572 CYCLES OF ORGANIC AND INORGANIC SUBSTANCES 



Hence, A'al and Kbl are the solubility products of pure AL 

 and pure BL (with the AL structure). 



If the activity coefficients are canceled out, we would have 



[B] _ xb ^ Kbl _ ns ^ i^BL 

 [A] -Va Kxh fix Kal 



The equations show that B may precipitate with AL, to some 

 extent, although the product of the B and L activities is much 

 lower than A^bl- The ratio B/A will be higher in the solid phase 

 than in the solution, if BL is less soluble than AL. If solid solutions 

 are formed, the effect is as if the B were extracted by the AL: the 

 more AL (?Ia moles) present, the more B (wb moles) will be in the 

 solid phase. 



In our equilibrium mixture, as in actual sea water, many of the 

 minor constituents may disappear almost completely into solid 

 solutions with the phases formed by the major constituents, 

 instead of forming a separate phase at equilibrium. 



Complex formation 



At the pH of sea water, hydroxide complexing is important for 

 all ions of oxidation number greater than 2. The chloride complex- 

 ing has to compete with the hydroxide. For a certain metal ion, 

 M (omitting the charge), let A'ci = [MC1]/[M][C1-], and Aqh = 

 [MOH]/[M][OH-]. It is easily seen that for MCI to predominate 

 over IVIOH, it is necessary that 



log Aci - log AoH > log [OH-] - log [C1-] = -5.4 

 A study of available complex formation constants shows that this 

 is true only for a few ions, grouped in the middle of the periodic 

 system : Ag+, Hg++, perhaps Cd+"'' and Pb+"^. These are likely to 

 exist predominantly as complexes: AgCL", AgCU , HgCU", 

 HgCli . To this group also belongs AuClo". However, we shall 

 see that for gold(III), the hydroxo complexes predominate. 



Similarly, for the fluoride complexes to predominate over the 

 hydroxo complexes we would have 



log Af - log AoH > log [OH] - log [F-] = -L5 



This requirement is fulfilled for Mg"*""*", for which log Aqh =2.1, 

 log Af = 1.3, and thus neither is very strong. Otherwise, there 

 seems to be no certain case of predominant fluoride complexing. 



