Commission Regulation (EEC) No 2568/91Show full title

ANNEX XVI U.K. DETERMINATION OF IODINE VALUE

1. SCOPE U.K.

This International Standard specifies a method for the determination of the iodine value of animal and vegetable fats and oils, referred to hereafter as fats.

2. DEFINITION U.K.

For the purposes of this International Standard, the following definition applies:

2.1. iodine value. The mass of iodine absorbed by the sample under the operating conditions specified in this International Standard. U.K.

The iodine value is expressed as grams of iodine per 100 g of sample.

3. PRINCIPLE U.K.

Dissolution of a test portion in solvent and addition of Wijs reagent. After a specified time, addition of potassium iodide solution and water, and titration of the liberated iodine with sodium thiosulfate solution.

4. REAGENTS U.K.

All reagents shall be of recognized analytical grade:

4.1. water, complying with the requirements of ISO 3696, Grade 3. U.K.

4.2. potassium iodide, 100 g/l solution, not containing iodate or free iodine. U.K.

4.3. starch, solution. U.K.

Mix 5 g of soluble starch in 30 ml of water, add this mixture to 1 000 ml of boiling water, boil for three minutes and allow to cool.

4.4. sodium thiosulfate, standard volumetric solution c (Na₂S₂O₃.5H₂O) = 0,1 mol/l, standardized not more than seven days before use. U.K.

4.5. solvent, prepared by mixing equal volumes of cyclohexane and acetic acid. U.K.

4.6. Wijs reagent, containing iodine monochloride in acetic acid. Commercially available Wijs reagent shall be used. U.K.

5. APPARATUS U.K.

Usual laboratory apparatus and, in particular, the following:

5.1. glass weighing scoops, suitable for the test portion and for inserting into the flasks (6.2). U.K.

5.2. conical flasks, of 500 ml capacity, fitted with ground glass stoppers and completely dry. U.K.

6. PREPARATION OF THE TEST SAMPLE U.K.

The homogenized sample is dried over sodium sulphate and filtered.

7. PROCEDURE U.K.

7.1. Test portion U.K.

The mass of the test portion varies according to its expected iodine value as shown in Table 1.

Table 1
Expected iodine value	Mass of test portion(g)
less than 5	3,0
5 to 20	1,0
21 to 50	0,4
51 to 100	0,2
101 to 150	0,13
151 to 200	0,1

Weigh the test portion to the nearest 0,1 mg in a glass weighing scoop (5.1).

7.2. Determination U.K.

Place the test portion in a 500 ml flask (6.2). Add 20 ml of the solvent (4.5) to dissolve the fat. Add exactly 25 ml of the Wijs reagent (4.6), insert the stopper, swirl the contents and place the flask in the dark. Do not use a mouth pipette for the Wijs reagent.

Similarily, prepare a blank with the solvent and the reagent but omitting the test portion.

For samples having an iodine valve below 150, leave the flasks in the dark for one hour; for those with an iodine value above 150 and for polymerized products or products oxidized to a considerable extent, leave for two hours.

At the end of the time, add 20 ml of the potassium iodide solution (4.2) and 150 ml of water (4.1) to each of the flasks.

Titrate with the standard volumetric sodium thiosulfate solution (4.4) until the yellow colour due to iodine has almost disappeared. Add a few drops of the starch solution (4.3) and continue the titration until the blue colour just disappears after very vigorous shaking.

Note: U.K.

Potentiometric determination of the end point is permissible. U.K.

7.3. Number of determinations U.K.

Carry out two determinations on the same test sample.

8. EXPRESSION OF RESULTS U.K.

The iodine value is given by the expression

where:

c

=

is the numerical value of the exact concentration, in moles per litre, of the standard volumetric sodium thiosulfate solution (4.4) used;

V₁

=

is the numerical value of the volume, in millilitres, of the standard volumetric sodium thiosulfate solution (4.4) used for the blank test;

V₂

=

is the numerical value of the volume, in millilitres, of the standard volumetric sodium thiosulfate solution (4.4) used for the determination;

m

=

is the numerical value of the mass, in grams, of the test portion (7.1).

Take as the result the arithmetic mean of the two determinations, provided that the requirement for repeatability (9.2) is satisfied.