THE PHYSICAL CHEMISTRY OF SEA WATER 565 



Fluoride 



Next we add 0.03 mole HF. At equilibrium, some of the fluoride 

 may enter the silicate phases, substituting O or OH" groups; at 

 present it is impossible to predict where these equilibria will stand. 

 It is conceivable that some of the fluoride might precipitate as 

 CaFo (s), fluorspar. For the reaction 



CaF. (s) ^ Ca++ + 2F-, log K = log [Ca++] + 2 log [F"] + log /./i^ 



zero activity values of about —10.7 at 0°C, and —10.4 at 25°C 

 have been given. With log [Ca++] = —2.0, and estimating log 

 /2/,2 _ _i_2, we find log [Fi = -3.75 (0°), -3.6 (25°) at equi- 

 librium with solid CaF2. 



Of the ions in the solution, it seems that only Mg++ would form 

 considerable amounts of fluoride complexes. For the comparable 

 medium 0.51f NaC104, Connick and Tsao (1954) give 



Mg++ + F- ^ MgF+, log K = 1.30 (25°), 1.20 (15°) 



Since log [Mg^"*"] = — 1.27, we would expect to have, in addition 

 to the lO^'^-'^ to lO'^-'^M of F~, an approximately equal concen- 

 tration of MgF+, giving a total F concentration of 10~'^-^ to 10~'^-\ 

 This is higher than the concentration found in sea water, 10~^-^"'3f, 

 so it may not be worth while to look out for fluorspar in marine 

 sediments. Probably most of the F is in silicate phases and some 

 in the apatite (see below). 



Sulfate 



The addition of 0.03 mole H2SO4 will allow a larger amount of 

 ions In the solution and cause some rearrangement in the silicate 

 phases. Considering the great amount of silicates present, we do 

 not expect any perceptible change in the equilibrium pH. (Those 

 who are very cautious might have saved some Na and added 

 sulfur as Na2S04 at this point.) 



One may now ask whether the conditions are such that calcium 

 sulfate will precipitate. For the reaction 



CaS04(H20)2 (s) ^ Ca++ + SO4— + 2H2O 



the zero activity product is lO^^-*''^ (25°). From available data on 

 the solubility of CaS04 (H20)2 In NaCl and NaC104 solutions, one 



