The iodine value measures the unsaturation of oils and fats by determining the number of carbon double bonds. Iodine reacts with these bonds, and the amount of iodine consumed is measured to calculate the iodine value. This value is expressed as grams of iodine used per 100 grams of fat. Vegetable oils are generally more unsaturated than animal fats, and therefore have higher iodine values. Saponification number and acid number are two other values associated with fats and oils.
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 the carbon atoms, there is less hydrogen in the molecule and the fat it is called unsaturated. Triglycerides with one double bond are known as monounsaturated and those with more than one double bond are known as polyunsaturated. Iodine can combine with fats that have carbon double bonds and, therefore, the number of such bonds can be inferred from the amount of iodine they will bond with.
Hydrogen and the halogen elements – fluorine, chlorine, bromine and iodine – resemble each other in that they are an 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 single electron of hydrogen and one electron of 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 form a bond with a halogen instead.
The more carbon double bonds an unsaturated fat has, the more halogen atoms it can combine with. It is therefore possible to determine the degree of unsaturation of a grease by allowing it to combine with a halogen. A simple test for unsaturated fat 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 when it combines with the fat. To determine the degree of unsaturation, however, iodine is normally used, as it is easy to measure precisely how much iodine has been consumed.
To obtain the iodine value – also called 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), since this reacts more easily. Where there is a carbon double bond, one carbon atom will form a single bond with the chlorine in iodine monochloride and the other with 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.
A solution of sodium thiosulphate of known concentration is then slowly added. Iodine reacts with this to form colorless I ions. Once all the iodine has reacted, the solution will become colorless. At this point it is possible to determine the quantity of sodium thiosulfate used and from this the quantity of iodine present. Once this amount is known, the amount of iodine that has 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 to 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 can be associated with fats and oils. 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 by the amount of alkali needed to neutralize it.
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