🍎 Iodine value of oils and fats — Wijs reagent, titration with thiosulfate
In short
A test portion of the fat (from 0. 13 to 10 g depending on the expected iodine value) is dissolved in cyclohexane with acetic acid, 25 ml of Wijs reagent is added, it is kept for 1–2 h in the dark, and the excess of halogen, after the addition of potassium iodide, is titrated with thiosulfate; the result is in g of iodine per 100 g of fat. The test is performed according to PN-EN ISO 3961:2018-09; Test portion: According to the table of the standard: 10 g for a value < 3, 0.846–1.586 g for 20, 0.254–0.317 g for 100, 0.127–0.159 g for 200; weighed to 0.001 g (to 0.0005 g at a value ≥ 100). The procedure comprises 8 steps (about 1–2 h in total); it is used for: Identification of the kind of fat and control of conformity with the specification (the ranges of the iodine value in the product standards), Assessment of the degree of unsaturation of vegetable oils, animal fats and hydrogenated fats, Control of the process of hydrogenation and refining.
At a glance
- Standard: PN-EN ISO 3961:2018-09
- Category: Food chemistry
- Procedure steps: 8
- Total time of stages: 1 h–2 h
- Test portion: According to the table of the standard: 10 g for a value < 3, 0.846–1.586 g for 20, 0.254–0.317 g for 100, 0.127–0.159 g for 200; weighed to 0.001 g (to 0.0005 g at a value ≥ 100)
- Wijs reagent: 25.00 ml; the I/Cl ratio within the limits 1.10 ± 0.1; an excess of 50–60 % of the amount added
- Solvent: Cyclohexane : glacial acetic acid 50 : 50 (v/v), 25 ml for a value ≤ 5, 20 ml for higher ones
Overview
The standard specifies the reference method of determining the iodine value of animal and vegetable oils and fats. The iodine value is the mass of halogen, expressed as iodine, taken up by the test portion under the conditions of the standard, divided by the mass of the test portion and expressed in grams per 100 g of fat. It is a measure of the content of unsaturated fatty acids, because they add halogen at the place of the double bonds, and for that reason it serves to identify fats: the laboratory instructions give the standard ranges among others for rapeseed oil 110–126, sunflower oil 136–148, palm oil 50–55, coconut oil 6–11 and lard 45–70.
The method consists in dissolving the test portion in a solvent, adding Wijs reagent (iodine monochloride in acetic acid), and after a defined time — potassium iodide and water, and titrating the iodine liberated with a solution of sodium thiosulfate. Annex B of the standard describes in addition the calculation of the iodine value from the composition of the fatty acids (it does not concern fish oils; cold-pressed, crude, unrefined and hydrogenated oils may give different results by the two methods). The sixth edition ISO 3961:2018 replaces the edition of 2013.
The mass of the test portion must ensure an excess of Wijs reagent of 50–60 % of the amount added (100–150 % of the amount taken up); the standard gives a table of masses for expected iodine values from below 3 (10 g) to 200 (0.127 g at an excess of 150 %). Fats with an iodine value of 20 or less (hard or hardened) are melted and dissolved in warm solvent at 60 °C.
STATUS OF THE STANDARD — CHECKED ON 11.09.2026. The edition PN-EN ISO 3961:2018-09, on which this description is based, was WITHDRAWN on 3 July 2025. The catalogue card indicates the successor expressly: PN-EN ISO 3961:2025-07 — the English version. This is a rare case, because the catalogue does not usually link a withdrawn standard with the next one; here the succession is certain and not inferred from a search by the subject of testing. The description below corresponds to the edition 2018-09 — before referring to the method in a test report, check what the edition 2025-07 has changed.
Method principle
The test portion in a glass boat is placed in a 500 ml conical flask with a ground glass stopper, 20–25 ml of solvent (cyclohexane with glacial acetic acid 50 : 50 v/v) and, with a pipette, 25.00 ml of Wijs reagent are added, after which the flask is closed, mixed and left in the dark: 1 h for an iodine value below 150, 2 h for a value above 150, for polymerised products, for oils with conjugated bonds (for example tung oil) and for heavily oxidised ones. Then 20 ml of a solution of potassium iodide (100 g/l) and 150 ml of water are added and it is titrated with a standardised solution of sodium thiosulfate 0.1 mol/l until the yellow colour has almost disappeared, a few drops of starch solution are added and the titration is continued until the blue colour disappears, with vigorous shaking; a potentiometric detection of the endpoint is permitted. In parallel a blank is carried out. The iodine value: wI = 12.69·c·(V1 − V2)/m, where c is the concentration of the thiosulfate (mol/l), V1 the volume for the blank (ml), V2 that for the sample (ml), m the mass of the test portion (g). Results ≤ 60 are rounded to 0.1, above 60 — to 1.
