REACTION OF FERRATE(VI) FERRATE(V) WITH HYDROGEN-PEROXIDE AND SUPEROXIDE ANION - A STOPPED-FLOW AND PREMIX PULSE-RADIOLYSIS STUDY/

The reduction of ferrate(VI) to ferrate(V) by superoxide ions was stud ied over the pH range 2.6-13.0 using the premix pulse radiolysis techn ique. The pH dependence indicates that only Me unstable protonated for ms of ferrate, H2FeO4 (pK(a) 3.5) and HFeO4- (pK(a) 7.3) are reactive, k(HFeO4- + O-2(-)) = (1.7 +/- 0.2) X 10(7) M(-1)s(-1). The stable fer rate ion, FeO42-, showed no significant reactivity towards either hydr ogen peroxide or superoxide anion. The rate constants for the spontane ous dimerization and decomposition of the protonated ferrates, e.g. k( HFeO4- + HFeO4-) approximate to 250 M(-1)s(-1), are orders of magnitud es lower than their corresponding reduction by superoxide indicating a n outer-sphere mode of electron transfer for the latter process. In co ntrast the ferrate(VI) species H3FeO4+ (pK(a) = 1.6 +/- 0.2), H2FeO4, and HFeO4- oxidize hydrogen peroxide, e.g. k(HFeO4- + H2O2) = 170 M(-1 )s(-1)), at rates which correspond closely to their dimerization rates suggesting an inner-sphere controlled mechanism.