Oxidations by aquo complexes of cobalt III

The course of the oxidation of V(IIl) by Co(IIl) has been the subject of an investigation by Rosseinsky and Higginson °. Fig. 7 illustrates the dependence of the concentrations of reactants and products upon time when Co(III) is in excess over V(III). Under the conditions applying in Fig. 7, V(III) is oxidised to V(IV) until 90% of V(IIl) has been used up i.e. at 28 min). Then V(V) begins to be produced, and after V(III) has been totally consumed ( 37 min) the reaction corresponds to the oxidation of V(IV) by Co(III). Co(III) reacts with V(III) and V(IV) at a similar rate under the same conditions. It is clear that Co(lll) is reduced by V(IV) as well as by V(ril) during the reaction, and that reduction by 

V(IV) becomes progressively more important as the reaction proceeds. The fact that V(V), the product of oxidation, is not detectable until all the V(III) is consumed is ascribed to the rapid reaction V(III) +V(V) 2 V(IV) with a rate coefficient of 5 X 10 l.mole min at 5 °C, [H ] = 3 M and = 3 M. When the initial concentration of V(III) is greater than that of Co(III), the reaction between V(in) and V(V) is unimportant and 

Values of were derived from plots of this equation using known values of k (see below). At 0 °C, [H ] = 1 Mand = 3 M, is equal to 0.192 l.mole sec k varies inversely with hydrogen-ion concentration, the precise form of the function being uncertain but of the type k = -l-k7[H ]. 

Although the oxidation of Cr(lII) by Ce(lV) is a rapid reaction in perchlorate solutions, the oxidation of Cr(III) by Co(III) in the same media occurs at a rate similar to or slower than that of the thermal decomposition of Co(lll) in perchloric acid (3 M). However, the Co(lII)- -Cr(III) reaction is subject to catalysis by Ag(I) ion and Kirwin et al have made a kinetic study of this system, viz. 

Kinetic data were obtained by following the rate of appearance of Cr(Vl) at 475 m/i, a wavelength where Cr(III), Co(Il) and Co(lII) absorb only slightly. Ag(Il) is said to exhibit an absorption maximum at 475 m/r but does not interfere with the observations since it is present in low concentrations. The concentration of Cr(III) was maintained in excess over Co(III) and Ag(I). Plots of log A -A,) versus time are linear where A and A, are the absorbancies due to Cr(VI) after complete reaction, and after time t. At constant Cr(IlI), Co(II) and Co(lll) 

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