can be also predicted from near infrared, FTIR and Raman spectroscopy data using the ratio between the intensities of |ν(C\dC) and |ν(CH2) bands. High resolution proton NMR provides also fast and reasonably accurate estimation of this parameter.

Although modern analytical methods (such as GC) provides more detailed molecular information including unsaturation degree, the iodine value still widely considered as an important quality parameter for oils and fats. Moreover, IV generally indicates oxidative stability of the fats which directly depend on unsaturation amount. Such a parameter have a direct impact on the processing, the shelf life and the suitable applications for fat based products. It is also of a crucial interest for lubricants and fuel industries. In biodiesel specifications, the required limit for IV is 120 g I2/100 g, according to standard EN 14214. IV is extensively used to monitor the industrial processes of hydrogenation and frying. However it must be completed by additional analyses as it does not differentiate cis/trans isomers. G. Knothe (2002) criticized the use of IV as oxidative stability specification for fats esterification products. He noticed that not only the number but the position of double bonds is involved in oxidation susceptibility. For instance, linolenic acid with two bis allylic positions (at the carbons no. 11 and 14 between the double bonds Δ9, Δ12 and Δ15) is more prone to autoxidation than linoleic acid exhibiting one bis allylic position (at C 11 between Δ9 and Δ12). Therefore, Knothe introduced alternative indices termed allylic position and bis allylic position equivalents (APE and BAPE), which can be calculated directly from the integration resultas of chromatographic analysis.