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Iodine value

From Wikipedia, the free encyclopedia

The iodine value (or "iodine adsorption value" or "iodine number" or "iodine index") in chemistry 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 amount of unsaturation in fatty acids. This unsaturation is in the form of double bonds, which react with iodine compounds. The higher the iodine number, the more C=C bonds 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 adrying oil, well suited for making oil paints.

Table of iodine values[edit]

Fat	Iodine number ^[1]
Tung oil	163 – 173
Grape Seed oil	124 – 143
Palm oil	44 – 51
Olive oil	80 – 88
Coconut oil	7 – 10
Palm Kernel oil	16 – 19
Cocoa butter	35 – 40
Jojoba oil	~80
Cottonseed oil	100 – 117
Corn oil	109 – 133
Wheat Germ Oil^[2]	115 – 134
Sunflower oil	125 – 144
Linseed oil	136 – 178
Soybean oil	120 – 136
Peanut oil	84 – 105
Ricebran oil	99 – 108

Methodology[edit]

This particular analysis is an example of iodometry. A solution of iodine ions is yellow/brown in color. When added to a complex solution however, any chemical group (usually C=C double bonds) in solution that react with iodine effectively reduce the strength, or magnitude of the colour (by taking iodine ions out of solution). Thus the amount of iodine required to make a solution retain the characteristic yellow/brown colour can effectively be used to determine the amount of iodine sensitive groups present in the solution.

In a typical procedure, the fatty acid is treated with an excess of the Hanuš or Wijs solutions, 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 titration with sodium thiosulfate.^[3]

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 colourless and so is the organoidine product (RCHI-CHIR').

Objective:

To determine the iodine value of fats and oils and thus estimate the unsaturation of the fats and oils.

Theory:

Fats and oils are a mixture of triglycerids. Triglycerides are made up of three fatty acids linked to glycerol by fatty acyl esters. Fatty acids are long chain hydrocarbons with carboxyl groups (COOH groups). These fatty acids can be classified into saturated or unsaturated based on the number of double bonds present in the fatty acid. Saturated fatty acids contain only single bond between the carbon atoms and are tend to be solids at room temperature. Unsaturated fatty acids contain double bonds between the carbon atom in addition to the single bonds present in the fatty acid chain. They are likely to exists as liquids at room temperature. The double bonds present in the naturally occurring unsaturated fats are in the Cis form. Trans fatty acids are associated with health problems and cardiovascular diseases.

Unsaturated fatty acids can be converted into saturated by the process of hydrogenation. Depending upon the degree of unsaturation, the fatty acids can combine with oxygen or halogens to form saturated fatty acids. So it is important to know the extend to which a fatty acid is unsaturated. There are different methods for checking the unsaturation level in fatty acids, one among them is by determining the iodine value of fats. Iodine value or number is the number of grams of iodine consumed by 100g of fat. A higher iodine value indicates a higher degree of unsaturation.

unsaturated fatty acids

Principle:

Fatty acids react with a halogen [ iodine] resulting in the addition of the halogen at the C=C double bond site. In this reaction, iodine monochloride reacts with the unsaturated bonds to produce a di-halogenated single bond, of which one carbon has bound an atom of iodine.

After the reaction is complete, the amount of iodine that has reacted is determined by adding a solution of potassium iodide to the reaction product.

ICl + KI ---------------> KCl + I₂

This causes the remaining unreacted ICl to form molecular iodine. The liberated I2 is then titrated with a standard solution of 0..1N sodium thiosulfate.

I₂ + 2 Na₂S₂O₃ -----------------> 2 NaI + Na₂S₂O₄

Saturated fatty acids will not give the halogenation reaction. If the iodine number is between 0-70, it will be a fat and if the value exceeds 70 it is an oil. Starch is used as the indicator for this reaction so that the liberated iodine will react with starch to give purple coloured product and thus the endpoint can be observed.

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