THE PHYSICAL CHEMISTRY OF SEA WATER 575 



[CO3 ] would then be about —0.7 at equilibrium with BaS04 + 

 BaCOa; and about 3.4 at equilibrium with SrS04 + SrCO,j. In sea 

 water, the value for this difference is approximately 3.71 — 1.55 = 

 2.2 at 5°C, and 3.49 - 1.55 = 1.9 at 25°C. So it seems that in sea 

 water, BaSOi and SrCO^ might be stable solid phases, but not 

 BaCOs or SrS04. 



For equilibrium with BaS04 we calculate log [Ba++] = — 10.2 + 

 (2 X 0.8) + 1.55 = —7.0, whereas the analytical values range 

 between —6.2 and —7.4. 



At equilibrium with SrCO^ we would'find log [Sr++] = — 10.0 + 

 (2 X 0.8) + 3.7 = —4.7, whereas the analytical value for sea 

 water is around —3.8. The actual concentration of Sr'^+is thus, as 

 for Ba^"*", higher than the calculated one. One reason may be that 

 we have underestimated the correction for activity factor, — log/o; 

 for SrCOs, Townley et al. (1937) give activity coefficients in NaCl 

 solution that would correspond to log/o ^ — 1.3 in sea water, and 

 give a practically perfect agreement with the observ'ations. 



In conclusion, it seems likely that vSrCOs and BaS04 would exist 

 as solid phases at equilibrium. Since these elements are present in 

 small amounts, it may be that a considerable part of Sr and Ba 

 is present in solid solution in, for instance, the silicate or CaCOs 

 phases. 



Cobalt. We may start with the equilibria 



CoOo (s) + HoO + g- ^ CoOOH (s) + 0H-, log A^ = 10 

 CoOOH (s) + HoO + £'- ^ Co(OH)2 (s) + 0H-, log K = 2.9 



In sea water, log jOH-} - log \e'} = 8.1 - 14.0 + 12.5 = 6.6, 

 which shows that the first reaction goes to the right and the second 

 to the left. Thus, of the stable phases mentioned, only CoOOH 

 would be stable. Combining the second formula with 



Co(OH)2 (s) + 2H+ ^ Co++ + 2H2O, log K = 12.4 

 H+ + OH- ^ H2O, log K = 14.0 

 gives 



CoOOH (s) + 3H+ + e-^ Co++ + 2H2O, log K = 29.3 



At the conditions of sea water, then, log [Co++] = 29.3 — 3 pH 

 - pE - log/2 ^ 29.3 - 24.3 - 12.5 + 0.8 = -6.7. The analytical 

 values for the total Co concentration in sea water are from 10~''^ 



