Hydroperoxides titration methods

Dialkyl peroxides (continued) colorimetry, 707-8 flame ionization detection, 708 NMR spectroscopy, 708 titration methods, 707 UV-visible spectrophotometry, 707-8 enthalpies of reactions, 153-4 graft polymerization initiation, 706 hydroperoxide determination, 685 peroxide transfer synthesis, 824-5 stmctural characterization, 708-16 electrochemical analysis, 715-16 electron diffraction, 713 mass spectrometry, 714 NMR spectroscopy, 709-11 thermal analysis, 714-15 vibrational spectra, 713-14 X-ray crystallography, 711-13 synthesis... 

TOCO see Thiol-olefin co-oxygenation Tocopherols, TEARS assay, 668 Torsion angles, hydroperoxides, 690 Tosylhydrazones, superoxide reactions, 1036 Total hydroperoxides see Peroxide value Total oxidative capacity, titration methods, 674 Total polar phenols, colorimetry, 664 Toxicity... 

Iodometric Titration Method Iodometric titration assay, which is based on the oxidation of the iodide ion (1 ) by hydroperoxides (ROOH), is the basis of current standard methods for determination of PV (9). In this method, a samrated solution of potassium iodide is added to oil samples to react with hydroperoxides. The liberated iodine (I2) is then titrated with a standardized solution of sodium thiosulfate and starch as an endpoint indicator (7, 9, 20). The PV is obtained by calculation and reported as milliequivalents of oxygen per kilogram of sample (meq/kg). The official determination is described by lUPAC (21). Chemical reactions involved are given below ... 

Carter et al. [402] have addressed the chemical assessment of automotive clearcoats (usually melamine-cross-linked systems), which requires evaluation of the cross-linker type, HALS and UV absorbers. Coating systems require a variety of chemical analytical techniques for their evaluation [403], including UV microspectroscopy [402, 404], /xFTIR [402,405], /xRS [406,407], NMR [408], ESR [409], ToF-SIMS [410,411] and hydroperoxide titration [412]. Ideally, what is needed for industrial evaluation purposes is a set of techniques that can follow chemical changes in individual layers of a full automotive paint system, typically consisting of 45 /xm clearcoat, 25 /xm basecoat, 35 /xm primer, and 35 /xm E-coat on metal. The clearcoat must shield underlayers from UV. Unlike the case for IR radiation, examination of 30 /xm and thinner clearcoat layers with 0.3 to 0.4 /xm radiation lends itself quite well to the use of microscopes and microspectroscopic techniques. UV microspectroscopy of 10 /xm paint system cross-sections is the method of choice cfr. also Chp. 1.1). UV microspectroscopy... 

The water (moisture) content can rapidly and accurately be determined in polymers such as PBT, PA6, PA4.6 and PC via coulometric titration, with detection limits of some 20 ppm. Water produced during heating of PET was determined by Karl Fischer titration [536]. The method can be used for determining very small quantities of water (10p,g-15mg). Certified water standards are available. Karl Fischer titrations are not universal. The method is not applicable in the presence of H2S, mercaptans, sulfides or appreciable amounts of hydroperoxides, and to any compound or mixture which partially reacts under the conditions of the test, to produce water [31]. Compounds that consume or release iodine under the analysis conditions interfere with the determination. 

The formation and role of hydroperoxide groups, particularly in the early stages of polymer oxidation is well discussed in the introduction to the next chapter and also features in many of the references cited in this chapter. Their detection and quantification is therefore important. Although this can be done directly or implicitly through many of the instrumentation techniques discussed in this chapter, there are several tests that have been developed, some of which are still widely used, that are based more on chemical methods, titration or staining. The majority have been applied to polyolefins, especially polyethylene. 

Oxidative stress Lipid oxidation Oxygen absorption Manometric, polarographic Diene conjugation HPLC, spectrophotometry (234 nm) Lipid hydroperoxides HPLC, GC-MS, chemiluminescence, spectrophotometry Iodine liberation Titration Thiocyanate Spectrophotometry (500 nm) Hydrocarbons GC Cytotoxic aldehydes LPO-586, HPLC, GC, GC-MS Hexanal and related end products Sensory, physicochemical, Cu(II) induction method, GC TBARS Spectrophotometry (532-535 nm), HPLC Rancimat Conductivity F2-iP GC/MS, HPLC/MS, immunoassays... 

