Iodine value

In fatty acids, unsaturation occurs mainly as double bonds which are very reactive towards halogens, the iodine in this case.

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 colorless and so is the organoiodine product (RCHI−CHIR’).

He used iodine alcoholic solution in presence of mercuric chloride (HgCl2) and carbon tetrachloride (CCl4) as fat solubilizer.

[Note 1] The residual iodine is titrated against sodium thiosulfate solution with starch used as endpoint indicator.

Afterward, the liberated I2 is titrated against sodium thiosulfate, in presence of starch, to indirectly determine the concentration of the reacted iodine.

One of the major limitations of is that halogens does not react stoichiometrically with conjugated double bonds (particularly abundant in some drying oils).

[8] For educational purposes, Simurdiak et al. (2016)[3] suggested the use of pyridinium tribromide as bromination reagent which is more safer in chemistry class and reduces drastically the reaction time.

It has been observed that Wijs/ Hanuš method gives erratic values of IV for some sterols (i.e. cholesterol) and other unsaturated components of insaponifible fraction.

[9] The original method uses pyridine dibromide sulfate solution as halogenating agent and an incubation time of 5 min.

[10] 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.

[16] 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.