574 CYCLES OF ORGANIC AND INORGANIC SUBSTANCES 



periodate, and 1 2 are negligible in comparison with lOs" at equi- 

 librium. 



Some authors ha\e given experimental evidence that sea water 

 would contain a certain proportion of the iodine as I~ ions. This 

 may be so because of nonequilibrium. However, it may be pointed 

 out that it is hard to make such an experimental proof that is not 

 open to criticism; for instance, IO3" may well coprecipitate with 

 AgCl, just as does I~. 



Arsenic. At pH = 8.1, arsenic(III) will exist mainly as the 

 uncharged species, say HAsOo [though As(0H)3 might be more 

 nearly correct], and arsenic(\') as the ion HASO4 . We combine 

 the equilibria 



As04^- + 2H2O + 2e- ^ AsO-r + 40H-, log i^ = -22.7 



H+ + AsOi- ^ HAsO., log K = 9.1 



HASO4— ^As04^- + H+, logX=-11.5 



4H+ + 40H- ^4H20, log i^ = 56.0 



HASO4— + 4H+ + 2f- ;=^ HASO2 + 2H2O, log K = 30.9 



By inserting the value for sea water: 



log[HAs02] - log[HAs04— ] = 30.9 - (4 X 8.1) - (2 X 12.5) = -26.5 



Thus, HASO4-' would be the predominating form at equilibrium. 

 Selenium. From the equilibria 



Se04— + H2O + 2e- ^ SeOs— + 20H-, log K = 1.7 

 2H+ + 20H- ^ 2H2O, log K = 28.0 



Se04— + 2H+ + 2e- ^ SeOa— + H2O, log K = 29.7 



By inserting pH = 8.1 and pE = 12.5, we find [Se03""]/[Se04— ] = 

 10-11-^, which would indicate that Se04 predominates at 

 equilibrium. 



Some Possible Solubility Equilibria 



Strontium and Barium. The logarithms for the zero activity 

 solubility products have been given as follows: SrS04 —6.6 



(5°C, 25°C); SrCOg -10.0 (25°C); BaS04 -10.2 (5°C), -10.0 

 (25°C); BaCO, -9.3 (25°C). The difference log [S04~] - log 



