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Oils & fats · Edible or industrial oil or fat (example: sunflower oil)

Iodine value (Wijs)

The oil reacts in the dark with a known excess of iodine monochloride (Wijs reagent). Unreacted ICl is converted to iodine with potassium iodide and titrated with sodium thiosulphate. The difference from a blank gives the iodine value: the grams of iodine taken up by 100 g of oil.

  • Chemicals & materials
  • Back titration (iodometric)
  • Burette & glassware
  • Analytical balance

Reference: Typical method based on ISO 3961, AOCS Cd 1d-92 and IS 548 (Part 1) (Wijs method).

PRINCIPLE

Why this method works

Iodine monochloride adds across carbon–carbon double bonds in fatty acid chains. The more double bonds, the more ICl is used up. After the reaction time, the excess ICl reacts with KI to release iodine (ICl + KI → KCl + I₂), which is titrated with sodium thiosulphate using starch. The blank minus sample titre equals the halogen absorbed, expressed as iodine. 12.69 is the equivalent mass of iodine (126.9 g/mol) divided by 10 to give g per 100 g.

Instruments used

MATERIALS

Reagents & standards

ReagentGrade / specification
Wijs reagentIodine monochloride in glacial acetic acid (commercially available)
SolventCyclohexane and glacial acetic acid 1:1 (older methods used carbon tetrachloride or chloroform)
Potassium iodide10% (100 g/L) solution, free of iodate
Titrant0.1 N sodium thiosulphate, standardised
IndicatorStarch solution, about 1%
SAMPLE PREPARATION

Preparing the sample

  1. 01

    Weigh

    Weigh oil into a dry iodine flask; mass depends on the expected IV.

  2. 02

    Dissolve

    Add solvent and swirl to dissolve.

  3. 03

    React

    Add exactly 25 mL Wijs reagent, stopper, swirl and keep in the dark.

  4. 04

    Add KI

    Add KI solution and water to release iodine from the excess ICl.

  5. 05

    Titrate

    Titrate with thiosulphate, adding starch near the end point.

Procedure

  1. 1Choose sample mass from the expected IV using the method table (for example about 0.13–0.25 g for oils with IV around 100–150). The excess ICl should be roughly 50–60% of the amount added.
  2. 2Weigh the sample accurately (to 0.1 mg) into a dry 500 mL iodine flask.
  3. 3Add about 20 mL of solvent and dissolve.
  4. 4Add exactly 25 mL of Wijs reagent from a pipette, stopper, swirl and store in the dark: typically 1 h for IV below about 150 and 2 h for higher IV, as the method states.
  5. 5Prepare two blanks the same way without sample.
  6. 6After the reaction time, add 20 mL of 10% KI and 150 mL of water.
  7. 7Titrate with 0.1 N sodium thiosulphate until the yellow colour almost disappears, add starch and continue until the blue colour just disappears after vigorous shaking.

Standard preparation

  1. 1Standardise sodium thiosulphate against potassium dichromate or potassium iodate and record its normality.

Notes for analysts

  • IV is a quick identity and quality check: a low IV in a sunflower oil may show blending with palm oil; a falling IV during frying shows oxidation.
  • Keep the reaction strictly in the dark and time it; light and long reaction times give high results.
  • Iodine value can also be calculated from the fatty acid composition by GC (AOCS Cd 1c-85), which avoids halogenated reagents.
TEST CONDITIONS

Method parameters (typical)

Wijs reagent volume25.0 mL, delivered by pipette
ReactionIn the dark at about 20–25 °C, typically 1 h (IV < 150) or 2 h (IV ≥ 150)
Titrant0.1 N sodium thiosulphate
BlanksTwo blanks per batch
BEFORE YOU REPORT

System suitability & sequence

ParameterLimit
Blank titresThe two blanks agree closely (for example within 0.1 mL)
Excess reagentSample titre at least about half the blank titre (excess ICl about 50–60%)

Typical sequence

  1. 1Standardise thiosulphate
  2. 2Prepare blanks and samples
  3. 3React in the dark
  4. 4Add KI and water
  5. 5Titrate blanks and samples
CALCULATION

Calculation with a worked example

Formula
Iodine value (g I₂/100 g) = 12.69 × (B − S) × N ÷ W

Where

B
Thiosulphate used for the blank (mL)
S
Thiosulphate used for the sample (mL)
N
Normality of sodium thiosulphate
W
Mass of sample (g)
12.69
Equivalent mass of iodine (126.9) ÷ 10

Given

Sample mass (W)
0.2050 g sunflower oil
Blank titre (B)
47.60 mL
Sample titre (S)
26.40 mL
Thiosulphate normality (N)
0.1002

Working

  1. 1

    B − S = 47.60 − 26.40 = 21.20 mL

  2. 2
    IV = 12.69 × 21.20 × 0.1002 ÷ 0.2050
  3. 3
    = 26.957 ÷ 0.2050 = 131.5 g I₂/100 g
  4. 4
    Excess check: 26.40 ÷ 47.60 = 55% of reagent left, within the 50–60% target

Iodine value = 131.5 g I₂/100 g

ACCEPTANCE CRITERIA

What result passes

  • Compare with the identity range for the oil. Typical published ranges: coconut about 6–11, palm about 50–55, olive about 75–94, groundnut about 86–107, sunflower about 118–141, soybean about 124–139 g I₂/100 g. Use the current Codex (CXS 210), FSSAI or customer ranges.
METHOD VALIDATION

Validation parameters (ICH Q2(R2))

ParameterTypical requirement
PrecisionRepeatability within the method's published limits
AccuracyAnalysis of a reference oil of known IV
In LimsPulse · Sample Management

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