Peroxide value Acetic Acid-Isooctane Method. Firestone, D. Plant oils are a good source of compounds that decrease the risk of some diseases. Growing consumer awareness has led to more interest in natural cold-pressed plant oils, which are often considered functional foods.
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The substances are generally assumed to be peroxides or other similar products of fat oxidation. This method is highly empirical, and any variation in the test procedure may result in erratic results.
Because this method gives erratic results at peroxide values 70, this method should not be used with the AOM test, AOCS Official Method Cd , with which peroxide values 70 may be encountered. KI in mL water and rotate to mix. Allow to stand for 5 min and then add mL of distilled water.
Titrate with sodium thiosulfate solution, shaking continuously until yellow color has almost disappeared. Add 12 mL of starch indicator and continue the titration, adding the thiosulfate solution slowly until the blue color just disappears. The strength of the sodium thiosulfate solution is expressed in terms of its normality.
Erlenmeyer flaskswith glass stoppers, mL. Burette25 mL or 50 mL, class A, graduated in 0. Balancetop loading, g capacity with 0. Potassium iodide KI solutionsaturated, prepared fresh each day analysis is performed by dissolving an excess of KI in recently boiled distilled water. Make certain the solution remains saturated during use, as indicated by the presence of undissolved KI crystals. Store in the dark when not in use. Test the saturated KI solution by adding 2 drops of starch solution to 0.
If a blue color is formed that requires more than 1 drop of 0. Weigh 0. Sodium thiosulfate solution0. This solution may be prepared by accurately pipetting mL of 0. Starch indicator solutiontested for sensitivity, prepared by making a paste with 1 g of starch see Notes, 1 and a small amount of cold distilled water.
Add, while stirring, to mL of boiling water and boil for a few seconds. Immediately remove from heat and cool. Salicylic acid 1. If long storage is required, the solution must be kept in a refrigerator at C. Fresh indicator must be prepared when the end point of the titration from blue to colorless fails to be sharp.
If stored under refrigeration, the starch solution should be stable for about 23 weeks. Test for sensitivityPlace 5 mL of starch solution in mL of water and add 0. Cd 8b Peroxide Value pared 0. The deep blue color produced must be discharged by 0. Weigh the test portion Table 1 into a mL Erlenmeyer flask with glass stopper and add 50 mL of the acetic acidisooctane solution. Swirl to dissolve the sample. Add 0. Allow the solution to stand for exactly 1 min, thoroughly shaking the solution at least three times during the 1 min, and then immediately add 30 mL of distilled water.
Titrate with 0. Continue the titration until the yellow iodine color has almost disappeared. Continue the titration with constant agitation, especially near the end point, to liberate all of the iodine from the solvent layer.
Add the thiosulfate solution dropwise until the blue color just disappears see Notes, 3 and 4. Conduct a blank determination of the reagents daily. The blank titration must not exceed 0.
Melt the sample by heating with constant stirring on a hot plate set at low heat, or by heating in an air oven at C. Avoid excess heating and particularly prolonged exposure of the oil to temperatures above 40C.
When completely melted, remove the sample from the hot plate or oven and allow to settle in a warm place until the aqueous portion and most of the milk solids have settled to the bottom.
Decant the oil into a clean beaker and filter through a. Whatman no. Do not reheat for filtration unless absolutely necessary. The sample must be clear and brilliant. Proceed as directed in Procedure for Fats and Oils, paragraphs The values presented may not be applicable to matrices other than those presented and may not be representative for other concentrations.
RepeatabilityThe difference between two test results on the same material, in the same laboratory under the same conditions, should not exceed the repeatability value, r.
ReproducibilityThe difference between two test results on the same material, under the same conditions in different laboratories, should not exceed the reproducibility value, R. Explosive limits in air are 1. It is toxic by ingestion and inhalation. A properly operating fume hood should be used when working with this solvent.
Acetic acid in the pure state is moderately toxic by ingestion and inhalation. It is a strong irritant to skin and tissue. The TLV in air is 10 ppm. Potassium dichromate is toxic by ingestion and inhalation. It is a strong oxidizing agent and a dangerous fire risk when in. Table 2 The results of international collaborative tests held between and see References, 5. Table 4 Test on lard, tallow and beef fat.
Potato Starch for Iodometry is recommended, because this starch produces a deep blue color in the presence of the iodonium ion. Soluble Starch is not recommended because a consistent deep blue color may not be developed when some soluble starches interact with the iodonium ion. Baker This delay is due to the tendency of isooctane to float on the surface of an aqueous medium, and the time necessary to adequately mix the solvent in large volumes of aqueous titrant, thereby liberating the last traces of iodine.
Based on collaborative study results References, 1,2 , the recommendation is to use 0. Erratic results reported for this method, especially at higher peroxide values, appear to be related to the isooctane floating on the surface of the aqueous layer References, 3. Rapid mechanical stirring e. If the titration is less than 0. Analysts may use 0. The test should be carried out in diffuse daylight or in artificial light shielded from a direct light source References, 4. Brooks, D. Brophy, B.
Hayden, and G. The International Standards Organization ISO successfully completed an international collaborative study of this method in The results are presented in ISO Learn more about Scribd Membership Home.
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It is a strong oxidizing agent and a dangerous fire risk when in Table 2 The results of international collaborative tests held between and see References, 5. Page 3 of 3. Elias Dammak.
AOCS CD 8b - 90 Peroxido