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).
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.
Potentiometric determination of the end point is permissible.U.K.
Carry out two determinations on the same test sample.