Oxidation by ferric ions

In the presence of sufficient FeY then / i[FeY ]Ar4[FeY ] and the kinetics observed with added FeY result. Phenanthroline removes all Fe(II) from solution thereby suppressing the back-reaction in (71) and the change in order is explained. Cu(II) exerts a catalytic effect, as in the oxidation by ferric ion, by oxidising N2H3, thereby reducing the importance of the back-reaction. 

The sulfide ion may be oxidized by ferric ions to form free sulfur ... 

The effect of EDTA in oxidation reaction is complicated. EDTA is known to accelerate ascorbic acid oxidation by ferric ions at neutral pH (21,25,26). In a medium at the same pH, EDTA also enhanced the formation of hydroxyl radicals by ferric ion in the presence of ascorbic acid (21,25). 

It is known that the redox potential of the Fe+3/Fe+2 pair can vary by complexing ligands (27). EDTA reduces the redox potential of Fe+2 (28) and this increases the rate constant transfer of the electron from Fe+2 to H202, which is formed during autooxidation of ascorbic acid (29), and decomposition of the latter to H0-. However, at low pH 3-4, EDTA was found to inhibit ascorbic acid oxidation by ferric ions (29). Thus, the form the metal chelate takes, as a function of pH, plays a key role in its effectiveness as a catalyst. Cupric ions are known to accelerate ascorbic acid oxidation however, EDTA inhibits its catalytic effect at both neutral and low pH (24). 

Ions and oxides of transition metals which may exist in different valence states have been shown to oxidize thiols. Most of the studies so far available on this topic deal with the oxidation by ferric ions careful investigations with many other metals have been carried out as well. The catalytic effect of these metal ions on the auto-oxidation of thiols has been pointed out (see section IV). The intervention of metals in a number of redox enzymes in which the metal is bound to a thiol group at the active site of the enzyme has been also suggested. 

Motekaitis determined the kinetics of hydrolysis and ammonolysis of EDTA at temperatures of interest for chemical cleaning activities. The rate increases with decreasing pH in the presence of ammonia. The rates are high enough at temperatures above 160°C (320°F) to affect the commercial use of ammonium EDTA. The molecule is cleaved at the C—N and C—C bonds. EDTA is rapidly oxidized by ferric ions. Motekaitis also determined that ferric EDTA would oxidize other EDTA molecules rapidly at temperatures as low as 125 C (257°F). The major reaction cuts off carboxy-late groups in a sequential fashion. NTA is oxidized more slowly than EDTA. 

A small solvent isotope effect was found by Bell and Onwood kuiolkoiO = 1.08) in contradiction to that of only 0.38 reported by Taylor and Halpern . Over one-third of the oxygen present in the carbonate originated from the oxidant when 0-labelled permanganate was used . The reaction is subject to pronounced catalysis by ferric ions . 

DeAtley WW (1970) Spectrophotometric determination of diphenylamine, 2-nitrodipheny-lamine, and 4-nitrodipheylamine by oxidation with ferric ion. Anal Chem 42(6) 662-664... 

Hydroxylamine (NH2 OH) is oxidized by ferric iron in boiling sulphuric acid - an oxide of nitrogen being amongst the products. 25.00 cm3 of a solution of hydroxylamine (2.00 g dm 3) were boiled with an excess of ferric chloride in dilute sulphuric acid. 30.30 cm3 of potassium permanganate solution (0.0200 M) were required to reoxidize the ferrous ions produced. Deduce the identity of the oxide of nitrogen. 

The oxidation of xanthenol and fluorenol showed a number of differences from the oxidation of benzhydrol. No induction period was observed (Figure 5). The rate was enhanced by ferric ion, and the stoichiometry was altered to 0.5 mole of oxygen per mole of fluorenol (Figure 5), apparently because of Reaction 25. 

A similar mechanism was invoked by Ohshima and Kawabata (2) to account for their results in the nitrosation of tertiary amines and amine oxides. In applying these concepts to the nitrosative dealkylation of tetraalkyltetrazenes, Michejda al. 5) introduced an interesting variant by suggesting that immonium ions could be formed in two successive one-electron oxidation steps (for example by ferric ion oxidation of tertiary amine to the radical cation followed by radical abstraction of a hydrogen atom from the alpha position), rather than exclusively through the one-step removal of a hydride ion as nitroxyl. The resulting immonium ion was again considered to react directly with nitrite to produce the N-nitroso derivative. These reactions are summarized in Fig. 2b. 

Oxidation reactions by ferric ions have been shown, with very few exceptions (20), to proceed by a mechanism involving the formation of a complex with the oxidized ligand,... 

The electron transfer in the excited state is reversed when the molecule returns to the ground state, the leuco dye being oxidized back by ferric ion. A similar system includes mixed inorganic solutions such as (I2/I ) + (Fes+/Fe2+). In such electron transfer reactions, the acceptor has a much lower electron affinity in the ground state than the donor. Thus, the... 

