Determination of iodine value of oil
The iodine value is a measure of the degree of unsaturation in an oil. It is constant for a particular oil or fat. Iodine value is a useful parameter in studying oxidative rancidity of oils since higher the unsaturation the greater the possibility of the oils to go rancid.
The oils contain both saturated and unsaturated fatty acids. Iodine gets incorporated into the fatty acid chain wherever the double bond exist. Hence, the measure of the iodine absorbed by an oil, gives the degree of unsaturation. Iodine value/number is defined as the ‘g’ of iodine absorbed by 100g of the oil.
» Hanus Iodine Solution
Weigh 13.6g of iodine and dissolve in 825mL glacial acetic acid by heating, and cool. Titrate 25mL of this solution against 0.1N sodium thiosulphate. Measure another portion of 200mL of glacial acetic acid and add 3mL of bromine to it. To 5mL of this solution add 10mL of 15% potassium iodide solution and titrate against 0.1N sodium thiosulphate. Calculate the value of bromine solution, to double halogen content of the remaining 800mL of the above iodine solution as follows:
» X = B/C, where X = mL of bromine solution required to double the halogen content, B = 800 x thiosulphate equivalent of 1mL of iodine solution and C = thiosulphate equivalent of one mL of bromine solution.
» 15% Potassium Iodide Solution
» 0.1% Sodium Thiosulphate
» 1% Starch
The quantity of thiosulphate required for blank minus the quantity required for sample gives thiosulphite equivalent of iodide adsorbed by the fat or oil taken for analysis.
B = mL thiosulphate for blank
S = mL thiosulphate for sample
N = normality of thiosulphate solution
Amount of fat/oil taken should be adjusted such that the excess iodine in the added 25mL of Hanus iodine solution has about 60% of excess iodine of the amount added, i.e., if (B – S) is greater than B/2, repeat the smaller amount of sample.
1. William Horowitz (ed) (1975) Official Methods of Analysis of AOAC Association of Official Analytical Chemists Washington (12th Ed) p 488.
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