568 CYCLES OF ORGANIC AND I.XORGAXIC SUBSTANCES 



Manganese 



Now we add the last of our major elements, 0.01 mole AIn in 

 the form of Mn02. From the equilibrium constants, and the pE 

 and pH of sea water, it is easily found that MnOo is the only 

 manganese oxide phase that would be stable.* One also finds that 

 of the soluble species that have been studied, Mn++ and Mn04" 

 would be the most important ones, though their concentrations at 

 equilibrium will be small indeed : 



MnO., (s) + 4H+ + 2e~ ^ Mn++ + 2H2O, log K = 41.6 



log {Mn++! = 41.6 - 4pH - 2pE = 41.6 - 32.4 - 25.0 = -16 



Mn04- + 4H+ + 3e- ^ MnO. (s) + 2H2O, log K = 86.0 



log {Mn04-1 = -86.0 + 4pH + 3pE = 32.4 + 37.5 - 86.0 = -16 



The total concentrations of manganese actually reported are 

 between lO"*^-' and 10~'-*M. It seems dubious that Mn++ or 

 ]Mn04" could exist for long periods at concentrations some 10^ 

 times the equilibrium value. 



If one does not prefer some unknown organic or phosphate 

 complex as a dens ex machina, the best guess is perhaps some 

 uncharged hydroxide species, such as ?vIn(0H)3 or Mn(0H)4. 



Nitrogen 



In our equilibrium mixture, to be strict, we have also around 

 0.10 mole N2, chiefly present as N2 in the air. This is an outrageous 

 case of nonequilibrium. The N2 molecule is certainly inert, and 

 whatever lightning is doing to approach equilibrium by forming 

 nitrite and nitrate, this is counteracted by biological activity. We 

 shall leave N out of the discussion, except to remind us for a 

 moment that there are limitations to the equilibrium treatment. 



It would have little relation to present conditions to calculate 

 what the equilibrium with N would be. On the other hand, the 

 present concentrations of NH3 and NOs" in the ocean are so small 



* In deep sea sediments, indeed, S-MnOa is an abundant constituent. There is 

 also a "manganite" phase (or perhaps two) described by Dr. Buser. It may be 

 described as a soUd solution phase containing Mn02 and oxides of various bipositive 

 metals. If it exists at equilibrium with Mn02. as it seems to do, its presence would 

 make no change in the discussion of the dissolved Mn species. 



