Oxidation of Mn ion

Rather than natural ores as in Leclanche batteries, electrolytic manganese dioxide (EMD), which is produced by anodic oxidation of Mn ions at graphite electrodes in solutions of manganese salts, is used as the active material for the positive... 

In a mechanism proposed by Hoganson and Babcock (Fig. 23-35) four successive transfers, each of one H+ + one e, leads to a three-electron oxidation of Mn ions, e.g., from the 2+ and 3+ oxidation states to all Mn4+, and to joining of the two water oxygens to form a manganese peroxide linkage. Oxidation of the peroxide dianion to 02 by the adjoining Mn4+ and Mn3+ ions completes the cycle. This mechanism is hypothetical, and various alternatives have been pre-... 

Manganese dioxide can be accumulated on the surface of a platinum disk electrode by oxidation of Mn " ions at 0.9 V vs SCE in 0.1 mol/L NH4CI [4,44]. The precipitate is subsequently reduced back to Mn at 0.3 V. This reaction is the best example of electroprecipitation on noble metal electrodes [2,45,46]. 

One-electron oxidation of carboxylate ions generates acyloxy radicals, which undergo decarboxylation. Such electron-transfer reactions can be effected by strong one-electron oxidants, such as Mn(HI), Ag(II), Ce(IV), and Pb(IV) These metal ions are also capable of oxidizing the radical intermediate, so the products are those expected from carbocations. The oxidative decarboxylation by Pb(IV) in the presence of halide salts leads to alkyl halides. For example, oxidation of pentanoic acid with lead tetraacetate in the presence of lithium chloride gives 1-chlorobutane in 71% yield ... 

Pipette 25 mL of the solution containing magnesium, manganese and zinc ions (each approx. 0.02M), into a 250 mL conical flask and dilute to 100 mL with de-ionised water. Add 0.25 g hydroxylammonium chloride [this is to prevent oxidation of Mn(II) ions], followed by 10 mL of the buffer solution and 30-40 mg of the indicator/potassium nitrate mixture. Warm to 40 °C and titrate (preferably stirring magnetically) with the standard EDTA solution to a pure blue colour. 

The apparent first-order rate coefficient obtained using excess oxidant increased exponentially with increase in acidity in the range 5 N < [H30" ] < 12 N. The reaction is first-order with respect to added manganous ions (k increasing sharply), but the activation energy (11.0 kcal.mole ) remains unchanged. At appreciable catalyst concentrations the reaction becomes almost zero-order with respect to bromide ion. The mechanism appears to be a slow oxidation of Mn(II) to Mn(III) followed by a rapid reduction of the latter by bromide. This reaction is considered further in the section on Mn(II)-catalysis of chromic acid oxidations (p. 327). 

Mn(II) oxidation is enhanced in the presence of lepidocrocite (y-FeOOH). The oxidation of Mn(II) on y-FeOOH can be understood in terms of the coupling of surface coordination processes and redox reactions on the surface. Ca2+, Mg2+, Cl, S042-, phosphate, silicate, salicylate, and phthalate affect Mn(II) oxidation in the presence of y-FeOOH. These effects can be explained in terms of the influence these ions have on the binding of Mn(II) species to the surface. Extrapolation of the laboratory results to the conditions prevailing in natural waters predicts that the factors which most influence Mn(II) oxidation rates are pH, temperature, the amount of surface, ionic strength, and Mg2+ and Cl" concentrations. 

This paper discusses the oxidation of Mn(II) in the presence of lepidocrocite, y-FeOOH. This solid was chosen because earlier work (18, 26) had shown that it significantly enhanced the rate of Mn(II) oxidation. The influence of Ca2+, Mg2+, Cl", SO,2-, phosphate, silicate, salicylate, and phthalate on the kinetics of this reaction is also considered. These ions are either important constituents in natural waters or simple models for naturally occurring organics. To try to identify the factors that influence the rate of Mn(II) oxidation in natural waters the surface equilibrium and kinetic models developed using the laboratory results have been used to predict the... 

All the ions studied, except phthalate, inhibit the oxidation of Mn(II) to some degree. The relative extent to which these ions (at the concentrations indicated) affect the rate of Mn(II) oxidation is as follows ... 

Hence, the passage of Mn between octahedral sites via an intermediate tetrahedral site (i.e., the open Oh Td Oh path of Figure 2) is expected to be greatly facilitated when the Mn can take on a +2 valence in the tetrahedral site. The amount of Mn ions that can become +2 is determined by the average degree of Mn oxidation which is determined by the Li content. 

A common method of synthesizing M-substituted oxides, particularly goethite and hematite is to add base to mixed M-Fe salt solutions to precipitate M-associated ferrihydrite. Most ions do not change their oxidation state, but incorporation of Mn and Co in goethite is preceded by oxidation of these ions to the trivalent state (Giovanoli Cornell, 1992). An indication of whether isomorphous substitution has occurred can be obtained from changes in the unit cell dimensions of the Fe oxides... 

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