Concentration and oxidation rate

Measurement of Concentration and Oxidation Rate of S(IV) in Rainwater in Yokohama, Japan... 

The dissolution of passive films is, in the main, controlled by a chemical activation step in contrast to film-free conditions at. Many protective anodic films are oxides and hydroxides whose dissolution depends upon the hydrogen ion concentration, and the rate follows a Freundlich adsorption equation ... 

The chromic acid oxidation of dithionic acid is independent of oxidant concentration and its rate is equal to that of the acid-catalysed hydrolysis to sulphite and sulphate, which must therefore constitute the rate-determining process . ... 

The reduction of persulphate by tris-[a-(2-pyridyl)-benzylideneaniline] iron(II) is, by contrast, independent of persulphate ion concentration , and the rates of reaction of several ring-substituted complexes of this type correspond exactly to the rates of acid-catalysed separation of one ligand. Clearly oxidation of the ligand... 

A recent study by Lee and Stewart confirms the previous observations that with rather low acid concentrations the oxidation rate of H2R depends only on the acidity of the medium and not on the nature of the proton-supplying mineral acid. On the contrary, at rather high acid concentrations the rate of oxidation depends not only on the acidity but also on the nature of the acids. This can be explained by the fact that the hydrogen chromate and the acid present interact with each other... 

The border conditions between chain and nonchain mechanisms of oxidation depends not only on temperature but also on the hydrocarbon concentration and the rate of chain initiation. The following equation describes this dependence ... 

The stability constants of the mono- and bis-complexes between Cu(II) and catecholate were determined under anaerobic conditions and were found to be the same as reported earlier, i.e. log p1 = 13.64 (CuC) and log p2 24.92 (CuC2+) (36,39). A comparison of the speciation and oxidation rate as a function of pH clearly indicated that the mono-catechol complex is the main catalytic species, though the effect of other complexes could not be fully excluded. The rate of the oxidation reaction was half-order in [02] and showed rather complex concentration dependencies in [H2C]0, [Cu(II)]0 and pH. The experimental data were consistent with the following rate equation ... 

Reagents to carry out the alkaline oxidation of the toxins were prepared as originally described ( ) with the exception that nitric acid was used in place of acetic acid. The reagent concentrations and flow rates are shown in Table I. 

Phenolic compounds naturally occurring in plants have induced many physiological responses that duplicate those reported for ozone and/or peroxyacetylnitrate (PAN). Chlorogenic acid is a competitive inhibitor of lAA-oxidase (35) and plant growth is adversely affected by increased concentrations of auxins (36). Concentrations of chlorogenic acid are increased in tobacco tissue exposed to ozone ( ) Phenols inhibit ATP synthesis (37), oxidative phosphorylation ( ) and SH enzyme activity (27) they increase respiration (38), reduce CO2 fixation (22), modify both membrane permeability (40) and oxidation rate of reduced NADH... 

Let us now consider what we mean by the reactivity of a hydrocarbon in autoxidation. One measure is how fast it oxidizes by itself at unit concentration and unit rate of initiation. (Rates of thermal oxidation at unknown rates of initiation are not useful enough to be considered.) The first two columns of figures in Table VII give such comparisons in terms of kp/(2kt)l/l at 30° and 60°C. and determine the order in which hydrocarbons are listed in the table. The results of Ingold and Sajus agree fairly well the orders of reactivity are identical except for a trivial difference with the xylenes. The stated quotients at 60°C. are uniformly 2 to 3 times as large as at 30°C. for sec-butylbenzene and more reactive hydrocarbons but 3 to 6 times as large for less reactive hydrocarbons. 

There have been many attempts to relate bulk electronic properties of semiconductor oxides with their catalytic activity. The electronic theory of catalysis of metal oxides developed by Hauffe (1966), Wolkenstein (1960) and others (Krylov, 1970) is base d on the idea that chemisorption of gases like CO and N2O on semiconductor oxides is associated with electron-transfer, which results in a change in the electron transport properties of the solid oxide. For example, during CO oxidation on ZnO a correlation between change in charge-carrier concentration and reaction rate has been found (Cohn Prater, 1966). 

Carlier fundamental studies of autoxidations of hydrocarbons have concentrated on liquid-phase oxidations below 100 °C., gas-phase oxidations above 200°C., and reactions of alkyl radicals with oxygen in the gas phase at 25°C. To investigate the transitions between these three regions, we have studied the oxidation of isobutane (2-methylpropane) between 50° and 155°C., emphasizing the kinetics and products. Isobutane was chosen because its oxidation has been studied in both the gas and liquid phases (9, 34, 36), and both the products and intermediate radicals are simple and known. Its physical properties make both gas- and liquid -phase studies feasible at 100°C. where primary oxidation products are stable and initiation and oxidation rates are convenient. 

As Table 19 shows, the product composition resulting from metal ion oxidation is highly variable. The reaction course depends not only on the metal ion and its ligands, but also on the concentration and addition rate of the reactants. The key intermediates in these reactions are the (3-propionate radicals, e.g. 140, which may be formed from highly unstable cyclopropyloxy radicals. The radical 140 has, in fact, been observed and characterized by ESR upon reaction of 1-methoxycyclopropanol with ceric sulfate.112 ... 

The PCFC technique utilizes traditional oxygen depletion calorimetry. The specimen is first heated at a constant rate of temperature rise (typically 1-5 K/s) in a pyrolyzer. The thermal decomposition products are swept from the pyrolyzer by an inert gas. The gas stream is mixed with oxygen and enters a combustor at 900°C, where the decomposition products are completely oxidized. Oxygen concentrations and flow rates of the combustion gases are used to determine the oxygen depletion involved in the combustion process, and the heat release, as well as the heat release capacity (HRC), is determined from these measurements. 

TEA masks the catalytic effect of metallic impurities found in the rain water. The concentrations and the rates of oxidation of S(IV) in rain waters from Yokohama, Japan measured by this method were 0.8-23.5 yM (16 samples) and 0.12-3.3 hr 1 (8 samples), respectively. 

Similarly, phenothiazine may be oxidized to the cation radical species which then dimerizes forming the 3,10 -diphenothiazinyl species (Tsujino, 1969). The product of the electron-transfer step may react, via a second-order process, with a species in solution to form a new product. An example of this type of mechanism involves the reduction of anthraquinone and its derivatives in the presence of oxygen (Jeziorek etal., 1997). To understand quantitatively an EC and EC2 process, the concentration and scan-rate dependence of the associated cyclic voltammograms is matched with theory deriving from the mass transport/kinetic equations for each species. 

It would appear from these series of investigations of nitric oxide preparation in plasma devices that, although reasonable conversions of oxygen to nitric oxide can be obtained under conditions of excess nitrogen, the final concentrations and production rates are low. The best conversions quoted in the foregoing are equivalent to approximately 200 kWh/lb nitric oxide produced, which is extremely high. The use of... 

Ward, B. B. (1985). Light and substrate concentration relationships with marine ammonium assimilation and oxidation rates. Mar. Chem. 16, 301—316. 

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