Applications
- Identification of the kind of fat and control of conformity with the specification (the ranges of the iodine value in the product standards)
- Assessment of the degree of unsaturation of vegetable oils, animal fats and hydrogenated fats
- Control of the process of hydrogenation and refining
- The reference method with respect to the calculation of the iodine value from the composition of the fatty acids (GC)
- Exercises in food analysis — the characteristic values of fats
Key parameters
| Parameter | Value |
|---|---|
| Test portion | According to the table of the standard: 10 g for a value < 3, 0.846–1.586 g for 20, 0.254–0.317 g for 100, 0.127–0.159 g for 200; weighed to 0.001 g (to 0.0005 g at a value ≥ 100) |
| Wijs reagent | 25.00 ml; the I/Cl ratio within the limits 1.10 ± 0.1; an excess of 50–60 % of the amount added |
| Solvent | Cyclohexane : glacial acetic acid 50 : 50 (v/v), 25 ml for a value ≤ 5, 20 ml for higher ones |
| Time of the reaction in the dark | 1 h (an iodine value < 150); 2 h (> 150, polymerised products, conjugated bonds, heavily oxidised ones) |
| Titrant | Sodium thiosulfate 0.1 mol/l, standardised not earlier than 7 days before use |
| Rounding of the result | To 0.1 g/100 g at a value ≤ 60; to 1 g/100 g above 60 |
Standard
- Standard number
- PN-EN ISO 3961:2018-09
- Title (PL)
- Oleje i tłuszcze roślinne oraz zwierzęce — Oznaczanie liczby jodowej
- Title (EN)
- Animal and vegetable fats and oils — Determination of iodine value
Step-by-step procedure
-
Preparation of the sample
Prepare the sample in accordance with ISO 661. If the expected iodine value is not known, carry out preliminary tests with different test portions.
-
Test portion
Weigh into a glass boat a mass from the table of the standard (for example 0.846–1.586 g for a value of 20, 0.254–0.317 g for 100) to the nearest 0.001 g or 0.0005 g; the mass is to ensure an excess of Wijs reagent of 100–150 % of the amount taken up.
-
Dissolution and addition of the reagent
Place the boat with the test portion in a 500 ml flask, add the solvent (25 ml for a value ≤ 5, 20 ml for higher ones) and with a pipette 25.00 ml of Wijs reagent; close it and swirl it. Melt fats with an iodine value ≤ 20 and dissolve them in warm solvent at 60 °C in closed vessels, heating the reagents as well.
-
Blank
Prepare in parallel a flask with the solvent and the reagent without a test portion and carry it through in the same way as the sample.
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Reaction in the dark
Leave the flasks in a dark place for 1 h (an iodine value < 150) or 2 h (> 150, polymerised products, oils with conjugated bonds or keto acids, heavily oxidised products).
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Addition of the iodide and of the water
After the time has elapsed add 20 ml of the potassium iodide solution and 150 ml of water, rinsing the stopper.
-
Titration
Titrate with thiosulfate 0.1 mol/l until the yellow colour of the iodine has almost disappeared, add a few drops of the starch solution and continue the titration until the blue colour disappears, with vigorous shaking; note V2 (the sample) and V1 (the blank). A potentiometric determination of the endpoint is permitted.
-
Calculation and test report
wI = 12.69·c·(V1 − V2)/m [g/100 g]; round to 0.1 (≤ 60) or to 1 (> 60). In the test report give the identification of the sample, the method of sampling, the reference to ISO 3961, the departures and the result; if the repeatability was checked — the final result.
