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Iodine value: rationale, procedure and applications

It is determined by a test that measures the amount of iodine that reacts under certain conditions with the C = C double bonds of the constituents of natural or processed fats or oils.

Olive oil has an iodine value of 75-95. Author: Steve Buissinne. Source: Pixabay.

The resulting number qualitatively expresses the concentration of unsaturated fatty acids, making it a simple and very useful parameter. Generally abbreviated IV ( Iodine Value ).

It is also known as “iodine number”, “iodine value” or “iodine absorption value” (translation from English iodine number , iodine value and iodine absorption value respectively).

It is used in processed food factories, in determining the quality of biodiesel and in general in characterizing oils and fats.

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Basis of the test

The iodine number is equal to the number of grams of iodine required to saturate the unsaturated fatty acids present in 100 g of oil or fat. Unsaturated fatty acids are so named because they can contain one or more C = C double bonds.

When the iodine value is determined, it is added to the oil or fat and reacts with the C = C double bonds. The C = C double bond takes two iodine atoms.

Reaction of iodine with double bonds. Author: Marilú Stea.

The higher the iodine number, the more unsaturations the sample has.

Oils rich in saturated fatty acids have low iodine numbers, while oils rich in unsaturated fatty acids have high iodine numbers.

There are several methods to measure the IV of fats and oils but the best known methods are the Wijs method and the Hübl method.

Hübl’s method

Arthur von Hübl was the first to come up with the concept of the iodine number in 1884.

To determine this, Hübl’s solution is prepared, which consists of iodine (I 2 ) and mercuric chloride (HgCl 2 ) dissolved in ethanol. The fat or oil sample is dissolved in chloroform.

Mercuric Chloride. This compound is used in Hübl’s method to measure the iodine number. Leiem / CC BY-SA (https://creativecommons.org/licenses/by-sa/4.0). Source: Wikimedia Commons.

Hübl’s solution is added to the dissolved sample so that the iodine reacts with the double bonds. It is then titrated with standard sodium thiosulfate solution (Na 2 S 2 O 3 ) to determine residual I 2 using starch as indicator.

The results are expressed as the number of grams of iodine absorbed per 100 g of the oil or fat sample.

Wijs method

The scientist JJA Wijs modified Hübl’s procedure in 1898 by changing the initial reagents.

According to this method, the Wijs reagent is prepared by dissolving iodine chloride (ICl) in a mixture of acetic acid and cyclohexane. This reagent is added dropwise to the sample dissolved in a solvent.

Iodine chloride is used in the Wijs method to measure the iodine number. W. Oelen / CC BY-SA (https://creativecommons.org/licenses/by-sa/3.0). Source: Wikimedia Commons.

Wijs considered that the advantage of this procedure is the formation of hypoiodinous acid HOI as an active species.

When the addition of iodine to the double bonds has occurred, the excess ICl is reduced to free iodine (I 2 ) by adding an aqueous solution of potassium iodide (KI).

The iodine thus liberated is titrated with a standard solution of sodium thiosulfate with starch as an indicator.

Other methods

Efforts have been made to implement procedures that do not require hazardous chemicals, especially in the food industries.

The most advanced methods use specialized instruments such as infrared, nuclear magnetic resonance, differential calorimetry, and gas chromatography equipment.

Much more information is obtained from these methods and they are also safer and more accurate.

Applications of the iodine number

This parameter is useful in a number of areas. It is used to determine the quality of the oil of various plant species , to study the effects of insecticides on plants and to determine the quality of diesel fuel derived from vegetable oils.

High IV oils contain large amounts of highly unsaturated fatty acids that can undergo rapid degradation reactions such as autoxidation or polymerization.

In the processed food industry

The thermal stability of oils and fats used in baked goods is connected with the tendency to rancidity (oxidation) and with a greater participation in the formation of residues in the baking molds.

During food baking processes oxidation and polymerization reactions occur more easily due to high temperatures and the presence of oxygen.

This is why fats with a low IV are better for baking wafers and sweet cookies.

On the quality of biodiesel

Biodiesel is a fuel used in diesel engines that is obtained from natural oils and fats through processes of conversion to esters.

The IV is a standard test used to indicate the oxidation tendency of biodiesel. This characteristic is due to polyunsaturated fatty acids, which are some of the compounds that cause the formation of deposits in the engine.

Biodiesel sample. The iodine number is one of the parameters that are measured in this fuel. Shizhao 拍摄 / CC BY-SA (http://creativecommons.org/licenses/by-sa/3.0/). Source: Wikimedia Commons.

In the vegetable oils and fats industry

Because melting point and oxidative stability are related to the degree of unsaturation, IV provides a qualitative estimate of these properties.

It is a very important parameter in the palm oil industry as it allows the fractionation process to be followed.

Here is the IV of some oils and fats:

Peanut Oil 82-107; corn oil 103-128; cottonseed oil 99-113; coconut oil 7.7-10.5; palm oil 44-54; linseed oil 155-205; butter 25-42.

Butter has a low iodine value. Author: Congerdesign. Source: Pixabay.

High iodine value

Oils with an iodine value greater than 115 are called drying oils . Flaxseed oil is a representative of this group.

They are oils that, when exposed to air for a certain time, harden as they undergo polymerization and form solid and resistant films.

An IV above 150 indicates that the oil dries very well and is suitable for use in printing inks, paints, wood surface treatments, floors, etc.

Flaxseed oil and its seeds. Author: Kamilla02. Source: Pixabay.

This is derived from the reactivity of the C – H bonds located next to the C = C bonds. By taking an H atom from these sites, free radicals are formed, then oxygen is added to give peroxide radicals which favor polymerization.

The greater the amount of C = C bonds in the oil, the greater its tendency to dry out.

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