Iodine liberation is one of the oldest and most commonly used methods for assessing lipid substrate oxidation. In this method, hydroperoxides and peroxides oxidize aqueous iodide to iodine, which is then titrated with standard thiosulfate solution and starch as endpoint indicator. The peroxide value is calculated as milliequivalents of peroxide oxygen per kilogram of sample. 

Hydroperoxides may be determined by measuring at 290 nm (e = 44100 M cm ) or 360 nm (e = 28000 cm ) the concentration of 13 formed in the presence of a large excess of ions. The reaction may be too slow for practical purposes, unless a catalyst is present. For example, an assay for lipid hydroperoxides conducted without a catalyst may require several measurements every 6 min until the absorbance reaches a maximum. Exclusion of air from the sample cuvette is important. The method is about 1000-fold more sensitive than thiosulfate titration The iodometric method with UVD at 360 was adopted for detecting the presence of hydroperoxides derived from protein, peptide or amino acid substrates subjected to y-radiation, after destroying the generated H2O2 with catalase. ... 

Oxidation indices, 656-72 peroxide determination, 762-3 peroxide value, 656, 657-64 colorimetry, 658-61 definition, 657 direct titration, 657 electrochemical methods, 663-4 IR spectrophotometry, 661-3 NIR spectrophotometry, 663 UV-visible spectrophotometry, 658-61 secondary oxidation products, 656, 665-72 tests for stability on storage, 664-5, 672 thermal analysis, 672 Oxidative amperometiy, hydroperoxide determination, 686 Oxidative cleavage alkenes, 1094-5 double bonds, 525-7 Oxidative couphng, hydrogen peroxide determination, 630, 635 Oxidative damage... 

Metal Catalyzed Reactions of a Cyclohexene Solution of Cyclohexenyl Hydroperoxide, V. Solutions of cyclohexenyl hydroperoxide in cyclohexene were prepared by the methods of Gould and Rado (24) and Van Sickle et al. (41). In either case a solution approximately 0.7-0.8M in cyclohexenyl hydroperoxide is obtained (24, 41). Smaller concentrations of VI (—0.01 M), VII (0.09M), and VIII (0.06M) are also present in solution (24, 41). A solution of cyclohexenyl hydroperoxide (8.0 mmoles by iodometric titration) in 10 ml of cyclohexene was rapidly added to 0.20 mmole of the metal complex and heated with stirring under nitrogen at 70°C for 2 hrs. Metal complexes used were [C5H5Fe(CO)2]2, [C5H5Mo(CO)3]2, and [C5H5V(CO)4]. The reaction mixture was then quickly vacuum transferred at 80°C/0.01 mm. Little or no residue remained. Yields in mmoles of the products in solution were obtained by GLPC analysis of the vacuum transferred reaction mixtures. Correction was made for the amount of the product initially present (24, 41). Results are listed in Table VI. 

Peroxide value. The oxidation of oils and fats leads to the formation of hydroperoxides. The hydroperoxides readily decompose to produce aldehydes, ketones, and other volatile products, which are characteristic of oxidation rancidity. The method for determination of peroxide concentration is based on the reduction of the hydroperoxide group with HI (or KI) to liberate free iodine, which may be titrated. The... 

Analytical methods for measuring hydroperoxides in fats and oils can be classified as those determining the total amount of hydroperoxides and those based on chromatographic techniques giving detailed information on the structure and the amount of specific hydroperoxides present in a certain oil sample (8). The PV represents the total hydroperoxide content and is one of the most common quality indicators of fats and oils during production and storage (9, 18). A number of methods have been developed for determination of PV, among which the iodometric titration, ferric ion complex measurement spectrophotometry, and infrared spectroscopy are most frequently used (19). 

