Effect of Formate Concentration on Radical Formation in the Radiolysis of Aqueous Methyl Viologen Solutions

The yields of methyl viologen radical cation (MV •+ ) in the radiolysis of nitrogen-saturated 0.5 mM methyl viologen (MV 2+ ) solutions containing formate were measured using 60 Co γ-rays, 1 H ions of 2−15 MeV, and 4 He ions of 5−20 MeV. The concentrations of formate were 1, 10, 100, and 1000 mM, which correspond to OH radical scavenging capacities of 3.2 × 10 6 to 3.2 × 10 9 s -1 . Methyl viologen radical cation yields should be stoichiometrically equivalent to the sum of the hydrated electron, H atom, and OH radical yields. This outcome is found in γ-ray radiolysis except at very high or low formate concentrations. At all formate concentrations, the 1 H and 4 He ion radiolyses give yields of methyl viologen radical cations that are significantly lower than expected from the results of other aqueous solutions. Possible reasons for the differences are discussed.