850 MILLER [chap. 32 



Using Pho = 1.5 X 10~3^ we have the ratio PcojPco^ = 5 x 10~'^, which is indepen- 

 dent of pressure. Furthermore, carbon monoxide is a relatively reactive com- 

 pound, and should any significant quantities appear in the atmosphere, it 

 would react in various ways to give organic compounds, carbon dioxide and 

 hydrogen, and formate. The hydration of carbon monoxide to formate is a base- 

 catalyzed reaction and proceeds at a significant rate, and the other reactions 

 would also be relatively rapid. 



Rubey (1955) and Abelson (1956) have argued that the surface carbon and 

 nitrogen came from out-gassing the interior of the Earth instead of coming from 

 the remainder of the cosmic dust cloud from which the Earth was formed. The 

 carbon from the out-gassing may be in the form of CO2 and CO or CH4, and 

 hydrogen may be present. While this may have been a significant process on 

 the primitive Earth, it does not require that the atmosphere was composed 

 of CO 2 and CO. The above thermodynamic considerations still apply. The 

 carbon dioxide would dissolve in the ocean to form HCOs" and CO32-, and 

 CaCOs would be deposited, and the CO would be unstable as demonstrated 

 above. The state of oxidation of gases from volcanoes is difficult to determine, 

 since they come in contact with the oxygen of the air and with oxides of iron, 

 which in turn may owe their oxidation state directly or indirectly to the 

 oxidizing atmosphere. 



The buffer systems of the ocean and the calcium silicate-calcium carbonate 

 equilibrium are of sufficient capacity to keep the partial pressure of the carbon 

 dioxide in the atmosphere at a low value, so that the principal species of car- 

 bon in the atmosphere would be methane even though the fraction of surface 

 carbon in the oxidation state of carbon dioxide was continuously increasing. 

 This would be true until the pressure of H2 fell below about 10"^ atm. At this 

 point the small partial pressure of carbon dioxide would have made it the 

 principal carbon species in the atmosphere. It is likely that shortly after this, 

 significant quantities of molecular oxygen would appear in the atmosphere. 



C. The Equilibrium of Nitrogen Compounds 



The equilibrium concentrations of ammonia can be discussed by considering 

 the reaction 



lN2 + fH2 = NH3, A'25^ = 7.6 X 102 



Using -^H2= 1.5 X 10~3, we have 



P^^JPl = 0-04. 



The nitrogen pressure could not be the present value and was probably much 

 lower. 



Ammonia is very soluble in water and this would displace the above reaction 

 toward the right, giving 



iN2 + fH2 + H+ = NH4+ 

 [NH4+]/Pi^^P^^ = 8.0xl0i3[H+], 



