Effect of temperature and scavenging power on the sum of G(e -aq) + G(H) + G(ȮH) in the range 20–200°C—A pulse radiolysis study

Abstract

The effect of temperature on the sum σ G = G(e - aq) + G(H) + G(·H) has been measured in N 2O saturated and Ar-saturated formate solutions for 2 × 10 -3 ⩽[HCO - 2]⩽ 1.0 mol dm -3 and 2.5 × 10 -3 ⩽[N 2O] ⩽ 3.6 × 10 -1 mol dm -3 in the temperature range 20–200°C. In most experiments 2 × 10 -4 mol dm -3 MV 2+ was added to the solutions to convert CO⋅ - 2 to MV⋅ + since MV⋅ +, unlike CO⋅ddot; - 2, has an absorption spectrum that shows little change with temperature. At low scavenger power for both ⋅ dot;OH and e - aq, where σ G is a measure of e - aq, H and OH that escape from the spurs, σ G increases from 0.60 μmolJ -1 to 0.76 μmolJ -1 at 200°C. Correspondingly less spur scavenging of ⋅OH by HCO - 2 occurs at the higher temperatures when the scavenging power of N 2O and MV 2+ for e - aq is low. These effects are attributed to an increase in spur size with temperature. σ G increases more with temperature when the scavenging power of both N 2O and HCO - 2 is high, but there is an unexpected decrease in σ G at 20°C when [N 2O] is raised from 2.5 × 10 -2 mol dm -3 to 3.6 × 10 -1 mol dm -3. A possible reason for this behaviour is discussed. The temperature dependence of k(e - aq + N 2O) is consistent with diffusion control, but that of k(⋅OH + HCO - 2) is less than expected for a diffusion controlled reaction.