THE PHYSICAL CHEMISTRY OF SEA WATER 563 



Sea water contains some iron in true solution. The analyses 

 vary from 10~'-^ to 10~^-°M, but for carefully filtered samples 

 10~'-- seems to be a good average (Lewis and Goldberg, 1954). It 

 is natural to ask what soluble iron species can be present. The 

 first guess is that it is perhaps some hydroxo complex of iron (I II). 

 For 3M NaC104, 25°C (Biedermann and Schindler, 1957; Hed- 

 strom, 1953) : 



Reaction log K 



FeOOH (s) + 3H+ ^ Fe='+ + 2H2O 3.96 



Fe3+ + H2O ^ FeOH++ + H+ -3.05 



FeOH++ + HoO ^ Fe(0H)2+ + H+ -3.26 



2Fe^+ + 2H2O ^ Fe2(OH)2^+ + 2H+ -2.91 



It is easily seen that the most important reaction will be 



FeOOH (s) + H+ ^ Fe(0H)2+ + H2O, log K = -2.35 



which will give, with pH = 8.1, [Fe(0H)2+] = IQ-^o ^^ For 

 FeOH++ and Fe2(OH)2^+, we find concentrations of the order of 

 10~^^ and 10"-'. Even if the formation constant for Fe(0H)2"'" is 

 the most uncertain of those given, it is not likely to be too small; 

 nor should equilibrium be very sensitive to a change in ionic 

 medium. The discrepancy between the equilibrium concentration 

 of the Fe(0H)2'^, 10~i°-^\ and the observed iron concentration, 

 around 10~"--, seems too great to be explained by activity factors, 

 or by any temperature influence on the equilibria. 



The concentration of Fe+"'" may be estimated from the equilibria: 



Reaction log K 



FeOOH (s) + 3H+ ^ Fe'+ + 2H2O 4.0 



Fe='+ + g- ^ Fe++ 13.0 



FeOOH (s) + 3H+ -\- e~ ^ Fe++ + 2H2O 17 

 Then: 



loglFe++] = 17 - 3pH - pE = 17 - 24.3 - 12.5 = -20 



We have been somewhat careless with the activity coefficients 

 in these formulas but it is obvious that the equilibrium concen- 

 tration of Fe++ cannot explain the observed iron concentration in 

 sea water. 