Consequently, in addition to hydroperoxides, a lot of secondary oxidized lipidic compounds, mainly short-chain aldehydes, may appear and represent late markers of lipid peroxidation. As an example, malondialdehyde (MDA, Fig. 6) is known to be the most abundant lipid peroxidation aldehyde whose determination by 2-thiobarbituric acid (TBA) is one ofthe most common assays in lipid peroxidation studies [20]. However, it can be noticed that the TBA assay method [21] is not specific of MDA titration since it also can detect a variety of peroxides and secondary degradation products of lipid peroxidation called... 

Hydroperoxide determination by iodometric titrations are quite common in the literature, the essential differences in the methods lies in the end point determination [Mielewski et al., 1989]. The propagation mechanism in the standard UV degradation process involves the production of hydroperoxides by the abstraction of a hydrogen atom from the polymer by the peroxy radical. 

One of the most commonly used methods for measuring rancidity is the peroxide value (POV), which is expressed in milliequivalents of oxygen per kilogram of fat or 011. This test is performed by iodometry, based on the reduction of hydroperoxide group (ROOH) with the iodide ion (I ). The concentration of the present peroxide is proportional to the amount of the released iodine (I2), which is assessed by titration against a standardized solution of sodium thiosulphate (Na2S203), using a starch indicator (Reactions 12.16 and 12.17) ... 

This method is one of the oldest and most commonly used measurements of the extent of oxidation in oils. The standard iodometric procedures measure by titration, or colorimetric or electrometric methods, the iodine produced by potassium iodide added as a reducing agent to the oxidized sample dissolved in a chloroform-acetic acid mixture. The liberated iodine is titrated with standard sodium thiosulfate to a starch endpoint. The peroxide value (PV) is expressed as milliequivalents of iodine per kg of lipid (meq/kg), or as millimole of hydroperoxide per kg of lipid (referred to as peroxide). PV expressed as meq/ Kg = 2 X PV mmol/kg. 

This method, described by T. Sarraf, involves a free radical scavenger diphenyl picryl hydrazil, DPPH. DPPH, stable at room temperature, is added in excess to the ozonized sample and reacted at 110°C. The cleavage of peroxides and hydroperoxides to RO and HO generates free radicals which are captured by DPPH. After precipitation of polyethylene, the excess of DPPH in the filtrate is back-titrated by colorimetry at 520 nm, and the amount of initial peroxides and hydroperoxides can be obtained from the result. 

Aralkyl hydroperoxides (ROOH) were purified according to Ref. [9]. Their purity (98.9%) was controlled by iodometry method. Tetraalkylammonium bromide (Et NBr) was reerystallized from acetonitrile solution by addition of diethyl ether exeess. The salt purity (99.6%) was determined by argentum metric titration with potentiometric fixation of the equivalent point. Tetraalkyl ammonium bromide was stored in box dried with P2O3. Acetonitrile (CH3CN) was purified according to Ref. [10]. Its purity was... 

Peroxide Value, The method for determination of peroxide concentration is based on the reduction of the hydroperoxide group with HI or Fe +. The result of the iodometric titration is expressed as the peroxide value. The Fe " method is more suitable for detemuning a low hydroperoxide concentration since the amount of the resultant Fe +, in the form of the ferrithiocyanate (rhodanide) complex, is determined photometrically with high sensitivity (Fe-test in Table 14.27). The peroxide concentration reveals the extent of oxidative deterioration of the fat, nevertheless, no relationship exists between the peroxide value and aroma defects, e. g. rancidity (already existing or anticipated). This is because hydroperoxide degradation into odorants is influenced by so many factors (cf. 3.7.2.1.9) which mdkc its retention by fat or oil or its further conversion into volatiles unpredictable. 

The advantages and limitations of these methods in solid polymers have been reviewed [683]. Table 10.7 gives a comparison of the different methods of titration with the determination of hydroperoxide groups by a spectroscopic method in which SO2 is used. (cf. section 10.17.2.3) The sulphur dioxide (SO2) method can be carried out easily, but with some polymers secondary effects are observed which perturb the measurements. 

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