Oxygen uptake measurement

Table 10.8 Comparison oE antioxidants in polyethylene in both the absence and presence of copper powder and carbon black (data based on ICI literature). Induction time assessed from oxygen uptake measurements using a Barcroft manometer...

Figure 3. Arrhenius plots for the formation of formaldehyde or acetaldehyde from methanol or ethanol, normalized by the number of vanadiums (open symbols) and by the amount of oxygen uptake measured at 625 K (filled symbols). Lines on the right panel are calculated from the data reported by Oyama and Somorjai [11].

Thermal reduction at 623 K by means of CO is a common method of producing reduced and catalytically active chromium centers. In this case the induction period in the successive ethylene polymerization is replaced by a very short delay consistent with initial adsorption of ethylene on reduce chromium centers and formation of active precursors. In the CO-reduced catalyst, CO2 in the gas phase is the only product and chromium is found to have an average oxidation number just above 2 [4,7,44,65,66], comprised of mainly Cr(II) and very small amount of Cr(III) species (presumably as Q -Cr203 [66]). Fubini et al. [47] reported that reduction in CO at 623 K of a diluted Cr(VI)/Si02 sample (1 wt. % Cr) yields 98% of the silica-supported chromium in the +2 oxidation state, as determined from oxygen uptake measurements. The remaining 2 wt. % of the metal was proposed to be clustered in a-chromia-like particles. As the oxidation product (CO2) is not adsorbed on the surface and CO is fully desorbed from Cr(II) at 623 K (reduction temperature), the resulting catalyst acquires a model character in fact, the siliceous part of the surface is the same of pure silica treated at the same temperature and the anchored chromium is all in the divalent state. 

In OIT determinations, the test samples are flushed continuously with oxygen during the course of the measurement [1], This is in contrast to oxygen-uptake measurements that are mostly undertaken in a static oxygen atmosphere (see below). Effects such as dynamic removal of volatile antioxidants in the OIT method can lead to these two methods yielding very different assessments of a polymer formulation s stability [1]. 

Oxygen uptake measurements can give considerable mechanistic information. In normal peroxyl radical reactions the G(02-uptake) will range between 3 x 10 7 and 6 x 10 7 mol J1 (Table 8.2). The lower value will be found when half of the 02 is reformed (for example, the formate system), the higher value when all 02 is consumed during the decay of the peroxyl radicals. As soon as G(02-uptake) exceeds the upper limit of 6 x 10 7 mol J, a chain reaction must prevail [examples are some polymers including poly(U)]. However, there is also the interesting situation that G(02-uptake) is below 3 x 10 7 mol JThis means that some of the radicals do not react with 02. A case in point are the OH-induced reactions of purines (Chap. 10.3). 

Analysis of the reaction rates is achieved in terms of oxygen uptake measured with an oxygen electrode, or by product separation and quantification. DPBF absorbs intensely at 415 nm and reacts rapidly with singlet oxygen to form a colorless intermediate endoperoxide. The DPBF reaction can be used as a benchmark against which the effect of an added quencher is compared. A note of caution must be applied the use of inhibitors and quenchers alone is not unambiguous in its outcome and should be strictly supplemented with flash photolysis experiments. Thus, if a photosensitized reaction is quenched by millimolar concentrations of azide ion, it should also be established that azide does not quench the triplet state of the sensitizer directly because that would also affect the reaction rate. 

Oxygen-Uptake Measurements. Combinations of phenyl-substituted phenols and / -activated thioethers were tested in unstabilized isotactic PP according to the method described by Pospisil, Taimr, and Kotulak (20). Powdered PP (1.000 g) was mixed with 2 mg of the phenol and 5 mg of the thioether. This mixture was homogenized with 5 mL of acetone. The solvent was evaporated carefully in vacuo at 40°C and the sample was placed in a tube connected witn a gas burette filled with oxygen. The oxygen uptake was measured at regular time intervals at 180°C (see Table I). 

