ANNEX IIIU.K.DETERMINATION OF PEROXIDE VALUE
1.SCOPEU.K.
This Standard describes a method for the determination of the peroxide value of oils and fats.
2.FIELD OF APPLICATIONU.K.
This Standard is applicable to animal and vegetable oils and fats.
3.DEFINITIONU.K.
The peroxide value is the quantity of those substances in the sample, expressed in terms of milliequivalents of active oxygen per kilogram, which oxidize potassium iodide under the operating conditions described.
4.PRINCIPLEU.K.
Treatment of the test portion, in solution in acetic acid and chloroform, by a solution of potassium iodide. Titration of the liberated iodine with standardized sodium thiosulphate solution.
5.APPARATUSU.K.
All the equipment used shall be free from reducing or oxidizing substances.
Note:U.K.
Do not grease ground surfaces.U.K.
5.1.3 ml glass scoop.U.K.
5.2.Flasks, with ground necks and stoppers, of about 250 ml capacity, dried beforehand and filled with a pure, dry inert gas (nitrogen or, preferably, carbon dioxide).U.K.
5.3.25- or 50-ml burette, graduated in 0,1 ml.U.K.
6.REAGENTSU.K.
6.1.Chloroform, analytical reagent quality, freed from oxygen by bubbling a current of pure, dry inert gas through it.U.K.
6.2.Glacial acetic acid, analytical reagent quality, freed from oxygen by bubbling a current of pure, [X1dry inert gas] through it.U.K.
6.3.Potassium iodide, saturated aqueous solution, recently prepared, free from iodine and iodates.U.K.
6.4.Sodium thiosulphate, 0,01 or 0,002 [X1Mol/L accurately] standardized aqueous solution, standardized just before use.U.K.
6.5.Starch solution, 10 g/l aqueous dispersion, recently prepared from natural soluble starch.U.K.
7.SAMPLEU.K.
Take care that the sample is taken and stored away from the light, kept cold and contained in completely filled glass containers, hermetically sealed with ground-glass or cork stoppers.
8.PROCEDUREU.K.
The test shall be carried out in diffuse daylight or in artificial light. Weigh in a glass scoop (5.1) or, failing this, in a flask (5.2), to the nearest 0,001 g, a mass of the sample in accordance with the following table, according to the expected peroxide value:
| Expected peroxide value(meq) | Weight of test portion(g) |
|---|---|
| 0 to 12 | 5,0 to 2,0 |
| 12 to 20 | 2,0 to 1,2 |
| 20 to 30 | 1,2 to 0,8 |
| 30 to 50 | 0,8 to 0,5 |
| 50 to 90 | 0,5 to 0,3 |
Unstopper a flask (5.2) and introduce the glass scoop containing the test portion. Add 10 ml of chloroform (6.1). Dissolve the test portion rapidly by stirring. Add 15 ml of acetic acid (6.2), then 1 ml of potassium iodide solution (6.3). Insert the stopper quickly, shake for one minute, and leave for exactly five minutes away from the light at a temperature from 15 to 25 °C.
Add about 75 ml of distilled water. Titrate the liberated iodine with the sodium thiosulphate solution [X1(6.4) (0,002 Mol/L solution for expected values less than 12, and 0,01 Mol/L solution] for expected values above 12) shaking vigorously, using starch solution (6.5) as indicator.
Carry out two determinations on the same test sample.
Carry out simultaneously a blank test. If the result of the blank exceeds 0,05 ml of [X10,01 mol/L sodium] thiosulphate solution (6.4), replace the impure reagents.
9.EXPRESSION OF RESULTSU.K.
The peroxide value (PV), expressed in milliequivalents of active oxygen per kilogram, is given by the formula:
where:
=
the number of ml of the standardized sodium thiosulphate solution (6.4) used for the test, corrected to take into account the blank test;
=
the weight in g, of the test portion.
Take as the result the arithmetic mean of the two determinations carried out.