Cure Terms Glossary

Iodine Value/Iodine Number

The iodine value is a measure of the level of unsaturation in a substance. One mole of iodine (MW 253.8) will react with one mole of C=C, and the iodine value is the weight of iodine which is consumed by 100 grams of the substance. Thus for oleic acid (MW 282.5), which contains one C=C per molecule, the iodine value is theoretically 89.8 g/100g.

The iodine value is commonly determined using Wijs' solution (iodine monochloride in glacial acetic acid). After treatment, the unreacted iodine monochloride is converted to iodine (with KI) which is then determined by titration against sodium thiosulphate.

Iodine value is useful index of activity in alkyd resins, which typically use mixed fatty acids containing one, two or (sometimes) three C=C bonds per molecule. These resins are classified according to their iodine value as: drying (>140 g/100g), semi-drying (125-140 g/100g) and non-drying (<125 g/100g).