The mechanism of hydrazine decomposition has been studied much less than that of hydrogen peroxide, and the mechanism actually is not known. We may take some clues however from studies of its oxidation in aqueous solution by metal salts, in which kinetic and isotope labelling techniques were used. As the main mechanism for the oxidation of hydrazine by ferric ion the following was proposed 57) ... 

The /V-formylmethionine of a nascent protein synthesized in bacteria is removed by the sequential activities of PDF and a methionine aminopeptidase to generate the mature protein. The gene encoding PDF was cloned and overexpressed in E. coli by Meinnel and coworkers (1993). The PDF enzyme has an unusual metal ion (Fe2+) as its catalyst. However, the ferrous ion in this enzyme is unstable and can be quickly and irreversibly oxidized to ferric ion, rapidly inactivating the enzyme. PDF-based assay development therefore depended on the ability of nickel ion to replace ferrous ion in vitro, increasing the stability of the enzyme and maintaining its enzymatic activity (Groche et al., 1998 Clements et al., 2001 Hackbarth et al., 2002). 

Weibel D B etal., 2005a, Oxidation of Coal by Ferric Ion at 100°C as the Basis for a Coal Fuel Cell. Angewandte Chemie, 117, 5828-5832. 

Iron can assume the oxidation states - -2, -f 3, and +6, the last being rare, and represented by only a few compounds, such as potassium ferrate, KoFeO. The oxidation states -f 2 and +3 correspond to the ferrous ion, Fe+ +, and ferric ion, Fe + +, respectively. The ferrous ion is easily oxidized to ferric ion, by air or other oxidizing agents. Both ferrous and ferric ion form complexes, such as the ferrocyanide... 

The reaction is approximately first-order with respect to each reactant (the second-order rate coefficient increases with increase of substrate concentration), and catalysis by hydroxide ions is observed. Henderson and Winkler studied the ferrous ion-catalysed oxidation of thioglycolicacidto dithioglycolic acid. The rate is sensitive to traces of metal ions, and reproducible results could not be obtained in the absence of the catalyst. The oxidation is first-order with respect to both peroxodisulphate and ferrous ions, and zero-order with respect to the substrate. The second-order rate coefficient is approximately equal to that determined in the absence of the substrate, so Henderson and Winkler suggested that the ratedetermining step is the oxidation of ferrous to ferric ions, as in reaction (96), and that this is followed by reaction (97) and then rapid oxidation of thioglycolic acid by ferric ions. 

Holmes P. R. and Cmndwell F. K. (2000) The kinetics of the oxidation of pyrite by ferric ions and dissolved oxygen an electrochemical study. Geochim. Cosmochim. Acta 64, 263 -274. 

Bromide and Iodide, The spectrophotometric determination of trace bromide concentration is based on the bromide catalysis of iodine oxidation to iodate by permanganate in acidic solution. Iodide can also be measured spectrophotometricaUy by selective oxidation to iodine by potassium peroxymonosulfate (KHSO ). The iodine reacts with colodess leucocrystal violet to produce the highly colored leucocrystal violet dye. Greater than 200 mg/L of chloride interferes with the color development. Trace concentrations of iodide are determined by its abiUty to catalyze ceric ion reduction by arsenous acid. The reduction reaction is stopped at a specific time by the addition of ferrous ammonium sulfate. The ferrous ion is oxidized to ferric ion, which then reacts with thiocyanate to produce a deep red complex. 

For quantitative studies in radiation chemistry, it is essential that the energy input into the irradiated volume should be accurately determined. For this purpose, the most versatile and reliable method is the ferrous sulfate dosimeter, proposed by Fricke and Morse. The method involves the use of an air-saturated solution of 10 M ferrous sulfate and 10 M sodium chloride in 0.8 N sulfuric acid. On exposure of the solution to ionizing radiations, the ferrous ion is oxidized to ferric ion, which may conveniently be determined accurately by spectrophotometry. The amount of chemical change is proportional to the total energy-input, independent of dose rate, and (within wide limits) independent of the concentration of ferrous ion, ferric ion, and oxygen. The main reactions involved are as follows. 

Ascorbic acid is a strong two-electron reducing agent that is readily oxidized in one-electron steps by metal ions and metal complexes in their higher valence states. An inner sphere mechanism for the stoichiometric oxidation of ascorbic acid by ferric ion in acid solution is illustrated by Scheme 1(8). The first step in the reaction is the formation of a monoprotonated Fe(III) complex similar to the monoprotonated ascorbate complexes listed in Table I. The intermediate monoprotonated Fe(III) complex is short-lived and rapidly undergoes an intramolecular one-electron transfer to give a deprotonated Fe(II) complex of the ascorbate radical anion, indicated by 7. This complex dissociates to the free radical anion, which may then combine with a second ferric ion to form the complex 9. Complex 9 in turn undergoes a second intramolecular electron... 

Scheme 1. Direct oxidation of ascorbic acid by ferric ion.

In the presence of an organism with iron-oxidizing ability such as T. ferro-oxidans, ferrous ions produced by the oxidation of a metallic sulfide can be re-oxidized to ferric ions and a cyclic mechanism established (eqn (11)) ... 

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