Iodine value

 ( Analytical Chemistry ) 

In chemistry, the iodine value ( IV; also iodine absorption value, iodine number or iodine index) is the mass of iodine in that is consumed by 100 grams of a chemical substance. Iodine numbers are often used to determine the degree of unsaturation in , and . In , unsaturation occurs mainly as Double bond bonds which are very reactive towards , 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 .

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Principle The determination of iodine value is a particular example of iodometry. A solution of iodine is yellow/brown in color. When this is added to a solution to be tested, however, any chemical group (usually in this test double bonds) that react with iodine effectively reduce the strength, or magnitude of the color (by taking out of solution). Thus the amount of iodine required to make a solution retain the characteristic yellow/brown color can effectively be used to determine the amount of iodine sensitive groups present in the solution.

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

R-CH=CH-R' + I2 -> R-CH(I)-CH(I)-R'

The precursor alkene () is colorless and so is the organoiodine product ().

In a typical procedure, the fatty acid is treated with an excess of the Hanuš or Wijs solution, 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 back-titration with sodium thiosulfate () standard solution.

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Methods for the determination of iodine value

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Hübl method 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 () and carbon tetrachloride () 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.

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Wijs/Hanuš method 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 reagent. Alternatively, J. Hanuš used iodine monobromide (IBr), which is more stable than ICl when protected from light. Typically, fat is dissolved in chloroform and treated with excess ICl/IBr. Some of the halogen reacts with the double bonds in the unsaturated fat while the rest remains.

: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 ().

ICl + 2 KI -> KCl + KI + I2

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

(2025). 9789383746224, S. Chand Publishing. . ISBN 9789383746224

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

is calculated from the formula :
     

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

Where:

* is the difference between the volumes, in Litre, of sodium thiosulfate required for the blank and for the sample, respectively;

* 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 mass of iodine is );

* 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 ). Therefore, Rosenmund-Kuhnhenn method makes more accurate measurement in this situation.

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Kaufmann method 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 Halonium ion intermediate as follows: Then the unused bromine is reduced to bromide with iodide ().

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.

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

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Rosenmund-Kuhnhenn method 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 (i.e. cholesterol) and other unsaturated components of insaponifible fraction. The original method uses pyridine Bromine sulfate solution as halogenating agent and an incubation time of 5 min.

(2025). 9783642649554, Springer Science & Business Media. . ISBN 9783642649554

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Other methods 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. Several non-wet methods have been proposed for determining the iodine value. For example, IV of pure fatty acids and Glyceride can be theoretically calculated as follows:

(1994). 9781461569985, Springer US. . ISBN 9781461569985

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

Accordingly, the IVs of Oleic acid, Linoleic acid, and are respectively 90, 181, and 273. Therefore, the IV of the mixture can be approximated by the following equation :

$\backslash textrm\{IV\}\_\{\backslash textrm\{mixture\}\}=\; \backslash sum\; A\_f\; \backslash times\; \backslash textrm\{IV\}\_f$

in which and are, respectively, the amount (%) and the iodine value of each individual fatty acid 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 Alkene substances in the unsaponifiable fraction. Therefore, this method is not applicable for fish oils as they may contain appreciable amounts of squalene.

Frank Gunstone (2025). 9781444302431, John Wiley & Sons. . ISBN 9781444302431

IV can be also predicted from near-infrared, FTIR and Raman spectroscopy data using the ratio between the intensities of and bands. High resolution proton-NMR provides also fast and reasonably accurate estimation of this parameter.

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Significance and limitations 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 Autoxidation of the fats which directly depend on unsaturation amount. Such a parameter has 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-Allyl group 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-Allyl group 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.

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Iodine values of various oils and fats Iodine value helps to classify oils according to the degree of unsaturation into , having IV > 150 (i.e. Linseed oil, Tung oil), IV : 125 – 150 ( Soybean oil, Sunflower oil) and with IV < 125 (Canola oil, Olive oil, Coconut oil). The IV ranges of several common oils and fats is provided by the table below.

Beef tallow (2016). 9781483164663, Elsevier. . ISBN 9781483164663	– 48
Beeswax (2002). 9780203908815, CRC Press. . ISBN 9780203908815	– 16
Butter (2025). 9780122270550, Elsevier. ISBN 9780122270550	– 42
Canola oil (2009). 9781405147927, John Wiley & Sons. . ISBN 9781405147927	– 126
Castor oil (2025). 9781420009675, CRC Press. . ISBN 9781420009675	– 91
Dehydrated castor oil (DCO)Laura M. Orozco, Sandra Cardona, Claudia Lorena Gómez Herrera, Helen Inciarte: Evaluation of KHSO4 as a recyclable catalyst in the production of dehydrated castor oil to be applied in alkyd resins, Progress in Organic Coatings, December 2021, 161(1):106467, DOI:10.1016/j.porgcoat.2021.106467Azcan Nezihe, Demirel Elif, Yılmaz Özlem, Erciyes Ahmet Tunçer: Microwave Heating Application To Produce Dehydrated Castor Oil American Chemical Society	- 140
Cocoa butter	– 40
Coconut oil	– 11
Cod liver oil	– 183
Corn oil	– 128
Cottonseed oil	– 115
Fish oil	– 205
Grape seed oil (2025). 9783030303143, Springer International Publishing. . ISBN 9783030303143	– 157
Hazelnut oil	– 90
Jojoba oil (2009). 9788190723756, New India Publishing. . ISBN 9788190723756	– 85
Kapok seed oil	– 110
Lard	– 68
Linseed oilLinseed and Flaxseed are varieties of the same species. A wide range of iodine values are found.	– 204
Olive oil	– 94
Oiticica oil	– 185
Palm kernel oil	– 21
Palm oil	– 55
Peanut oil	– 107
Pecan oil (2025). 9780122270550, Elsevier. ISBN 9780122270550	– 106
Pistachio oil	– 98
Poppyseed oil	– 158
Rapeseed oil	– 120
Rice bran oil	– 108
Safflower oil (2008). 9781420061673, CRC Press. . ISBN 9781420061673	– 150
Sesame oil	– 120
Sunflower oil	– 145
Soybean oil	– 139
Tung oil	– 175
Walnut oil	– 162
Wheat germ oil	– 128

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Related methods of analysis

• Acid number
• Amine value
• Argentation chromatography
• Bromine number
• Epoxy value
• Hydroxyl value
• Peroxide value
• Saponification value

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Notes

The interaction between mercuric chloride and iodine chloride is supposed to produce the active agent of halogenation, the ICl as follows : HgCl₂ + I₂ → HgClI + ICl

Chloroform is replaced in modern protocols by less hazardous and more available solvents such as cyclohexane and 2,2,4-trimethylpentane (ASTM D5768).

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