550 CYCLES OF ORGANIC AND INORGANIC SUBSTANCES 



These are average values for the bulk of the ocean; in isolated 

 pockets of the ocean, or in places with strong biological activity, 

 larger deviations may occur. The average temperature seems to 

 be about 5°C, the density is taken as 1.024 g m\^\ and the average 

 depth as 3,800 m, which gives an average pressure of nearly 

 200 atm. 



The total volume of the ocean has been estimated as 1.37 X 10^ 

 km'' = 1.37 X 10-^ liters. The volume of this solution and of the 

 gas phase and solids with which it may react seems somewhat 

 unwieldy to an ordinary chemist. With your permission, in the 

 following I shall divide all these amounts by 1.37 X 10-\ and thus 

 consider 1 liter of average ocean water and its share of the atmos- 

 phere and the sediments. 



Atmosphere 



The total mass of the atmosphere has been estimated as 

 5.13 X 10-^ g. With an average molecular weight of 28.97 and with 

 the partial pressures 0.7809 atm for N., 0.2095 atm for O., 0.0093 

 atm for Ar, and 0.00030 atm for COo, we find that the share of 

 1 liter of sea water will be 0.1293 mole dry air (3.16 liters at 25°C, 

 1 atm or 2.90 liters at 0°C, 1 atm), containing 0.1010 mole No, 

 0.0271 mole Oo, 0.0012 mole Ar, and 0.000039 mole CO.. 



What Determines the pH? 



One of the first questions that an equilibrium chemist will ask 

 is this: What makes the pH equal 8.2? A common answer is 

 that pH is determined by the buffering of the H2C03~HC03~~ 

 COs"", and to some extent also by the B(OH)3~B(OH)4~ system. 

 This answer is correct in one respect. If one removes 1 liter of sea 

 water from its surroundings and adds strong acid, almost 3 mM H''' 

 will be used up in transforming HCOg" to HoCO^, and in the mean- 

 time the pH remains about 8 to 6; only after that, the H+ added 

 will remain as such in the solution, and pH will drop to 3 or 2. 

 Similarly, if strong alkali is added, almost 3 ml\l 0H~ is needed 

 to transform B(0H)3 to B(0H)4-, and HCO3- to CO3--, which, 

 incidentally, precipitates as CaC03.* Now, a buffer concentration 



* If more OH" is added, Mg(0H)2 precipitates. I have been told, however, that 

 the ocean never seems to have been at equilibrium with solid Mg(0H)2. 