Required equipment and apparatus
| Equipment | Example | Indicative price |
|---|---|---|
| Glass weighing boats | Fitting into the flask; the boat stays in the flask during the reaction | — |
| Conical flasks 500 ml | With ground glass stoppers, without traces of moisture | — |
| Analytical balance | A reading of 0.0001 g, an accuracy of weighing of 0.001 g | — |
| Pipette 25 ml | Automatic (ISO 8655-2) or class A with a bulb — never pipette Wijs reagent by mouth | — |
| Burette 25 ml or 50 ml | A scale division of 0.1 ml, class A; alternatively an autotitrator | — |
| Volumetric flask 1000 ml | Class A, for the thiosulfate solution | — |
Reagents, media and consumables
| Reagent | CAS | Details |
|---|---|---|
| Wijs reagent (iodine monochloride in acetic acid) | 7790-99-0 | Commercial; the I/Cl ratio 1.10 ± 0.1; sensitive to temperature, moisture and light — keep it in the dark below 30 °C, observe the expiry date |
| Cyclohexane | 110-82-7 | The solvent: 50 ml of cyclohexane + 50 ml of glacial acetic acid |
| Glacial acetic acid | 64-19-7 | A component of the solvent |
| Potassium iodide | 7681-11-0 | Solution 100 g/l, without iodates and free iodine |
| Sodium thiosulfate pentahydrate | 10102-17-7 | Standardised solution 0.1 mol/l, standardised not earlier than 7 days before use |
| Soluble starch | — | Mix 5 g with 30 ml of water, add it to 1000 ml of boiling water, boil for 3 min; prepare daily. Water of grade 3 according to ISO 3696 |
Health and safety (OHS)
- Wijs reagent causes severe burns, and its vapours damage the lungs and the eyes — work only under a fume hood, protective goggles, gloves; never pipette by mouth
- Cyclohexane is flammable, glacial acetic acid corrosive — away from sources of ignition, under a fume hood
- The dissolution of hard fats at 60 °C in closed vessels — open the heated flasks carefully
Frequently asked questions
Which standard describes this test?
The test is performed according to PN-EN ISO 3961:2018-09 — “Animal and vegetable fats and oils — Determination of iodine value”.
How does this method work?
The test portion in a glass boat is placed in a 500 ml conical flask with a ground glass stopper, 20–25 ml of solvent (cyclohexane with glacial acetic acid 50 : 50 v/v) and, with a pipette, 25.00 ml of Wijs reagent are added, after which the flask is closed, mixed and left in the dark: 1 h for an iodine value below 150, 2 h for a value above 150, for polymerised products, for oils with conjugated bonds (for example tung oil) and for heavily oxidised ones.
What is the measuring range and accuracy?
Test portion: According to the table of the standard: 10 g for a value < 3, 0.846–1.586 g for 20, 0.254–0.317 g for 100, 0.127–0.159 g for 200; weighed to 0.001 g (to 0.0005 g at a value ≥ 100); Wijs reagent: 25.00 ml; the I/Cl ratio within the limits 1.10 ± 0.1; an excess of 50–60 % of the amount added; Solvent: Cyclohexane : glacial acetic acid 50 : 50 (v/v), 25 ml for a value ≤ 5, 20 ml for higher ones; Time of the reaction in the dark: 1 h (an iodine value < 150); 2 h (> 150, polymerised products, conjugated bonds, heavily oxidised ones).
How long does the test take?
The times given for the individual stages add up to approximately 1 h–2 h. The procedure comprises 8 steps.
What equipment is required?
Glass weighing boats, Conical flasks 500 ml, Analytical balance, Pipette 25 ml, Burette 25 ml or 50 ml, Volumetric flask 1000 ml.
Where is this test used?
Identification of the kind of fat and control of conformity with the specification (the ranges of the iodine value in the product standards); Assessment of the degree of unsaturation of vegetable oils, animal fats and hydrogenated fats; Control of the process of hydrogenation and refining; The reference method with respect to the calculation of the iodine value from the composition of the fatty acids (GC); Exercises in food analysis — the characteristic values of fats.
What safety precautions apply?
Wijs reagent causes severe burns, and its vapours damage the lungs and the eyes — work only under a fume hood, protective goggles, gloves; never pipette by mouth; Cyclohexane is flammable, glacial acetic acid corrosive — away from sources of ignition, under a fume hood; The dissolution of hard fats at 60 °C in closed vessels — open the heated flasks carefully.
Which laboratory can perform this test?
The test is performed by laboratories accredited to ISO/IEC 17025. On LabCoda you can find them by the standard number PN-EN ISO 3961.