Iodine Value

The "kitchen test" for evaluating fats is this, "The more solid a fat is at room temperature the more saturated it is." This is because unsaturated fats contain double bonds which "kink" the molecules and prevent them lining up in an orderly fashion. Chemists describe the relative saturation of a fat with an "Iodine Value". Iodine reacts with a double bond by breaking the carbon-carbon double bond into a carbon-carbon single bond by adding two iodine atoms to the molecule in the place where the carbon-carbon double bond was. The higher the iodine value the more unsaturated the fat (or oil). Highly unsaturated oils contain so many of these carbon-carbon double bonds that they cross-link with themselves, forming a varnish. These highly unsaturated, cross linking vegetable oils are called "drying oils" and are the traditional ingredient in paints, varnishes, and inks. Linseed oil (from flax) and hemp oil are drying oils. Soybean oil is a semi-drying oil. Cottonseed oil has an iodine value of only 105.7. It is not used in the production of drying inks.

The traditional method for determining iodine value makes use of the solvent carbon tetrachloride. In a number of countries this solvent is now banned for use in laboratories because of its carcinogenic properties. Consequently this method of analysis has been modified using cyclohexane as a solvent.

The results from the ring test o peroxide value show that replacement of glacial acetic acid: chloroform as the solvent with glacial acetic acid: iso-octane in the analysis of fish oils resulted in the formation of an emulsion which made determination of the end point of titration difficult and resulted in a small increase in PV and an increase in the within laboratory standard deviation or repeatability of the assay. Filtering the oils prior to analysis to remove impurities and residual moisture and addition of emulsifiers to aid the titration appear to be helpful in reducing variability and should be made mandatory in the new method.