Acid value and iodine number of oils and fats (lab visit)
Acid value counts free fatty acids (rancidity); iodine value counts double bonds (unsaturation). Both are titrations with exact formulas.
Aim and principle
Lab visit: observed at a food analysis laboratory, not performed by you (but definitions, formulas and calculations are asked). Aim: To find the acid value and the iodine value of an edible oil or fat. Principle of acid value: Free fatty acids, released when fat is hydrolysed, are titrated with standard potassium hydroxide in a neutralised alcohol-ether solvent, with phenolphthalein. Acid value is the number of mg of KOH needed to neutralise the free fatty acids in 1 g of fat or oil. Principle of iodine value (Wijs method): Iodine monochloride (ICl) adds across the C=C double bonds of unsaturated fatty acids. A known excess is added; the unused ICl is then made to release iodine with potassium iodide (ICl + KI → KCl + I₂), and this iodine is titrated with sodium thiosulphate using starch. A blank without fat shows the total ICl added, so the difference is what the fat took up. Iodine value is the number of g of iodine absorbed by 100 g of fat or oil, a measure of unsaturation. (Alternative: the Hanus method uses iodine monobromide.)
Requirements
| Item | What it does |
|---|---|
| Oil or fat sample | Material tested |
| Neutralised solvent: ethanol + diethyl ether (1:1), or hot neutral ethanol | Dissolves the fat; neutralised first so its own acidity is not counted |
| 0.1 N potassium hydroxide (or sodium hydroxide) | Titrant that neutralises the free fatty acids; keep 56.1 in the formula even with NaOH, because acid value is defined in mg KOH |
| Phenolphthalein (1%) | Indicator: faint pink at the end point |
| Chloroform (or the solvent your manual names) | Dissolves the fat for the iodine value |
| Wijs solution: iodine monochloride in glacial acetic acid | Halogen that adds across double bonds |
| 10% potassium iodide | Turns unused ICl into free iodine for titration |
| 0.1 N sodium thiosulphate | Titrates the free iodine |
| 1% starch solution | Indicator: blue with iodine, colourless at the end point |
| Apparatus | Conical flasks, glass-stoppered iodine flasks, burettes, pipettes with a filler, analytical balance, dark cupboard, fume hood |
Procedure: acid value
Typical quantities are given; follow the exact volumes in your lab manual.
- 1Weigh about 5 g of oil accurately into a conical flask.
- 2Add 50 mL of neutralised alcohol-ether solvent and about 1 mL phenolphthalein; swirl to dissolve (warm gently if the fat is solid).
- 3★Titrate with 0.1 N KOH, swirling, until a faint pink lasts for about 15-30 seconds.
- 4Note the titre (V, in mL) and calculate the acid value.
★ one-way, controls the pace
Procedure: iodine value (Wijs)
Typical quantities are given; follow the exact volumes in your lab manual. Use less sample for highly unsaturated oils.
- 1Weigh 0.2-0.3 g of oil accurately into a dry iodine flask; dissolve in about 10 mL chloroform.
- 2Add exactly 25 mL Wijs solution (burette or pipette with a filler), stopper and swirl.
- 3★Keep in the dark for 30 minutes (60 minutes for very unsaturated oils). Set up a blank without oil at the same time.
- 4Add about 15 mL of 10% KI and 100 mL of water; mix.
- 5★Titrate with 0.1 N sodium thiosulphate until the yellow is pale; add 1 mL starch (blue) and continue, shaking well, until the blue just disappears.
- 6Note the sample titre (S) and the blank titre (B).
★ one-way, controls the pace
Worked example 1: acid value of fresh and used oil
5.00 g of fresh groundnut oil needed 0.45 mL of 0.1 N KOH. 5.00 g of the same oil after repeated frying needed 2.40 mL. Find the acid values and the free fatty acids as % oleic acid.
- Acid value = 56.1 × N × V ÷ W. (56.1 is the molar mass of KOH in g/mol, so N × V, in milliequivalents, × 56.1 gives mg of KOH.)
- Fresh oil: 56.1 × 0.1 × 0.45 = 2.5245; ÷ 5.00 = 0.50 mg KOH/g.
- Used oil: 56.1 × 0.1 × 2.40 = 13.464; ÷ 5.00 = 2.69 mg KOH/g.
