Iodine value.

Background

Iodine values serve as a common metric to assess the unsaturation level in substances like fats, oils, and waxes. The unsaturation in fatty acids primarily manifests as double bonds, which exhibit a high reactivity towards halogens, particularly iodine. Consequently, a higher iodine value indicates a greater number of unsaturations in the fat.

The iodine value is a measure that quantifies the amount of halogen, accounted as iodine, that 100 grams of a substance can absorb under specified conditions. The iodine value test is governed by the general chapters of various pharmacopoeias, including EP 2.5.4, JP 1.13-10, USP <401>, and CP <0713>.

Procedure

1. Preparation of the sample performed in parallel with a blank.
2. The sample is dissolved in chloroform (EP, USP) or cyclohexane (JP)
3. Addition (in excess) of iodine bromide (BrI) in acetic acid glacial (EP, USP and CP) or iodine monochloride (ICl) in acetic acid glacial (JP)
4. Keep in the dark for 30 min (or up to 1 hour in JP)
5. Add KI (in excess) and water.
6. Titrate with Sodium thiosulfate, using starch as indicator.

Science behind

As Iodine value is performed in fats and oils, an organic solvent is required to ensure solubility. It also helps to dissolve the iodine (I₂).

The reaction, via double bond with the BrI or ICl, is:

R₁CH = CHR₂ + BrI → R₁CHBr – CHR₂I 

R₁CH = CHR₂ + ICl → R₁CHCl – CHR₂I 

Storage in the dark allows the reaction to complete.

The remaining BrI or ICl reacts with the KI added to form diiodide (I₂) and potassium chloride or potassium bromide:

KI + BrI → KBr + I₂

KI + ICl → KCl + I₂

The excess of I₂ is titrated with sodium thiosulfate (Na₂SO₃) using starch as indicator. The I₂ oxidates the thiosulfate to tetrathionate (S₄O₆^2-):

I₂ + starch (blue) +2Na₂SO₃ → 2I^–+ starch (colourless) + S₄O₆^2-

(A recommendation from the CP is to start the titration without starch until the colour changes from dark brown (due to the I₂) to pale brown, when the concentration of I₂ is lower and easier to see the blue colour).

The iodine taken by the sample is calculated by subtracting the volume of thiosulfate required by the sample from the volume of thiosulfate required by the blank.

Sources

1. Iodine value – Wikipedia
2. Iodine value | Definition, Procedure, Units, Values, & Facts | Britannica
3. https://chemistnotes.com/biochemistry/iodine-value-definition-principle-procedure-formula-and-2-reliable-applications/