Fats and oils are part of a group of compounds knows as lipids which are characterized by their solubility in organic solvents and the insolubility in water. Most fat and oil molecules consist of glycerol thrice esterified to fatty acid and are known as triglycerids, represented by the following formula:

R₁’ R₂’ R₃ are fatty acid chains. The most common fatty acids in nature are straight-chained unsaturated or saturated compounds containing an even number of carbon atoms. Since there is nor preferential bonding to glycerol, fat or oil molecules will vary widely in molecular mass, and the difference between oils and fats lies in the saturated and unsaturated fatty acid composition of the triglycerids. Generally the triglycerids of oils have fatty acids unsaturated to a greater degree than those in fats and therefore have lower molecular masses.

One can determine the average molecular mass of these triglycerides by using by saponification number (see later) and the degree of unsaturation by the iodine number where I₂ adds across a double bond. The amount of iodine used in this reaction may be determined by titration and expressed as the number of grams of iodine absorbed by 100 grams of oil.

METHOD

Add 20 ml 15% KI and 100 ml freshly boiled and cooled water. Titrate with 0.1N Na2S₂O₃ until the yellow has almost disappeared (magnetic stirring permissible). Add a few drops of starch indicator and titrate till blue disappears. As the end of the reaction is approached shake the flask vigorously to remove iodine from the chloroform phase.

Conduct a blank determination without fat or soil simultaneously under the same conditions (temperature appreciably affects the titre due to the high co-efficient of expansion of acetic acid).

Na2S₂O₃ equivalents of iodine absorbed by the sample is equal to ml standard solution required by sample minus that required by the blank.

I number (Wijs) = [(B-S) X N X 12,69] / g sample where n is normality of standard solution.

Calculate iodine numbers and compare with literature.

1. Place the lipids, used in the determination of the iodine number, in order of decreasing unsaturation.
2. Does the degree of unsaturation bear any relationship to the solid or liquid nature of these lipids?
3. Is the claim that margarine is "high in polyunsaturate" true?
4. In terms of health, why is margarine used in preference to butter?

MATERIALS AND EQUIPMENT

Commercial Wijs solution – may be prepared in lab.

15% KI.

Chloroform.

Starch indicator.

Ca. 0.1N Na2S₂O₃ standard solution.

Burettes and stands.

500 ml g-s flasks.

Assorted oil samples.