- Free fatty acids as % oleic acid = 28.2 × N × V ÷ W (28.2 is the molar mass of oleic acid, 282, ÷ 10).
- Fresh: 28.2 × 0.1 × 0.45 ÷ 5.00 = 0.25%. Used: 28.2 × 0.1 × 2.40 ÷ 5.00 = 1.35%.
- Check: acid value ≈ 1.99 × FFA %: 1.99 × 0.25 ≈ 0.50 and 1.99 × 1.35 ≈ 2.69.
Fresh oil: acid value 0.50 mg KOH/g (FFA 0.25% as oleic acid). Used frying oil: 2.69 mg KOH/g (FFA 1.35%), more than five times higher, a sign of hydrolytic rancidity.
Worked example 2: iodine value (Wijs)
0.2700 g of groundnut oil was treated with 25 mL Wijs solution. Blank titre (B) = 48.0 mL and sample titre (S) = 28.0 mL of 0.1 N sodium thiosulphate. Find the iodine value.
- Iodine value = (B − S) × N × 12.69 ÷ W.
- Basis of 12.69: 1 mL of 1 N thiosulphate ≡ 0.1269 g of iodine (equivalent weight 126.9 ÷ 1000); × 100 to express per 100 g of fat.
- B − S = 48.0 − 28.0 = 20.0 mL.
- 20.0 × 0.1 × 12.69 = 25.38.
- Iodine value = 25.38 ÷ 0.2700 = 94.0.
- Check: 28.0 of 48.0 mL, about 58% of the reagent, was left unused. The standard Wijs method (AOCS) asks for 50-60% of the reagent to be left over, so the sample weight was right.
Iodine value = 94.0 g I₂ per 100 g, within the usual range for groundnut oil.
What the values mean
| Value | High value means | Low value means |
|---|---|---|
| Acid value | More free fatty acids: hydrolytic rancidity from moisture, lipase, heat or repeated frying; poorer quality and lower smoke point | Fresh, well-refined oil |
| Iodine value | More double bonds (more unsaturated, e.g. sunflower, linseed): usually liquid, but oxidises and turns rancid faster | More saturated: solid or semi-solid (coconut oil, ghee, vanaspati) and more stable |
Typical iodine values (approximate)
| Oil | Iodine value (g I₂/100 g) | Usual description |
|---|---|---|
| Coconut oil | About 6-11 | Highly saturated; non-drying |
| Groundnut oil | About 86-107 | Non-drying |
| Sunflower oil | About 118-141 | Semi-drying |
| Linseed oil | Above 170 | Drying |
Precautions and result
- Neutralise the alcohol-ether solvent before use, and stop at the first faint pink that lasts 15-30 s.
- For iodine value use dry glass-stoppered flasks, keep them in the dark for the full time, and always run a blank alongside.
- Add starch only near the end point (pale yellow), and shake well so iodine in the chloroform layer is titrated.
- Wijs solution and glacial acetic acid are corrosive; chloroform is toxic and ether is highly flammable: gloves, goggles, fume hood, no flames, never pipette by mouth.
- Result: Acid value of the given oil = ___ mg KOH/g; iodine value = ___ g I₂ per 100 g.
Acid value is a key quality and food safety check for edible oils: it rises with poor storage and repeated frying, and food standards such as FSSAI set maximum limits. Iodine value shows how unsaturated an oil is, helps detect adulteration (a value outside the normal range for that oil) and tracks hydrogenation (vanaspati has a lower iodine value than the oil it was made from). Dietitians use these ideas when advising on choosing cooking oils and not reusing frying oil.
Short answer: “Define acid value.” The number of mg of KOH needed to neutralise the free fatty acids present in 1 g of fat or oil. Acid value = 56.1 × N × V ÷ W.
Short answer: “Define iodine value. What does a high value show?” The number of g of iodine absorbed by 100 g of fat or oil. A high value shows many double bonds, i.e. a highly unsaturated oil such as sunflower or linseed oil.
Structured question: “Explain the determination of iodine value by the Wijs method, with the calculation.” Principle: ICl adds across C=C; unused ICl + KI releases I₂, titrated with thiosulphate using starch. Blank and sample, 30 min in the dark. IV = (B − S) × N × 12.69 ÷ W, with the basis of 12.69. Worked example. Meaning of high and low values.
Acid = Age, Iodine = Unsaturation: acid value climbs as oil ages and is reused; iodine value climbs with every extra double bond.