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The iodine value, or iodine adsorption value, is used in analytical chemistry to measure the amount of unsaturation of oils and fats. Animal and vegetable oils and fats — known chemically as triglycerides — have chains of carbon atoms that can bond with hydrogen. When the carbon atoms in these chains are bonded to the maximum possible number of hydrogen atoms, the triglyceride is said to be saturated, but when there are one or more double bonds between carbon atoms, there is less hydrogen in the molecule, and the fat is said to be unsaturated. Triglycerides with one double bond are known as monounsaturates and those with more than one double bond are known as polyunsaturates. Iodine can combine with fats that have carbon double bonds and, therefore, the number of such bonds can be deduced from the amount of iodine with which they will combine.
Hydrogen and the halogen elements — fluorine, chlorine, bromine and iodine — resemble one another in that they are one electron short of a stable configuration and can form stable compounds by sharing an electron pair with another atom. In a carbon-hydrogen bond, the hydrogen’s single electron and one electron from the carbon are shared to form a single covalent bond. Where there is a carbon double bond in an unsaturated fat, each of the carbon atoms can instead form a bond with a halogen.
The more carbon double bonds an unsaturated fat has, the more halogen atoms with which it can combine. It is therefore possible to determine the degree of unsaturation of a fat by allowing it to combine with a halogen. A simple test for unsaturated fats is to mix the fat with a solution of bromine in carbon tetrachloride; if the fat is unsaturated, the brown or yellow color of the bromine disappears as it combines with the fat. For determining the degree of unsaturation, however, iodine is normally used, as it is easy to measure precisely how much iodine has been used up.
To obtain the iodine value — also known as the iodine number — of a fat, a known quantity is dissolved in a suitable solvent, such as chloroform, and mixed with an excess of iodine in the form of iodine monochloride (ICl), as this reacts more easily. Where there is a carbon double bond, one carbon atom will form a single bond with the chlorine in the iodine monochloride and the other with the iodine. When the reaction is complete, potassium iodide is added to the remaining iodine monochloride to release the iodine: ICl + 2KI → KCl + I2. The remaining iodine is reacted with a starch to form a dark blue compound.
Sodium thiosulfate solution at a known concentration is then slowly added. The iodine reacts with this to form colorless I- ions. Once all the iodine has reacted, the solution will become colorless. At this point, the amount of sodium thiosulfate used can be determined, and from this, the amount of iodine that was present. When this amount is known, the amount of iodine that reacted with the fat can be calculated, giving the iodine value, which is expressed as grams of iodine used per 100 grams of fat.
Animal and vegetable fats and oils are mixtures of triglycerides. A fat or oil that is high in unsaturated triglycerides will have a high iodine value. Many vegetable oils are rich in unsaturated triglycerides. Sunflower oil, for example, has an iodine value of 110-143, compared with 35-48 for a typical animal fat. Coconut oil, in contrast, is highly saturated, with an iodine value of only 6-11.
There are two other numbers that may be associated with fats and oils. The saponification number is an indication of the average molecular weight of a fat and is determined by breaking it down into glycerol and a fatty acid salt by treatment with a strong alkali. The acid number indicates how much free fatty acid a fat contains and is estimated from the amount of alkali required to neutralize it.