ELEMENTARY PROCESSES OF WATER 119 



Thus, reaction between hydroxyl radicals 



OH + OH -^ H2O2 (5) 



is quite probable when they are formed close together, as in slow-particle 

 irradiation. The reaction 



OH + OH -> H2O + (6) 



might also occur under such conditions. However, although reaction 6 



is '■^9 kcal mole~^ exothermal, the difference in activation energies 



Eq — E5 may favor reaction 5. An unpublished estimate indicates that 



£"5 < 4 kcal mole~^ 



The reactions 



xl + M -^ II2 (7) 



H + OH -^ H2O (8) 



occur readily under any circumstances, and the reaction 



OH + H2 -> H2O + H (9) 



which is -^9 kcal mole~^ exothermal, is an important back reaction which 

 serves to decrease the yield of electrolytic gas in pure water. According 

 to Allen (1, 2), reaction 9 is one step of a two-step chain, of which re- 

 action 10 



H + H2O2 -^ H2O + OH (10) 



is the second, which in general reduces gas production under gamma, x- 

 ray, and fast-electron irradiation to barely detectable levels. Ghormley 

 and Allen (3) have shown, however, that with slow-electron irradiation, 

 as with 5.6-kev betas from tritium, the yield of electrolytic gas resembles 

 what might be expected from our knowledge of effects of alpha-particle 

 irradiation. 



Introduction of impurities into the water may have a considerable 

 effect on the yield of gas. Such anions as SO4"", P04~, and Cl~~ are 

 without effect, but Br~ and I~ are successively more effective in inducing 

 production of gas. The obvious point noted by many acquainted with 

 these facts is that the electron affinity of the negative ion involved 

 relative to hydroxyl ion is the decisive factor. Where we can write, for 

 aqueous systems, 



X- + OH -^ X ^- OH- (11) 



the anion X" is effective in production of electrolytic gas to an extent 

 related to the weakness of its electron affinity. Of course, in reaction 11 

 the electron affinities involved pertain to the solvated ions. However, the 



