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.[1] 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.

Principle

An example of triglyceride occurring in the saponifiable fraction of oils with a saturated fatty acid residue, a monounsaturated fatty acid residue and a triple unsaturated (polyunsaturated) fatty acid residue. The triple esterified glycerol (marked black) can be seen in the center of the structure. Such a triglyceride has a high iodine value (approx. 119). Below, the reaction product after the addition of four equivalents of iodine or bromine to the four C=C double bonds of the unsaturated fatty acid residues.

The determination of iodine value is a particular example of iodometry. A solution of iodine I2 is yellow/brown in color. When this is added to a solution to be tested, however, any chemical group (usually in this test −C=C− double bonds) that react with iodine effectively reduce the strength, or magnitude of the color (by taking I2 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):

The precursor alkene (RCH=CHR’) is colorless and so is the organoiodine product (RCHI−CHIR’).

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 I2, whose concentration can be determined by back-titration with sodium thiosulfate (Na2S2O3) standard solution.[2][3]

Methods for the determination of iodine value

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 (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.

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 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.

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).

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

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

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]

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 bromonium intermediate as follows:[7]

Bromination of unsaturated fats

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

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.

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 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]

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.[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]

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]

in which and 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]

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 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.[17]

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) [12] 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.

Iodine values of various oils and fats

Iodine value helps to classify oils according to the degree of unsaturation into drying oils, having IV > 150 (i.e. linseed, tung), semi-drying oils IV : 125 – 150 ( soybean, sunflower) and non-drying oils with IV < 125 (canola, olive, coconut). The IV ranges of several common oils and fats is provided by the table below.

Fat Iodine value (gI/ 100g)
Beef tallow[18] 42 – 48
Beeswax[19] 7 – 16
Butter[20] 25 – 42
Canola oil[21] 110 – 126
Castor oil[22] 81 – 91
Cocoa butter[22] 32 – 40
Coconut oil[22] 6 – 11
Cod liver oil[23] 148 – 183
Corn oil[22] 107 – 128
Cottonseed oil[22] 100 – 115
Fish oil[1] 190 – 205
Grape seed oil[22][24] 94 – 157
Hazelnut oil[24] 83 – 90
Jojoba oil[25] 80 – 85
Kapok seed oil[22] 86 – 110
Lard[18] 52 – 68
Linseed oil[24][22][26] 170 – 204
Olive oil[22] 75 – 94
Oiticica oil[24][27] 139 – 185
Palm kernel oil[24][22] 14 – 21
Palm oil[24] 49 – 55
Peanut oil[20] 82 – 107
Pecan oil[28] 77 – 106
Pistachio oil[24] 86 – 98
Poppyseed oil[29] 140 – 158
Rapeseed oil[21] 94 – 120
Rice bran oil[22] 99 – 108
Safflower oil[22][30] 135 – 150
Sesame oil[24] 100 – 120
Sunflower oil[22][30] 110 – 145
Soybean oil[30] 120 – 139
Tung oil[24] 160 – 175
Walnut oil[28] 132 – 162
Wheat germ oil[24] 115 – 128

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 [31]
^ Chloroform is replaced in modern protocols by less hazardous and more available solvents such as cyclohexane and 2,2,4-trimethylpentane (ASTM D5768).

