Critical levels, oxidation

The rate of H2 escape from the upper atmosphere and thus oxygenation from decomposition of H2O vapour is directly dependent on the density of H2 at the critical level of escape. It may be that the Great Oxidation Event from 2.4 Ga to 2.0 Ga reflects elevated H2 in the hydrosphere / atmosphere during much of this period. 

We believe that the primary reason of deactivation is the formation of irreversibly adsorbed species and oxygen poisoning occurs when the blocking of active sites reaches a critical level. By-products can be formed during the oxidation reaction. A frequently observed side-reaction is the aldol-dimerization of the carbonyl compound and a further oxidation of the product. The process is catalyzed by bases, including the basic functional groups of a carbon support (25). 

There seems to be special promise in oxidizing liquefied hydrocarbon gases at temperatures and pressures approximating critical levels. That such reactions are highly effective is attested, for example, by the liquid-phase oxidation of butane, one of the simplest and most efficient methods of producing acetic acid and methyl ethyl ketone. 

Supercritical fluids, most commonly carbon(IV) oxide, occasionally modified by a small addition of a polar solvent (methanol, acetonitrile, or water). Supercritical fluid extraction (SEE) uses water as the most popular additive, because increasing the temperature from 50 to 400 °C at a pressure exceeding the critical level makes it possible to achieve transition of extractant from the subcritical to the supercritical state and leaching of the compoimds in the order of polar to moderately polar [86]. 

Once the aldehyde concentration builds up to a critical level (and even in the oxidation of secondary alcohols, small quantities of aldehydes are produced by side-reactions), then autocatalysis is observed. It is suggested that the aldehydes bring about chain-branching by reacting directly with oxygen... 

Muramoto S, Nishizaki H, Aoyama I. The critical levels and the maximum metal uptake for wheat and rice plants when applying metal oxides to soil. J Environ Sci Health B. 1990 25(2) 273-80. 

Tocopherols are important fat-soluble vitamins. They contribute to the stability of an oil and have an important role as quenchers of free radicals in vivo. Blekas and co-workers (1995) and Blekas and Boskou (1998) examined the role of a-tocopherol and its contribution to olive oil triacylglycerol stability. They found that a-tocopherol acts as an antioxidant at all levels but the antioxidant effect is greater at low (100 mg/kg) than at higher concentrations (500 and 1000 mg/kg). In the presence of more effective antioxidants such as o-diphenols, a-tocopherol did not show any significant antioxidant activity during the period of low peroxide accumulation but acted well when the primary oxidation products reached a critical level. 

After the initial work on asymmetric sulfonated polyphenylene oxide membranes, sulfonated polysulfone was subsequently examined by several research groups. However, the appearance of patents in 1973 and 1977 by Rhone-PoulencM 8S covering asymmetric membranes of sulfonated polysulfones appeared to dampen such research efforts. Also, a basic problem with sulfonated polysulfone was the fact that salt rejections fell rapidly when a critical level of sulfonation was exceeded, but water fluxes were too low until this sulfonation level was exceeded. 

In fungi, xylose is reduced to xylitol by NADH- or NADPH-dependent xylose reductase (XR) and thereafter is oxidized to xylulose by NAD -dependent xylitol dehydrogenase (XDH). The xylulose is phosphorylated, channeled into the pentose phosphate pathway [3]. XR of most fungi, including most yeasts, prefers NADPH to NADH. Because of the cofactor preference of XR (NADPH) and XDH (NAD, redox imbalance occurs under anaerobic condition [4]. Therefore, the oxygen-limited rather than anaerobic condition is ideal for bioconversion of xylose to ethanol, so that the accumulated reduced cofactor can be oxidized to reach redox balance. A critical level of oxygen should exist for the highest ethanol yield and productivity. 

A common method of predicting product lifetimes assumes a single, thermally activated degradation process, with rate given by Eq. (11.8). If failure occurs when oxidation or hydrolysis reaches a critical level, the failure time is given by... 

The exponential increase of oxidation products, when stabiliser concentration falls below a well defined critical level, is, however, only an artefact in this model consideration due to the incomplete treatment of the reaction equations. The termination reaction Eq. 5.13 can no longer be neglected at low stabiliser concentrations which would slow down an exponential increase. In reaUty, the rise of oxidation rate is not sudden but gradual, and no sharp transition can be observed at a certain critical stabiliser concentration. On the one hand it is fairly reasonable for the interpretation and description of experimental data to separate the phase of stabiliser consumption from the phase of incipient auto-oxidation. On the other hand, such a separation should, in a strict sense, not be considered as one specified by a well-defined stabiliser concentration. 

To ensure that the temperature of the gas stream leaving the top of the furnace does not fall below a critical level where re-oxidation of zinc occurs, some air is introduced to bum CO and generate additional heat ( top air ). Generally the critical temperature to prevent re-oxidation is around 950°C. 

It remains uncertain, however, whether oxidative alterations of biomembranes exerted by ascorbate is related to toxicity rather than to physiological events. Ohyashiki et al. (1994) followed the Fe +Zascorbate/t-butyl hydroperoxide-induced oxidation of membrane thiols, a consequence of massive lipid peroxidation, by the incorporation of fluorescence dyes. The thiol reactivity of the membrane proteins appeared to change characteristically before and after a critical level of lipid peroxidation occurred. If lipid peroxidation and oxidation of membrane thiols are indeed involved in various ascorbate effects, a shift from benefit to toxicity observed in ascorbate-treated cultured cells might just represent two sides of the same coin. 

The clinical significance of in vivo oxidation continues to be a controversial topic for total hip replacement. In vivo oxidation, as a solitary factor, does not govern clinical failure modes of wear, osteolysis, and late implant loosening observed in historical, cemented total hip replacements (Chapter 5). The mechanical behavior of the UHMWPE liner in THA is but one variable in a complex multifactorial problem (Chapter 5). On the other hand, there is clear evidence of critical levels of oxidation in retrieved... 

The apparently superior resistance of the Haynes 214 honeycomb foils may, at first sight, appear to be contrary to what would be expected, since this alloy has even less almninium than the FeCrAlY materials tested, i.e. 4.5% compared to 5.5-5.8%. Therefore,the aluminium level should be depleted below a given critical level sooner in the case of the Haynes 214. However, PM2000, Aluchrom YHf and PM2Hf are iron-based alloys whereas Hornes 214 is anickel-based material. As breakaway oxidation results largely from die very rapid formation of oxides which are more defective/less protective than AI2O3, the slower growth of Ni oxides (compared to Fe oxides) will favour the Ni-based alloy. Other factors that may also contribute to the observed difference in behaviour include ... 

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