Palmitic and stearic acids are the most common saturated fatty acids, and oleic and linoleic acids are the most common unsaturated fatty acids. A way to measure the relative degree of unsaturation of a fat or an oil is to determine its iodine number.
The Iodine Number is defined as the number of grams of iodine taken up by 100g of fat. The amount of iodine consumed is determined by titrating the iodine released, after addition of excess potassium iodide with standard sodium thiosulphate solution and comparing with a blank in which the fat is omitted (Pocklington, 1990).
In general, fats have lower iodine numbers than oils because oils have greater percentages of carbon-carbon bonds that are double bonds. For example, typical iodine numbers for butter are 25 to 40, and for corn oil, 115 to 130.
The fat and oil samples with 0.3 g weight were placed inside a clean and dry conical flask. Chloroform (10 cm3) was then added into the reaction vessel. Using a burette, 25cm3 of Wij's solution was carefully added dropwise. The flask was kept in the dark for 30 - 45 minutes for the reaction to occur. After stirring the solution, a spatula of potassium iodide was added. The solution was mixed until the dissolution was complete. De-ionised water (100cm3) was then added into the solution. Titration was performed using the 0.1mol dm-3 solution of sodium thiosulphate provided until a pale straw colour was observed. After this, a starch indicator was added until the blue colour disappears. A blank was also prepared separately. The iodine numbers (I2 No) of the samples were computed using the Equation (1):
I2 No = (B-T) x 0.1 x 127 x 100 (1)
1000 x W
where B and T are the respective blank and test values from titration (cm3), and W is the weight of the fat and oil (g). The B-T values represent the difference between the volumes of sodium thiosulfate required for the blank and for the sample, respectively.
Table 2. Experimental and calculated I2 No. values of the fat and oil samples.
The I2 No for fat and oil are 66.96 and 115.42, respectively. Oil has lower T value compared to that of fat.
IV. Discussion and Conclusion
The iodine number is the mass of iodine, in grams, that is consumed by (reacts with) 100 grams of a fat or oil. Iodine reacts with the carbon-carbon double bonds.