References

  1. 1 2 Thomas A (2002). "Fats and Fatty Oils". Ullmann's Encyclopedia of Industrial Chemistry. Weinheim: Wiley-VCH. doi:10.1002/14356007.a10_173. ISBN 3527306730.
  2. ↑ Firestone D (May–Jun 1994). "Determination of the iodine value of oils and fats: summary of collaborative study". Journal of AOAC International. 77 (3): 674–6. doi:10.1093/jaoac/77.3.674. PMID 8012219.
  3. 1 2 3 Simurdiak M, Olukoga O, Hedberg K (February 2016). "Obtaining the iodine value of various oils via bromination with pyridinium tribromide". Journal of Chemical Education. 93 (2): 322–5. doi:10.1021/acs.jchemed.5b00283.
  4. 1 2 "Arthur von Hübl and the Iodine Value". lipidlibrary.aocs.org. Retrieved 2020-09-04.
  5. ↑ Das S, Dash HR (2014). Laboratory Manual for Biotechnology. S. Chand Publishing. p. 296. ISBN 978-93-83746-22-4.
  6. ↑ Panda H (2011). The Testing Manual of Paints, Varnishes and Resins. Asia Pacific Business Press Inc. ISBN 978-81-7833-141-6.
  7. ↑ Hilp M (2002). "Determination of iodine values according to Hanuš using 1,3-dibromo-5,5-dimethylhydantoin (DBH)". Journal of Pharmaceutical and Biomedical Analysis. 28 (1): 81–86. doi:10.1016/S0731-7085(01)00632-X. PMID 11861111.
  8. ↑ "Explaining the methane - bromine free radical substitution mechanism". www.chemguide.co.uk. Retrieved 2020-09-07.
  9. ↑ Copping AM (1928). "The iodine values of some sterols by Dam's method". The Biochemical Journal. 22 (4): 1142–4. doi:10.1042/bj0221142. PMC 1252234. PMID 16744111.
  10. ↑ Paech K, Tracey MV (2013). Modern Methods of Plant Analysis / Moderne Methoden der Pflanzenanalyse. Vol. 2. Springer Science & Business Media. p. 335. ISBN 978-3-642-64955-4.
  11. ↑ Pomeranz Y, ed. (1994). Food Analysis: Theory and Practice. Springer US. ISBN 978-1-4615-6998-5.
  12. 1 2 Knothe G (2002-09-01). "Structure indices in FA chemistry. How relevant is the iodine value?". Journal of the American Oil Chemists' Society. 79 (9): 847–854. doi:10.1007/s11746-002-0569-4. ISSN 1558-9331. S2CID 53055746.
  13. ↑ Gunstone F (2009). Oils and Fats in the Food Industry. John Wiley & Sons. ISBN 978-1-4443-0243-1.
  14. ↑ Dymińska L, Calik M, Albegar AM, Zając A, Kostyń K, Lorenc J, Hanuza J (2017-09-02). "Quantitative determination of the iodine values of unsaturated plant oils using infrared and Raman spectroscopy methods". International Journal of Food Properties. 20 (9): 2003–2015. doi:10.1080/10942912.2016.1230744. ISSN 1094-2912.
  15. ↑ Xu L, Zhu X, Yu X, Huyan Z, Wang X (2018). "Rapid and simultaneous determination of the iodine value and saponification number of edible oils by FTIR spectroscopy". European Journal of Lipid Science and Technology. 120 (4): 1700396. doi:10.1002/ejlt.201700396.
  16. ↑ Miyake Y, Yokomizo K, Matsuzaki N (1998-01-01). "Rapid determination of iodine value by 1H nuclear magnetic resonance spectroscopy". Journal of the American Oil Chemists' Society. 75 (1): 15–19. doi:10.1007/s11746-998-0003-1. S2CID 96914982.
  17. ↑ "Biodiesel Standards & Properties". dieselnet.com. Retrieved 2020-10-26.
  18. 1 2 Andersen AJ, Williams PN (4 July 2016). Margarine. Elsevier. pp. 30–. ISBN 978-1-4831-6466-3.
  19. ↑ Akoh CC, Min DB (17 April 2002). Food Lipids: Chemistry, Nutrition, and Biotechnology (Second ed.). CRC Press. ISBN 978-0-203-90881-5.
  20. 1 2 Sanders TH (2003). "Ground Nut oil". Encyclopedia of Food Sciences and Nutrition. Elsevier. pp. 2967–2974. doi:10.1016/b0-12-227055-x/01353-5. ISBN 978-0-12-227055-0.
  21. 1 2 Gunstone F (12 February 2009). Rapeseed and Canola Oil: Production, Processing, Properties and Uses. John Wiley & Sons. pp. 80–. ISBN 978-1-4051-4792-7.
  22. 1 2 3 4 5 6 7 8 9 10 11 12 13 Gunstone FD, Harwood JL (2007). The Lipids Handbook (3 ed.). CRC Press. p. 68. ISBN 978-1420009675.
  23. ↑ Holmes AD, Clough WZ, Owen RJ (1929). "The chemical and physical characteristics of cod oil". Oil & Fat Industries. Springer Science and Business Media LLC. 6 (10): 15–18. doi:10.1007/bf02645697. ISSN 0003-021X. S2CID 101771700.
  24. 1 2 3 4 5 6 7 8 9 10 Krist S (2020). Fats and Fatty Oils. Springer International Publishing. doi:10.1007/978-3-030-30314-3. ISBN 978-3-030-30314-3. S2CID 213140058.
  25. ↑ Nagaraj G (15 June 2009). Oilseeds: Properties, Processing, Products and Procedures. New India Publishing. pp. 284–. ISBN 978-81-907237-5-6.
  26. ↑ Linseed and Flaxseed are varieties of the same species. A wide range of iodine values are found. Zhang, Zhen-Shan; Wang, Li-Jun; Li, Dong (13 October 2011). "Characteristics of Flaxseed Oil from Two Different Flax Plants". International Journal of Food Properties. 14 (6): 1286–1296. doi:10.1080/10942911003650296. hdl:11511/30872. S2CID 98298810.
  27. ↑ "Oiticica oil". CAMEO. 2020-09-03. Retrieved 2020-09-03.
  28. 1 2 Prasad RB (2003). "Walnuts and Pecans". Encyclopedia of Food Sciences and Nutrition. Elsevier. pp. 6071–6079. doi:10.1016/b0-12-227055-x/01269-4. ISBN 978-0-12-227055-0.
  29. ↑ "Poppy seed oil". CAMEO. 2020-09-03. Retrieved 2020-09-03.
  30. 1 2 3 O'Brien RD (5 December 2008). Fats and Oils: Formulating and Processing for Applications (3 ed.). CRC Press. ISBN 978-1-4200-6167-3.
  31. ↑ Gill AH, Adams WO (1900-01-01). "On Hübl's iodine method for oil analysis". Journal of the American Chemical Society. 22 (1): 12–14. doi:10.1021/ja02039a003.
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