Nitric acid, autocatalytic oxidation

Oxidation of [Fe(phen)3] + by concentrated nitric acid is autocatalytic. " T e hen)3] + reacts with bromate by rate-determining oxidation at high bromate concentration, [Fe(bipy)3] + and ligand-substituted [Fe(phen)3] + cations react with perbromate by rate-limiting dissocia-587 Reactions of [Fe(diimine)3] + with peroxodiphosphate also involve rate-limiting dissocia-... 

In his work on the nitration kinetics, which has already been referred to, Mar-tinsen [145] found that in the nitration of phenol with nitric acid, nitrous acid, if present, acted as a catalyst on the nitration process. The presence of nitrous acid was necessary for initiating the reaction, further quantities being formed in the reaction of oxidation of phenol with nitric acid, which accompanied the nitration reaction. Thus the reaction of nitrating phenol is an autocatalytic one. 

Orban and Epstein (1982-1) studied the autocatalytic oxidation of Fe(II) in nitric acid in a CSTR. In this preliminary work, although the possibility still remains, they did not observe oscillations. 

Autocatalytic behavior is a common observation in nitric acid oxidations of various substrates in mixed acids, (5, 6) and generally the catalyst has been found to be nitrous acid. Acid catalysis, or an increase in oxidation rate over at least some portion of the acidity ranges studied, is also commonly observed. 

The kinetics and mechanism of the autocatalytic oxidation of [Fe(phen)3] , [Fe(bipy)3], and [Fe(H20)6] complexes by nitric acid have been examined. 

The autocatalytic oxidation reactions of nitric acid have been studied by Bazsa et al They have examined the oxidation of formaldehyde, and suggest a rate-determining step involving H2C(OH)2 and N2O4. A much more complex system is the oxidation of bromide. The reaction has been studied in both forward and backward directions, and has been found to exhibit bistability when studied in a continuous-flow stirred tank reactor. A detailed mechanism has been proposed [see Eqs. (25)-(31)] together with values for the rate constants. The reverse reaction, the oxidation of nitrous acid by bromine, gives rate law (24), a much simpler... 

A. Komlosi, G. Porta, and G. Stedman [Int J. Chem. Kinet., 27,911-917 (1995)] studied the autocatalytic oxidation of the hydrated form of formaldehyde hy nitric acid ... 

Hydrazine is also used in conjunction with HNA to impede hydroxylamine oxidation by nitrous acid, always present in nitric acid media, which increases the HNA availability for the plutonium reduction. Both hydrazine and HNA interact with nitrous acid (HNO2), but the hydrazine oxidation is much faster. On the other hand, the use of HAN in conjunction with nitric acid introduces the possibility of an autocatalytic reaction resulting in the overpressurization of the system or explosion in a reprocessing facility as pointed out in Barney s report (Barney, 1998). The main function of HNA is to reduce plutonium from the tetravalent state to the trivalent state and, thus, separate the plutonium from the uranium, which is retained in the hexavalent oxidation state and, hence, in the organic phase. The reduction reaction by HNA is the result of two irreversible reactions (Equations 14.10 and 14.11) ... 

Substitution and oxidation can often both be involved in reactions of tris(diimine)-iron(II) complexes with oxidizing agents. Thus, for example, reaction with hydrogen peroxide involves rate-determining dissociation as the first step. Similarly, initial dissociation seems to be the first step in the predominant pathway for superoxide oxidation of the [Fe(phen)3] cation. Dissociation may also be involved in reactions of diimine-iron(II) complexes with nitrous acid. Here and elsewhere it is recognized that these complexes react with nitric acid—in the initial stages aquation may be the only important path, but autocatalytic redox processes usually become dominant before aquation is complete, especially for the more easily oxidizable ligands and complexes. ... 

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