72 PROCESSES OUTSIDE THE LIVING CELL CHAP. 4 



Wood (1925) found that water vapor containing mercury decomposes when illumi- 

 nated with the resonance Une (253.6 mn) of mercury. Senftleben and Rehren (1926) 

 investigated the reaction more closely by measuring the heat conductivity of the illumi- 

 nated mixture. In a closed vessel, the illumination leads to a photostationary state. 

 Ga viola and Wood (1928) have given spectroscopic proofs of the presence in this state 

 of free hydroxyl radicals (OH) and of mercury hydride molecules (HgH). According 

 to Beutler and Rabinowitch (1930) these are the primary products of the reaction: 



light 



(4.8) (Hg)g + (H20)g V (HgH)g + (OH)g - 95 kcal 



dark 



However, through the recombination of OH radicals, the dissociation of the unstable 

 HgH molecules, and other processes competing with (4.8), numerous other products are 

 formed, among others, H2, O2, H2O2, HgO and free H atoms. Melville (1936) found 

 that the uncondensable fraction of the illuminated Hg/H20 mixture consists mainly of 

 hydrogen, and suggested that the equivalent quantity of oxygen is bound in mercurous 

 oxide. 



Reaction (4.8) is of the type (4.6), a photoxidation of water by mercury, with the 

 transfer of only one hydrogen atom. This reaction is possible, despite the high energy 

 of dissociation of water into H and OH (109 kcal), because the absorption of the line 

 253.6 m/j. brings mercury into a state ('Pi) with an excess energy of 112 kcal per mole. 

 This is the first example of how hght energy can be utilized for hydrogen transfer against 

 the gradient of chemical potential. 



The efficiency of conversion of light energy into chemical energy in reaction (4.8) 

 is 90%. However, most of this energy is dissipated by secondary processes. If the 

 hydroxyl radicals decompose into water and oxygen, while the mercury hydride decom- 

 poses into mercury and hydrogen, the ultimate result is that a quantum equivalent to 

 112 kcal per mole, has produced the reaction § H20^ ^ H2 -I- i O2, with a heat effect 

 of 27 kcal, corresponding to the conversion of only 25% of hght energy into chemical 

 energy. This result is actually achieved when the reaction between Hg and H2O is 

 carried out in a streaming system. Bates and Taylor (1927) found that the decom- 

 position products contain 73% H2 and 27% O2. The deficiency of oxygen may be due 

 to the incomplete decomposition of hydrogen peroxide. 



3. Sensitization of Water Decomposition by Solids (ZnO and AgCl) 



The photochemical decomposition of water can be extended further 

 towards longer waves by the use of solid sensitizers, e. g., zinc oxide and 

 silver chloride. However, it is not certain whether the sensitization by 

 zinc oxide goes beyond sensitized photautoxidation of water, according 

 to equation (4.7). This reaction was discovered by Baur and Neuweiler 

 (1927). After oxygen-containing water has been shaken with zinc oxide 

 for 10-15 hours in full sunlight, the liquid is found to contain about 

 1 X 10~^ mole per liter of hydrogen peroxide. According to Baur and 

 Neuweiler, no peroxide is formed in air-free solutions; the oxidant is 

 thus apparently molecular oxygen. The active light belongs to the near 

 ultraviolet (the absorption limit of ZnO lies at 380 m/x), and zinc oxide 

 seems to act as a true photocatalyst, promoting reaction (4.7) without 

 participating in it. 



