THE PHVSICAI. CHEMISTRY OF SEA WATER 567 



Thus, at equilibrium between the first two calcium phosphate 

 phases, we would have 2 log {H+} — log {Ca+"'"f = —8; in the 

 second equilibrium, the difference would be 6. In sea water, we 

 have 2 log {H+1 - log {Ca++} = -16.2 + 2.8 = -13.4. It seems 

 that no reasonable error in the experiments quoted, activity, 

 pressure, or temperature correction, would change the fact that 

 the two last reactions given would proceed completely to the right, 

 under the conditions of sea water, so that Ca5(P04)30H (s), 

 hydroxyapatite, would be the only stable calcium phosphate phase. 

 This agrees with experience on oceanic sediments, I have learned. 

 The important equilibrium will then be, since phosphate exists 

 mainly as HPO4 



CasCPOO^OH (s) + 4H+ = 5Ca++ + 3HPO4— + H.O, log X = -5.0 



Inserting pH = 8.1, log [Ca++] = —2.0, and estimating log 

 /, = -0.8 we find log [HPO4— ] = -7.0. 



This is admittedly a very rough estimate; for instance, the 

 influence of temperature on the equilibria could not be considered. 

 The total amount of P in the equilibrium solution should be larger, 

 because in addition to 10~''^M free HPO4 there will be complexes 

 of this ion with Mg"*""^ and Ca++, which are slightly stronger than 

 the sulfate complex. 



The total P concentration found in deep sea water, 30 — 90 

 mg/m^ = lO"*^-** to 10~''-'iVf, together with this very approximate 

 calculation, would indicate that the lower layers may be at 

 equilibrium with hydroxyapatite; in the upper layers, as well 

 known, the P content is decreased locally by organic activity. 



Phosphate ions are said to be adsorbed on FeOOH and, to some 

 extent, on the silicate phases. Unfortunately, very little is known 

 about the solubility equilibria of various phosphates, such as 

 FeP04. In the scanty measurements that exist it seems that the 

 solubility equilibrium has not been kept distinguished from 

 complex formation and hydrolysis equilibria. More work on 

 phosphate equilibria would be desirable. At present it cannot be 

 said if any more phosphate, such as FeP04, could exist at equi- 

 librium as a separate solid phase. 



