THE CHEMISTRY OF PHOTOSYNTHESIS 299 



from that of energetics the reduction of carbon dioxide is by far the 

 most important and difficult step in photosynthesis. The reduction of 

 carbon dioxide to the energy level of carbon or what is perhaps more 

 accurate, the reduction of carbonic acid to the energy level of carbohy- 

 drates, requires a great deal of energy. As to the means by which this 

 is accomplished in the plant we have but a few clews. We know that 

 it takes place in the visible portion of the spectrum, that chlorophyll, 

 water and oxygen are essential and that the action is dependent upon con- 

 ditions necessary for the maintenance of the life activities of the living 

 organism, such as a moderate temperature and the presence of certain 

 inorganic salts. In what manner the light acts and just what the rela- 

 tions are to the other activities of the living cell are largely matters of 

 conjecture. All information regarding analogous reactions are therefore 

 not without interest to the problem of photosynthesis and may contribute 

 directlv or indirectly to a clearer understanding of the phenomenon. 



The reduction of carbonic acid to formic acid has been accomplished 

 by various means. Lieben °^ and Ballo ^°° were able to do this with sodium 

 and other amalgams and Moisson "^ by the action of potassium hydride 

 on carbon dioxide obtained potassium formate. 



W'ieland ^''" showed that in the presence of palladium, carbon monoxide 

 can be oxidized in the absence of oxygen according to CO + H^O = 

 COo + Ho. Bredig and Carter ^°^ were able to reverse this reaction, 

 so that under certain conditions they obtained 100 per cent formic acid. 

 This required high pressures of hydrogen (50 atm.) and of carbon diox- 

 ide (20 atm.) and the presence of some inorganic carbonate, besides 

 the palladium. They used KHCO3, Na.COs, CaCOg, and Na4B407 and 

 found that the yield of formic acid with sodium carbonate was much 

 lower than with potassium bicarbonate, the velocity of formic acid forma- 

 tion depending apparently not only on the partial pressure of the hydro- 

 gen, but also on that of the carbon dioxide and on the undissociated 

 molecule of the base. The catalyst, palladium, is also essential to the 

 reaction. The determination of the equilibrium, CO2 -f Ho ^ CO + 

 HoO ^ HCOoH gave no indication of the formation of formaldehyde. 

 Lieben also concluded that bicarbonates are most easily reduced while 

 carbonates are reduced with difficulty, and free carbon dioxide is reduced 

 by sodium amalgam only in traces. It is apparently the HCOg" ion 

 which is most easily attacked. The amount of hydrate which is formed 

 when carbon dioxide is dissolved in water is small. The proportion of 

 dissolved CO. which is in the form of H0CO3 varies with dilution. Car- 

 bonic acid, which may be considered as hydroxy formic acid, is accord- 



" Lieben, Momitsh. Chew.. 16. 211 (1895) ; 18, 582 (1897) ; 19, ?,?,?, (1898). 

 •""Ballo, Ber. chem. Gcs., 17. 7 (1884). 



"•Moisson, Compt. retid., 134, 18, 261 (1902); 136, 723 (1903). 

 •"•^Wieland, Ber. chem. Ges., 45. 679 (1912) ; 46, 3327 (1913). 

 "'Bredig and Carter. ;7;i(/.. 47, 541 (1914); Zeit. elektrochem., 20, 489 (1914); 

 Chem. Ztg., 39, 72 (1915). 



