The Iodine Number is determined experimentally by measuring how much iodine reacts with the double bonds in a fat or oil sample. The standard method involves halogen addition using iodine monochloride (ICl) or iodine monobromide (IBr) in a process called iodometric titration.
Principle
Unsaturated fatty acids contain carbon-carbon double bonds (C=C) that react with iodine via addition reaction:
R–CH=CH–R' + I2 → R–CHI–CHIR'
The amount of iodine absorbed is proportional to the number of double bonds. Excess iodine is then back-titrated with sodium thiosulfate to determine how much was consumed.
Common Methods
- Wijs Method (Most common) – Uses ICl in glacial acetic acid.
- Hanus Method – Uses IBr in glacial acetic acid.
- Kaufmann Method – Uses IBr in methanol.
Step-by-Step Procedure (Wijs Method)
Step 1: Sample Preparation
Weigh accurately about 0.1–0.5 g of the fat or oil (depending on expected iodine value) into a dry iodine flask.
Step 2: Dissolve Sample
Dissolve the sample in 10–15 mL of chloroform or carbon tetrachloride.
Step 3: Add Wijs Solution
Add 25 mL of Wijs reagent (iodine monochloride in glacial acetic acid). Include a blank (same volume of solvent + Wijs solution, no sample).
Step 4: Reaction in Dark
Stopper the flask, swirl gently, and keep in the dark for 30 minutes (1 hour for highly unsaturated oils) at room temperature (~25°C).
Step 5: Add KI and Water
Add 20 mL of 15% potassium iodide (KI) solution and 100 mL of distilled water. This liberates free iodine from excess ICl:
ICl + 2KI → KCl + KI + I2
Step 6: Titrate with Thiosulfate
Titrate the liberated iodine with standardized 0.1 N sodium thiosulfate (Na2S2O3) using starch indicator (added near endpoint). The blue color disappears at the endpoint.
I2 + 2Na2S2O3 → 2NaI + Na2S4O6
Step 7: Record Volumes
Let:
- V1 = Volume of Na2S2O3 used for blank
- V1 = Volume of Na2S2O3 used for sample
- N = Normality of Na2S2O3
- W = Weight of sample (in grams)
Calculation
Iodine absorbed (in grams) per 100 g of sample:
$$\text{Iodine Value (IV)} = \frac{(V_1 - V_2) \times N \times 0.1269 \times 100}{W}$$
Where:
- 0.1269 = Equivalent weight of iodine (126.9 g/mol ÷ 1000)
- (V1 – V2) = Volume of thiosulfate corresponding to iodine reacted with sample
- N = Normality of Na2S2O3
- W = Weight of sample in grams
Example Calculation
| Parameter | Value |
|---|---|
| Blank titration (V1) | 45.0 mL |
| Sample titration (V2) | 25.0 mL |
| Normality of Na2S2O3 | 0.1 N |
| Weight of oil (W) | 0.25 g |
$$\text{Iodine Number} = \frac{(45 - 25) \times 0.1 \times 0.1269 \times 100}{0.25} = 101.52$$
Precautions
- Conduct in subdued light to prevent side reactions.
- Use dry glassware and solvents.
- Ensure complete dissolution of sample.
- Standardize thiosulfate solution accurately.
- Avoid excess reaction time for low-unsaturation samples.
Significance of Accurate Measurement
- Ensures correct classification of oils (drying, semi-drying, non-drying).
- Critical for quality control in food, paint, and cosmetic industries.
- Helps predict oxidative stability and shelf life.