The main constituents of natural fats and oils are triglycerides. Triglycerides are formed from three fatty acids which are linked to glycerol by fatty acyl esters. Fatty acids are long aliphatic chains with carboxyl groups. These are grouped into saturated or unsaturated based on the number of double bonds present in fatty acid. Saturated fatty acids consist of only single bonds between the carbon atoms and are likely to exist as solids at room temperature. Unsaturated fatty acids consist of single bonds as well as double bonds and tend to be liquids at room temperature. Naturally occurring unsaturated fats contain double bonds in the cis form. Trans fatty acids are linked with cardiovascular diseases and health problems.Lets go in deep about Iodine Value Of Fats And Oil.
Hydrogenation leads to the conversion of unsaturated fatty acids to saturated fatty acids. The fatty acids combine with oxygen or halogens depending upon the extent of unsaturation to form saturated fatty acids. So, it is significant to know the degree of unsaturation. There are different methods to know the level of unsaturation in fatty acids, one among them is by determining the iodine number of fats.
What is iodine value?
Iodine value, also called iodine number or iodine index, is the mass of iodine in grams absorbed by 100 grams of oil/fat. It is used in the estimation of the degree of unsaturation in fatty acids. The double bonds present in unsaturated fatty acids react with the iodine; the more the iodine number, the more the number of C=C bonds.
Theory | Iodine Value Of Fats And Oil
Iodine number is a measure of the total number of double bonds present in fats and oils. It is expressed as the iodine in grams that will react with the double bonds in 100 grams of fats or oils. The determination is carried out by dissolving a weighed sample in a non-polar solvent such as cyclohexane, then adding glacial acetic acid. The double bonds are reacted with surfeit of a solution of iodine monochloride in glacial acetic acid (Wjis solution). Mercuric ions are added to fasten the reaction. The excess iodine monochloride is then decomposed to iodine by adding aqueous potassium iodide solution after completion of the reaction which is then titrated with standard sodium thiosulphate solution
The oil/fat sample taken in carbon tetrachloride is reacted with an excess of iodine monochloride solution (Wjis solution). Unsaturated fatty acids undergo halogenation reaction resulting in the addition of an iodine atom to one carbon of the double bond. On completion of the reaction, the remaining iodine monochloride reacts with potassium iodide leading to the formation of molecular iodine. The liberated iodine is then evaluated by titration with a standard solution of sodium thiosulphate.
Reaction flasks: Standard 250 ml iodine flasks are advocated. Erlenmeyer flask, 500 ml capacity, with standard taper-covers may be used as a substitute.
1) Carbon tetrachloride, analytical reagent grade
2) Iodine monochloride, Wjis’s reagent, 0.22 N
3) Potassium Iodide, 30% – Dissolve 30 g of KI in purified water and dilute to 100 ml.
4) Sodium Thiosulphate solution, 0.1 N – Standard
5) Starch Indicator solution, 1%
Oil/fat may be weighed precisely following the table hereinafter described: Expected weight of iodine to be taken for value estimation (g)
Expected Iodine value Weight to be taken for estimation
- An appropriate quantity of the oil/fat to be weighed accurately as indicated in the table above, into a 500 ml conical flask with glass stopper, to which 25 ml of carbon tetrachloride has been added. Mix it well. There should be surplus of 50 to 60 percent of Wjis’s solution over that actually needed and so forth the weight of the sample should be taken accordingly.
- Pipette out 25 ml of Wjis’s solution and replace the glass stopper after wetting with potassium iodide solution.
- Mix it properly and place the flasks in dark for half an hour for non-drying and semi-drying oils and one hour for drying oils.
- Carry out a blank concurrently. After standing, add 15 ml of potassium iodide solution, followed by 100 ml of recently boiled and cooled water, rinsing in the stopper also.
- The liberated iodine is titrated with standardized sodium thiosulphate solution, using starch as indicator at the end until the blue colour formed disappears after thorough shaking with the stopper on.
- Conduct blank determinations in a similar manner as test sample but without oil/fat. Subtle variations in temperature appreciably affect titre of molecular iodine solution as chloroform has a high coefficient of expansion. It is thus necessary that blanks and determinations are made simultaneously.
Iodine value = 12.69 (B – S) N/W
B = volume in ml of standard sodium thiosulphate solution required for the blank.
S = volume in ml of standard sodium thiosulphate solution required for the sample.
N = normality of the standard sodium thiosulphate solution.
W = weight in g of the sample.