Abstract
Iron in the +5 oxidation state was generated by the reduction of aqueous potassium ferrate(VI), K{sub 2}FeO{sub 4}, using pulse radiolytically generated free radicals. The predominant mode of decay is first-order, in the pH range 3.6 to 7, with the rate constant decreasing from 7 x 10{sup 4} s{sup {minus}1} to about 100 s{sup {minus}1}. The instability of K{sub 2}FeO{sub 4} under the conditions used required the use of the premix pulse radiolysis technique in which the pulse is delivered 100-200 ms after mixing of K{sub 2}FeO{sub 4} solution with buffers. The rate law and the observation of small pH-dependent spectral shifts indicate that ferrate(V) exists in at least three protonated forms that, formulated as tetrahedral species, form the following equilibria: H{sub 3}FeO{sub 4} {rightleftharpoons} H{sub 2}FeO{sub 4}{sup {minus}} + H{sup +}, 5.5 {le} pK{sub 1} {le} 6.5; H{sub 2}FeO{sub 4}{sup {minus}} {rightleftharpoons} HFeO{sub 4}{sup 2{minus}} + H{sup +}, pK{sub 2} {approx} 7.2; HFeO{sub 4}{sup 2{minus}} {rightleftharpoons} FeO{sub 4}{sup 3{minus}} + H{sup +}, pK{sub 3} = 10.1. The rate-limiting process in the first-order decay is the aquation of a tetrahedral ferrate(V) species into a species which is probably six-coordinate. The subsequent first order decay of the octahedral species is somore » fast that a second-order decay mode for H{sub 3}FeO{sub 4} and H{sub 2}FeO{sub 4}{sup {minus}} is not observed. The species present in alkaline solution, FeO{sub 4}{sup 3{minus}} and HFeO{sub 4}{sup 2{minus}}, aquate very slowly ({le}10 s{sup {minus}1}) decay predominantly by a second-order process.« less
Published Version
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