Fixation of oxygen

The authors suggest that this is more than a purely physical fixation of oxygen, being in all probability the outcome of a physicochemical attraction between oxygen and carbon. Physical, inasmuch as it seems hardly possible to assign any definite molecular formula to the complex formed, which, indeed, shows progressive variation in composition chemical, in that no isolation of the complex can bo effected by physical means. 

Carbons outgassed at high temperatures and exposed to oxygen between 200 and 700 °C have been shown to adsorb appreciable amounts of strong bases but very little of strong acids. The optimum temperature for development of maximum capacity to adsorb bases is the same as that for the maximal fixation of oxygen (approximately 400 C). 

From the reaction cycle of cytochrome P-450, three main steps can be defined (1) the formation of an enzyme-substrate complex that can be monitored by the spin variation of the ferric enzyme (2) the reduction of Fe3+—S, either enzymically or chemically (Fe3+—S - Fe2+—S and (3) the fixation of oxygen to give a ternary oxycompound by addition of oxygen to the ferrous form Fe2+—S + 02 — Fe2s—02. 

The effect on adsorption of the fixation of oxygen surface complexes is shown in Table 8. In addition to the decrease in V stated before, the adsorption is less energetic after the treatment [28-30] because partial blocking of the pores hinders the access of pores with similar dimension to the molecular size of the adsorbates. Moreover, it is interesting to point out that benzene is adsorbed more strongly on the original than on the oxidized... 

Saturation and Oxidation of Double Bonds It is known that radicals can directly add on double bonds [32], which leads in turn to saturation of the double bonds and formation of a new radical. After fixation of oxygen, a peroxy radical is formed followed an hydroperoxide by the classical hydrogen abstraction. Thermal and photochemical decomposition of the hydroperoxides gives alkoxy radicals (Scheme 15.4). 

Provided they reach equilibrium fast enough at measuring temperature (- 1600 C) and do not react with the solid electrolyte, metal-oxide couples allow fixation of oxygen pressure to desired value. The equilibrium can be written ... 

The large surplus of maganese(/7) hydroxide forces the formation of manganese(///) instead of manganese(/V). Owing to the instability of manganese(/7) in an alkaline medium the fixation of oxygen is rapid and quantitative. 

This process is sometimes called fixation of oxygen). 

The D.O. ill a sample is utilised to oxidise to [MnO(OH)2 Mn02 H20 reaction 3.52 or 3.52 a] in alkaline medium. This process is known as fixation of oxygen. When acidifed, MnO(OH)2 gives back an equivalent amount of oxygen ... 

The mechanism of the oxidation of sulfide sulfur to sulfate is probably much more complex than a simple fixation of oxygen. Fromageot and Royer (51) have presented the hypothesis that the oxidation of hydrogen sulfide to sulfate involves formation of thiosulfate as intermediary. They base this hypothesis on the constant presence of thiosulfate in the urine of higher animals, and also on the ease with which hydrogen sulfide is oxidized to thiosulfate by inorganic catalysts in the presence of air (106). 

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