PHOTOXIDATION OF WATER BY CATIONS 75 



where in this region is effective in bringing about the reaction 



Ught 



(4.11) Ce++++ + h H2O > Ce+++ + H+ + i O2 



Since the oxidation potential of eerie ions is more negative than that of 

 molecular oxygen, reaction (4.11) does not convert light into chemical 

 energy. 



It is known or suspected (see Rabinowitch 1942) that the light 

 absorption by many other cations also leads to a primary oxidation of 

 water, probably, according to the equation: 



Ught 



(4.12) M+-H20 )-M-H20+ 



A. and L. Farkas (1938), who suggested this primary process, pointed 

 out that the final state in (4.12) is unstable, and is terminated either by 

 a return of the electron to water: 



(4.13) M -1120+ > M+-H20 ("primary back reaction") 



or by a chain of transformations of the ion H2O+, e. g.: 



f H2O+ + OH- > H2O + OH 



(4.14) OH 4- OH >H202 



I H2O2 > H2O + \ O2 



At any stage of (4.14), the reaction may be reversed by a "secondary" 

 back reaction, that is, the reoxidation of M by hydroxyl, peroxide or 

 oxygen. 



In the case of the eerie ions, reaction sequence (4.14) has a good 

 chance of occurring in preference to a primary or secondary back reaction. 

 With other cations, no oxygen evolution has been observed upon illumi- 

 nation (at least, as far as attention has been paid to this point) and this 

 can be taken as an indication that the back reactions are more probable 

 than the oxidation chain (4.14). The cause most probably lies in the 

 relative energies of the different states involved in the process. For 

 eerie ions, oxidation releases more energy than the return into the initial 

 state, and the probability of the metastable state Ce+++- H2O+ undergoing 

 a development according to (4.14) is correspondingly high. In the case 

 of other ions (ferric ions, for example) much more energy can be gained 

 by the transformation of the metastable complex (Fe++-H20+) back 

 into Fe+++-H20, than by the completion of oxidation according to 



(4.15) Fe++-H20+ + (OH-)aq. > Fe++-H20 + \ H2O2 > • • • (as in 4.14) 



The reaction with ferrous oxalate observed by Hill and described on 

 page 63 is probably of type (4.12) - (4.14), although it requires sensi- 

 tization (by chloroplasts). In this case, the oxygen liberation occurs 

 with a considerable yield, despite the unfavorable position of the energy 



