One-electron redox kinetics of aqueous transition metal couples Zn, Co, and Ni using pulse radiolysis.

The one-electron redox potentials for aqueous metal couples Co and Ni have been investigated by using pulse radiolysis using their reactions with a series of reference compounds to establish the most positive upper limits of E. Experiments with Zn were also carried out to confirm the characteristic shape of the expected reduction kinetics. Both formate ions and t-BuOH were employed to scavenge ˙OH radicals and ˙H atoms. Kinetics and fitted first and second order reaction rates have been reported for reactions with methyl viologen, fluorescein, Ru(NH), Co(en), Co(sepulcrate), Ru(bpy), Cr(bpy), and Ni(Me[14]4,11-dieneN). Previous work demonstrated that both Co and Ni can be reduced by CO˙ radicals, giving a negative E limit of -1.9 V vs. SHE. A definite reaction of Ni with fluorescein di-anions provides a new upper limit of the Ni couple as -0.906 V vs. SHE. The reaction of Co with Ru(bpy) has been confirmed, giving E = -1.3 V vs. SHE as a rigorous upper limit of the Co couple. In the case of Co, kinetics were complicated by a self-catalyzed metal clustering phenomenon. Initiation rate constants of this process have also been reported.