566 CYCLES OF OR(3AXIC AND INORCAXIC SUBSTANCES 



would estimate for sea water a concentration solubility product of 

 10"-•^ which would vary rather little with temperature. 



In average sea water we have [Ca++][S04 ] = 10~^-''\ which 

 indicates that no solid CaS04(H.20)2 will be present at equilibrium. 

 The sulfate found in some marine sediments is probably adsorbed 

 to the phases previously mentioned. 



Sulfate ions seem to form complexes with bivalent metal ions: 



I\I++ + SO4— ^ MSO4 



with zero activity constants that are approximately equal, around 

 10--- at 25°C for most bivalent cations. In the ionic medium of 

 sea water, the stability constant should be considerably less, 

 perhaps of the order of 10"- '\ At any rate, a minor part of Ca++ 

 and IMg++ will be present as uncharged complexes, CaS04 and 

 MgS04 in solution. 



Phosphate 



Next addition is 0.02 mole H3PO4, or Na.2HP04, if we want a 

 minimum rearrangement in the silicate phases. At the final 

 equilibrium, we may expect much of the phosphate to be precipi- 

 tated in the form of some calcium phosphate. The literature data 

 on the solubility of calcium phosphates do not agree as well as one 

 could wish : there are difficulties in getting true equilibria, and 

 identifying the phases. The following estimates (25 °C, zero 

 activities) are based on data of Farr (1950), Kauko and Eyubi 

 (1955): 



CaHP04 (s) ^ Ca++ + HPO4— , log K = -7.0 

 Ca3(P04)2 (s) ^ 3Ca++ + 2P04^-, log K = -26.0 

 Ca5(P04)30H (s) ^5Ca++ + 3P04^- + OR- log X = -55.9 



In adding 



HPO4— ^ P04^^- + H+, log K = -12.3 



HoO ^ H+ + 0H-, log A' = -14.0 

 we find 



3CaHP04 (s) + 2Ca++ + H.O ^ Ca5(P04)30H (s) + 4H+, log K = - 16 

 3Ca3(P04)2 (s)+Ca++ + 2H20^2Ca5(P04)30H (s)+2H+, \og K = 6 



