Track effects in radiation chemistry. Concentration dependence for the scavenging of OH by ferrocyanide in N₂O-saturated aqueous solutions

The radiation chemical yield for oxidation of ferrocyanide by OH produced in the fast electron radiolysis of N₂O-saturated aqueous solutions is found to increase from a low concentration limiting yield of 5.2 to values in excess of 6.7 for ferrocyanide concentrations above 0.1 M. This observed dependence reflects competition between the scavening reaction and track recombination processes involving OH radicals which occurs on a time scale greater than 10^-9 s. By extrapolation an additional yield of ∼1.5 is estimated for OH radicals which recombine at still shorter times. These scavenging studies show that significant amounts of recombination occur within the track even at times as long as 10^-6 s. Secondary radicals produced within the track are also subject to recombination processes as is illustrated by the low concentration limit which shows that most OH radicals produced from N₂O within the track do not escape from the track. In general, for concentrations of reactive solutes greater than ∼10^-4 M, i.e., for k[S] > 10⁶ s^-1, scavenging of OH from the track will contribute significantly to the observed yields and this effect must be taken into account in comparing the results from different radiolytic systems.