74 PROCESSES OUTSIDE THE LIVING CELL CHAP. 4 



It is thus an interesting question whether hydrogen peroxide can be 

 obtained by the illumination of zinc oxide suspensions in the absence of 

 oxygen. Baur and Neuweiler (1927) gave a negative answer to this 

 question, while Yamafuji, Nishioeda, and Imagawa (1939) asserted that 

 a small quantity of hydrogen peroxide is formed even if all oxygen had 

 been removed. 



Vogel (1863), Eder (1906) and Sichling (1911), found that water 

 over a silver chloride precipitate evolves oxygen if exposed to daylight, 

 and this was confirmed by Baur (1908) and Baur and Rebmann (1921). 

 However, the amount of gas evolved is small, and the reaction soon stops. 

 In contrast to water decomposition by zinc oxide, this reaction is probably 

 not a true photocatalysis, but a photoxidation of water hy silver chloride: 



light 



(4.10a) Ag+Cl- > Ag + CI 



(4.10b) CI + H2O > Cl-aq. + H+aq. + OH 



(4.10c) OH > I H2O + i O2 



light 



(4.10) Ag+Cl- + h H2O > Ag + Cl-aq. + i O2 + H+aq. - 25 kcal 



This reaction deserves attention because of the apparent conversion 

 of a large part of light energy into chemical energy. If brought about 

 by a single quantum at 400 mn, it would lead to the conversion of about 

 35% of absorbed light energy, a yield not attained by any other known 

 photochemical reaction in visible light. It is, however, not certain 

 whether the observations of Vogel, Baur, and Rebmann are correct, 

 whether interpretation (4.10) applies to them and, if it does, what the 

 quantum yield of this reaction may be. 



4. Photoxidation of Water by Cations 



We have considered mercury vapor and ionic crystal powders as 

 sensitizers which enable the photochemical liberation of oxygen from 

 water to occur in the medium or near ultraviolet. A third group of 

 such sensitizers is found in dissolved cations. 



The only cation whose capacity for photochemical water oxidation has 

 been demonstrated by experiments, is the eerie ion, Ce++''"+. The normal 

 oxidation-reduction potential of the system Ce+~^+-Ce"''+"'' is close to 

 — 1.5 volt, that is, far below that of the oxygen electrode. Consequently, 

 Ce++++ ions liberate oxygen from water even in the dark; but this 

 process is slow. Baur (1908) noticed that this oxidation can be acceler- 

 ated by light, and Weiss and Porret (1937) found that oxygen is produced 

 with a quantum yield as high as 0.5. 



The absorption bands of the eerie ions extend from the far ultraviolet 

 to the blue-violet region of the visible spectrum. (The molar extinction 

 coefficient is about 150 at 400 mn.) It is likely that absorption every- 



