Iodine value

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. ^{[Note 1]} The residual iodine is titrated against sodium thiosulfate solution with starch used as endpoint indicator.[4] This method is now considered as obsolete.

J. J. A. Wijs modified the Hübl method by using iodine monochloride (ICl) in glacial acetic acid, which became known as Wijs's solution, dropping the HgCl₂ reagent.[4] Alternatively, J. Hanuš used iodine monobromide (IBr), which is more stable than ICl when protected from light. Typically, fat is dissolved in chloroform ^{[Note 2]} and treated with excess ICl/IBr. Some of the halogen reacts with the double bonds in the unsaturated fat while the rest remains.

${\displaystyle {\ce {R-CH=CH-R' + {\underset {(excess)}{ICl}}-> R-CH(I)-CH(Cl)-R' + {\underset {(remaining)}{ICl}}}}}$

Then, saturated solution of potassium iodide (KI) is added to this mixture, which reacts with remaining free ICl/IBr to form potassium chloride (KCl) and diiodide (I₂).

${\displaystyle {\ce {ICl + 2 KI -> KCl + KI + I2}}}$

Afterward, the liberated I₂ is titrated against sodium thiosulfate, in presence of starch, to indirectly determine the concentration of the reacted iodine.[5]

${\displaystyle {\ce {I2{}+ {\underset {(blue)}{starch}}{}+ 2 Na2S2O3 -> 2 NaI{}+ {\underset {(colorless)}{starch}}{}+ Na2S4O6}}}$

IV (g I/ 100 g) is calculated from the formula :

${\displaystyle {\textrm {IV}}={\frac {({\textrm {B}}-{\textrm {S}})\times {\textrm {N}}\times 12.69}{\textrm {W}}}}$

Where:

• (B – S) is the difference between the volumes, in mL, of sodium thiosulfate required for the blank and for the sample, respectively;
• N is the normality of sodium thiosulfate solution in Eq/ L;
• 12.69 is the conversion factor from mEq sodium thiosulfate to grams of iodine (the molecular weight of iodine is 126.9 g/mol);
• W is the weight of the sample in grams.

The determination of IV according to Wijs is the official method currently accepted by international standards such as DIN 53241-1:1995-05, AOCS Method Cd 1-25, EN 14111 and ISO 3961:2018. One of the major limitations of is that halogens does not react stoichiometrically with conjugated double bonds (particularly abundant in some drying oils). Therefore, Rosenmund-Kuhnhenn method makes more accurate measurement in this situation.[6]

Proposed by H. P. Kaufmann in 1935, it consists in the bromination of the double bonds using an excess of bromine and anhydrous sodium bromide dissolved in methanol. The reaction involves the formation of a bromonium intermediate as follows:[7]

Then the unused bromine is reduced to bromide with iodide (I⁻).

${\displaystyle {\ce {Br2 + 2 I- -> 2 Br- + I2}}}$

Now, the amount of iodine formed is determined by back-titration with sodium thiosulfate solution.

The reactions must be carried out in the dark, since the formation of bromine radicals is stimulated by light. This would lead to undesirable side reactions, and thus falsifying a result consumption of bromine.[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.

This method is suitable for the determination of iodine value in conjugated systems (ASTM D1541). 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.[3] Several non-wet methods have been proposed for determining the iodine value. For example, IV of pure fatty acids and acylglycerols can be theoretically calculated as follows:[11]

${\displaystyle {\text{IV}}={\frac {2\times 126.92\times {\text{no. of double bonds}}\times 100}{\text{molecular weight}}}}$

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 :[12]

${\displaystyle {\textrm {IV}}_{\textrm {mixture}}=\sum A_{f}\times {\textrm {IV}}_{f}}$

in which ${\displaystyle A_{f}}$ and ${\displaystyle {\text{IV}}_{f}}$ are, respectively, the amount (%) and the iodine value of each individual fatty acid f in the mixture.

For fats and oils, the IV of the mixture can be calculated from the fatty acid composition profile as determined by gas chromatography (AOAC Cd 1c-85; ISO 3961:2018). However this formula does not take into consideration the olefinic substances in the unsaponifiable fraction. Therefore, this method is not applicable for fish oils as they may contain appreciable amounts of squalene.[13]

IV can be also predicted from near-infrared, FTIR and Raman spectroscopy data using the ratio between the intensities of ν(C=C) and ν(CH₂) bands.[14][15] High resolution proton-NMR provides also fast and reasonably accurate estimation of this parameter.[16]