Iodometric determination of active oxygen

Iodohexane, 31, 67 Ji-Iodomandelic acid, 35,14 Iodometric determination of active oxygen, 34, 92... 

By definition POV is the number of miliequivalents of active oxygen per kilogram of sample" , or in some cases the number of micrograms of active oxygen in one gram of sample, capable of oxidizing iodide to iodine" °°. Many of the methods described in Section V for determination of hydroperoxide classes or individual compounds can also be applied for determination of POV, as total hydroperoxides. The iodometric determination of hydroperoxides in lipids and proteins has been reviewed . 

Apart from the activation of the anode no reagent has to be produced. Nickel peroxide, however, has to be prepared by oxidation of nickel(II)sulfate with sodium hypochlorite. Subsequently the reagent has to be carefully dried and the amount of active oxygen determined by iodometric titration. This must be kept in mind, because small amounts of alcohol need already a relative large amount of nickel peroxide, e.g. 100 mmol alcohol more than 75 g nickel peroxide. For that reason the use of the relative expensive, commercial nickel peroxide is restricted. 

A solution of dibenzoyl peroxide (10 g) in toluene (20 ml) is cooled, with stirring, to —5° and then treated during 5 min with a pre-cooled (—2°) solution of sodium (2 g) in 96% ethanol (50 ml). Sodium peroxybenzoate separates at once. To this mixture is added ice-water, in which the sodium peroxybenzoate dissolves. The aqueous peroxybenzoate solution is separated from the toluene in a separatory funnel and washed twice with ether which removes residual toluene and ethyl benzoate. This purified aqueous solution is cooled and treated with a mixture of concentrated sulfuric acid (5 g) and ice-water (60 g), and the per-oxybenzoic acid liberated is shaken into chloroform (60 ml in two portions). The chloroform extracts are united and dried over sodium sulfate they may then be used for oxidations. For preparation of crystalline peroxybenzoic acid the chloroform is evaporated in a stream of dry air at ca. 30 mm and 25-30°. The content of active oxygen is determined by iodometric titration. 

Active oxygen content is determined iodometrically 3 In an iodine flask, an accurately weighed sample (0.1-0.3 g.) is dissolved in 20 ml. of an acetic acid-chloroform solution (3 2 by volume), and 2 ml. of saturated aqueous potassium iodide solution is added. The flask is immediately flushed with nitrogen, stoppered, and allowed to stand at room temperature for 15 minutes. Fifty milliliters of water is then added with good mixing, and the liberated iodine is titrated with 0.1 A sodium thiosulfate, employing starch as indicator. A blank titration, which usually does not exceed 0.2 ml., is also run. One milliliter of 0.1 N sodium thiosulfate is equivalent to 0.00821 g. of tetralin hydroperoxide. 

ALTERNATE PROTOCOL DETERMINATION OF PEROXIDE VALUE BY MEASUREMENT OF IRON OXIDATION The ferrous oxidation/xylenol orange (FOX) method is based on the ability of lipid peroxides to oxidize ferrous ions at low pH. The resulting oxidation is quantitated by using a dye that complexes with the generated ferric ions to produce a color that can be measured spectrophotometrically. Peroxide values (PVs) as low as 0.1 meq active oxygen/kg sample can be determined with this method, providing a distinct advantage over iodometric titration. 

Oishi et al. (1992) compared the results from classical iodometric PV determinations of edible oils and fats to those using a coulometric detector. Results from each technique expressed as meq active oxygen/kg sample, were consistent with one another. Typical results were sesame oil (4.1), corn oil (8.7), cottonseed oil (14.5), rapeseed oil (33.2), peanut oil (30.5), olive oil (17.0), palm oil (8.9), beef tallow (2.5), and lard (35.0). 

The determination of the active oxygen in ozonides with sodium iodide in glacial acetic acid gives reliable values only in the case of ketozonides. The reaction products are ketones.96 Iodometric peroxide determination in the case of aldozonides gives less than 60% of the theoretical value 112 carboxylic acids are formed as well as aldehydes. The reduction with iodide ions probably suffers competition from the reaction shown in Eq. (7). 

This reaction is a key part of iodometric titrations, which are used for quantitative determination of oxidants in aqueous samples, such as oxygen saturation in ecological samples or active chlorine in swimming pool water. 

Oxygen-sensitive membrane electrodes are commercially available. The electrode in such a system is covered with an oxygen-permeable plastic membrane, thus protecting it from impurities. The current is proportional to the activity of dissolved molecular oxygen, and at low concentration, to the amount of DO. Before the measurement of DO in the sample, calibrate the electrodes using standards of known DO concentrations (determined from iodometric titrations). 

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