562 CYCLES OF ORGANIC AND INORGANIC SUBSTANCES 



Oxygen: pE 



Now that pH is determined, we shall add 0.027 mole O2, which 

 will mainly stay in the gas phase. Some will, however, dissolve in 

 the water (concentration of the order of 0.0002i\/), and will be 

 important for maintaining the oxidation potential, which is 

 conveniently expressed by means of the quantity pE, which is a 

 measure of the electron activity in a solution, in analogy to pH 

 which measures its proton activity. 



pE = -log [e-\, pH = -log IH + I 



To obtain pE, divide the oxidation potential of a solution (on the 

 hydrogen scale) by the factor RTF'^ In 10, which is 0.05915 v 

 (25°C), 0.05419 V (0°C). In sea water, as in our equilibrium 

 mixture, pE is determined by the equilibrium with atmospheric 

 oxygen (25°C, zero activity) : 



IO2 (g) + 2H+ + 2e- ^ HoO, log K = 41.55 

 log K = log [H.,0} -i log po,-2 log {H + 1 -2 log {e-\ 

 = -0.01 -^ log 0.21 + 2pH + 2pE 

 = -0.01 + 0.34 + 16.4 + 2pE 

 pE = M25.0) = 12.5 



This value for pE is not very sensitive to the usual small \ari- 

 ations of pH and oxygen concentration. A change of pH with 

 + 0.1 unit will change pE by ±0.1. A deficiency of oxygen will 

 decrease pE, but in order to lower pE by 0.1 unit, one has to lower 

 the oxygen content to 10-°- or 40 per cent of the saturation value. 



Iron 



We now add 0.55 mole iron to our mixture, mainly in the form 

 of FeOOH (strictly speaking, a certain percentage should be added 

 as FeO). At equilibrium, some iron will have reacted with the 

 silicate phases, but a considerable part will remain as FeOOH. 

 The amount of the silicate phases will have increased, and they 

 will contain some Fe^+ in Al''+ positions, and perhaps some Fe+"'" 

 in Mg++ positions. This adjustment will probably cause some 

 slight change in the pH, and it might have been more logical to 

 add the iron before the CaCO.<. 



