Oxidation rates of cation by Mn

Oxidation Rates of Cations by Mn(III/IV) Oxides 167 Oxidation Kinetics of As(III) 167 Cr(III) and Pu(III/IV) Oxidation Kinetics 169 Supplementary Reading 172... 

OXIDATION RATES OF CATIONS BY Mn(III/IV) OXIDES Oxidation Kinetics of As(III)... 

Specifically adsorbed cations and anions may lower reductive dissolution rates by blocking oxide surface sites or by effecting release of Mn(II) into solution. Stone and Morgan (1984a) found that PO4- considerably inhibited the reductive dissolution of Mn(III/IV) oxides by hydroquinone. For example, addition of 10-2 M PO4- at pH 7.68 resulted in the dissolution rate being only 25% of the rate in the absence of PO4-. The dissolution rate was affected more by PO than by Ca2+. 

Unfortunately, it is far from trivial to obtain oxidation potentials for commonly encountered 17-electron metalloradicals M, because many such radicals dimerize at rates approaching diffusion-control, rendering it nearly impossible to observe such species by cyclic voltammetry. The use of ultramicroelectrodes was shown [41] to give a reversible signal for the oxidation of Mn(CO)5 at scan rates of ca 5000 V s , but the fmther oxidation of this radical to the 16-electron cation was not reported. There are, however, certain frequently encountered systems for which such radicals are stable at least on the time-scale of normal voltammetric measurements. Figure 4 shows an example, the oxidation of CpCr(CO)3 in acetonitrile. 

It has been proposed that the extent to which mixed-cation hydroxide compounds actually do form in aquatic and terrestrial environments is limited more by slow rates of soil mineral dissolution, a necessary preliminary step, than by lack of thermodynamic favorability (57). Because the dissolution rates of clays and oxide minerals are fairly slow, the possibility of mixed-cation hydroxide formation as a plausible "sorption mode" in 24 hour-based sorption experiments (and also most long-term studies) containing divalent metal ions such as Mg, Ni, Co, Zn, and Mn and Al(III)-, Fe(III)-, and Cr(III)-(hydr)oxide or silicate minerals has been ignored in the literature 16,17). This study and others recently published (77), however, suggests that metal sorption onto mineral surfaces can significantly destabilize surface metal ions (A1 and Si) relative to the bulk solution, and therefore lead to an enhanced dissolution of the clay and oxide minerals. Thus, predictions on the rate and the extent of mixed-cation hydroxide formation in aquatic and terrestrial environments based on the dissolution rate of the mineral surface alone are not valid and underestimate the true values. 

Radiochromatographic techniques have been used to determine the rates of oxidation of cysteine by pertechnate ion, Tc04. The technetium(vu) is reduced by the thiol (and cysteine ethyl ester) to form a Tc complex which involves both S- and 7V-co-ordination of the amino-acid. The rate law is first order with respect to both [Tc ] and [RSH]. A hydrogen-ion dependence observed is attributed to the formation of pertechnic acid, the rate-determining step being the nucleophilic attack by the thiol at the metal centre of HTCO4. The oxidation of RSH (R=Et, Pr, or Bu) has been studied over the range 20—40 °C in aqueous alkaline solutions in the presence of metal phthalo-cyanines. The reaction is zero order with respect to [thiol], first order in phthalocyanin and decreases in the order M = Co>Mn> V>Feii. No effects are observed from the nature of the alkali cation. 

There appears to be no significant inner-sphere pathway via the Br end of the molecule since no [Co LBr] " product is detected. In an extension of the work to other metal ions, aminopolycarboxylates, and oxidants it is reported that, whereas the rates of oxidation of [Fe L] by Brg, Bra , and Ig appear to be controlled by the rate at which the oxidant enters the primary co-ordination sphere of the metal ion (Table 4), the rates of oxidation of [Fe L] by Ig and of [Co L] and [Mn L] by Ig, Ig", Brg, and Brg are limited by electron-transfer steps subsequent to co-ordination of the oxidant. Linear free-energy relationships for these reactions have been presented in terms of a Marcus correlation. In the cleavage reactions of alkyl-chromium(m) cations [RCr(HgO)6] + by Brg (R = alkyl, halogeno-alkyl, or 4-pyridinomethyl) the overall stoicheiometric reaction,... 

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