The relative rates of autoxidation of different unsaturated fatty acids and esters were compared on the basis of oxygen absorption measurements (Table 1.1). In neat systems without added initiator, linoleate was 40 times more reactive than oleate, linolenate was 2.4 times more reactive than linoleate, and arachidonate was 2 times more reactive than linolenate. The oxidizability of polyunsaturated fatty acid (PUFA) esters was also compared on the basis of oxygen uptake measured kinetically by the induction period method described above, in solution in the presence of azo initiators. The oxidizability of 18 2,18 3,20 4, and 22 6 was linearly related to the number of bis-aUyUc positions present in the fatty esters. From this relationship, the oxidizability of each PUFA was increased approximately two fold for each active bis-allylic methylene group. Thus, the oxidizability of 22 6 was 5 times greater than that of 18 2. 

The adsorption of linoleic acid onto silica gel from petroleum ether solution conformed to a Langmuir isotherm, consistent with the formation of a monolayer (Porter etal. 1972). Confirming the finding of Honn etal. (1951) with soybean oil, it was found that the most rapid uptake of oxygen occmred at a linoleic acid-silica ratio close to that for the monolayer. Without included antioxidant, oxidation commenced at a nearly linear rate without observable induction period. Time for consumption of one-half mole of oxygen per mole of linoleic acid was ca. 60 min on acid-washed silica. If very small amounts of a-tocopherol were included in the layer, virtually no oxygen uptake measurable in this system occurred during the induction pe-... 

DMA solutions of the analogous iridium(I) complex, [IrQ(C8Hi4)2]2 absorb O2 irreversibly with expulsion of free cyclooctene to the solution [480]. No evidence of any olefin oxidation products was found. Oxygen uptake measurements in benzene solution gave O2 Ir ratios of 0.6 to 1.1 depending on the conditions, and the reaction products exhibited IR bands at 830 cm (coordinated dioxygen) and 1260cm" (possibly the OH of hydroperoxide). This iridium system was not effective at ambient conditions for olefin oxidation. 

In the past, oxygen-uptake measurements at elevated temperatures were used to determine oxidation resistance. Today, other types of testing, such as oxidative induction time (OIT), by differential scanning calorimetry or chemiluminescence, are used. 

The evaluation of thermal degradation may be suggestively done by the comparison between chemiluminescence and oxygen uptake measurements (Figs. 22a and b) which both of them are based on the chemical modifications caused by oxygen diffusion. They similarly allow to order several polymeric materials based on their stability. Ozawa s group found for some polyolefins the following sequence of stability [9001] ... 

This sequence is in a good agreement with the results obtained by oxygen uptake measurements [6801]. 

Thermal oxidation of plastics can be assessed by various methods, amongst which heat measurement (DSC and DTA). Accelerated methods such as DTA and DSC and oxygen uptake measurements have been used quite extensively in studies of thermal oxidative stability of plastics [113]. The definition of thermal stability is very vague and is interpreted differently. Nikulicheva et al. [114] have summarised the diversity of methods of thermal stability determination using TA methods. Problems associated with the use of thermal analysis to determine the thermal stability of plastics have been discussed in detail [115,116]. 

Oxygen Uptake Measurements to Identify the Cause of Unexpected Differences between Accelerated and Outdoor Weathering, Die Angewandte Makromolekulare Chemie,... 

STUDIES OF THE KINETICS OF OXIDATION BY OXYGEN UPTAKE MEASUREMENTS... 

The use of oxygen uptake measurements is a very common method for studying the kinetics of polymer photo-oxidation (and/or thermal photooxidation). A number of oxygen uptake measuring devices have been described in the literature. The oxygen consumption can be measured at ... 

One of the simplest oxygen uptake measuring devices is shown in Figure 10.35. Before irradiation, oxygen is introduced and its pressure is controlled on the U-manometer. The quantity of oxygen consumed on irradiation is directly derived from the equation ... 

Fig. 4.5. Influence of the sucrose concentration in the external medium upon the rate of respiration of cultured tomato roots. The roots were grown in presence of 2% sucrose, maintained in a sugar-free solution for 48 hr and then transferred to the test media and their oxygen uptake measured for 1 hr commencing 67 hr after transference to the test medium.

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