SOP: Iodine Value of Oils and Fats (Wijs Procedure)
GIFSQ — Global Institute for Food Safety & Quality
Principle
The iodine value (I.V.) is the number of grams of iodine absorbed per 100 g of the oil or fat, when determined using Wijs solution. The material is treated in carbon tetrachloride medium with a known excess of iodine monochloride in glacial acetic acid; the excess iodine monochloride is treated with potassium iodide, and the liberated iodine is estimated by titrating with standard sodium thiosulphate solution. It is a measure of the unsaturation of the fat.
Apparatus required
- 250 ml glass-stoppered conical flask (clean, dry)
- Dark cupboard
Reagents
a. Wijs solution: dissolve 8 g of iodine trichloride in 200 ml of glacial acetic acid and mix with 9 g of iodine dissolved in 400 ml of glacial acetic acid.
b. 10% potassium iodide solution: dissolve 10 g of potassium iodide in 90 ml of water.
c. 0.1 N Na₂S₂O₃ (sodium thiosulphate) solution.
d. Starch indicator, freshly prepared.
e. Carbon tetrachloride.
f. Distilled water.
Procedure
Weigh accurately, by difference, an appropriate quantity of the oil or fat (previously melted) into a clean dry 250 ml glass-stoppered conical flask — 2.5–3.0 g for coconut oil, 0.15–0.6 g for other oils depending upon the iodine value — and add 10 ml of carbon tetrachloride
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Add 25 ml of Wijs solution; replace the stopper after moistening it with potassium iodide solution; mix
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Store in a dark cupboard for 30 min in the case of non-drying and semi-drying oils, or 60 min in the case of drying oils
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Add 15 ml of 10% potassium iodide solution and 100 ml of distilled water
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Titrate with 0.1 N Na₂S₂O₃ solution using starch as the indicator near the end point; carry out a blank determination alongside without the fat
Calculation
(Blank titre − Sample titre) × N of Na₂S₂O₃
I.V. = ───────────────────────────────────────────────── × 12.69
Wt of sample (g)
References
Ranganna, S., Handbook of Analysis and Quality Control for Fruit and Vegetable Products, 2nd edition.