120 ELEMENTARY CHEMICAL PROCESSES 



explanation of the effect was not so readily apparent. Many efforts were 

 made to explain the result in terms of a chain involving X and X~. 

 Allen (1,2) suggested the explanation that the redox potential of reaction 

 11 controls the steady-state concentration of free radical OH and thus 

 determines the effectiveness of the back-reaction sequence 9 and 10. 

 It is, as a matter of fact, known that with sufficient Br~ or I~ present 

 the rate of gas production is proportional to the intensity of irradiation 

 and that the concentration of Br~ or I~ determines the maximum rate 

 of such production, I~ being the more effective. So far as this author 

 is informed, Allen's theory of this effect has not been subjected to 

 quantitative tests. For the purpose of this presentation it is sufficient 

 to note that the presence of anions of low electron affinity reduces the 

 free-hydroxyl-radical concentration at any intensity level of irradiation 

 and thus decreases the effectiveness of steps 9 and 10 for the back re- 

 action. As a result, H2 gas escapes from the liquid. The H2O2 which 

 escapes reaction 10 may, however, decompose by an overall reaction we 

 note simply as 



H2O2 -^ H2O + i02 (12) 



Allen (1, 2) has suggested as the chief mechanism of this reaction the 

 chain 



H2O2 + OH -> H2O -\- HO2 (13) 



H2O2 + HO2 -> O2 + H2O + OH (14) 



OH + HO2 -^ H2O + O2 (15) 



Presence of oxygen also assists production of electrolytic gas under 

 fast-particle irradiation. In this case the effective reaction presumably 

 involves formation of free hydroperoxyl radical 



H -t- O2 -> HO2 (16) 



The process reduces the free-hydrogen-atom concentration (and thus 

 the effectiveness of the back sequence 9 and 10) and also provides a 

 substance itself capable of capturing hydrogen atoms. 



HO2 + H -^ H2O2 (17) 



The simplest satisfactory explanation of the effectiveness of dissolved 

 * oxygen in assisting production of electrolytic gas and hydrogen peroxide 

 lies in those two facts. However, as Allen has noted, the hydroperoxyl 

 radical enters also into its own back-reaction sequence (13 and 17) and 

 the total effect consequently depends in quite complicated fashion on 

 intensity of irradiation, concentration of dissolved oxygen, pressure 

 above the liquid, and relative volumes of gas and liquid. 



