6 GOLDBERG [CHAP. 1 



exist is essential. Further, coherence in geochemical behavior can often he 

 postulated for dissolved species of the same form and charge. 



Sillen (1961) has attempted to define the thermodynamically stable dissolved 

 species in sea-water, where the appropriate physico-chemical data exist. His 

 computations rely upon an oceanic model in which the dissolved ions and 

 molecules are in equilibrium with known or assumed solid phases. He used 

 average values for the acid-base and redox equilibria as 



pH = 8.1 ±0.2 and pE = 12.5 ±0.2. 



Table I lists the most probable major forms of the elements, based in part upon 

 this work of Sillen. 



He points out that hydroxide complexes are significant for all ions of oxida- 

 tion number greater than 2. The more abundant chloride ion can compete with 

 hydroxide in forming complexes with metals under the following conditions. 

 Let the stability constants of the chloride and hydroxide be given by Kc\- 

 [MC1]/[M][C1-] and A O h = [M0H]/[M][0H-] respectively, where M is a given 

 metal ion. For MCI to be more abundant than MOH, it is clear that 



log K cl - log K oh > log [OH-] - log [C1-] = - 5.4. 



Sillen noted that this equation can be satisfied by only a few metal ions, grouped 

 in the middle of the periodic table : Ag+, Hg 2+ and Au+ and perhaps Cd 2+ and 

 Pb 2+ . Mg 2+ is possibly the only major ion involved in fluoride complexing. 



Ion-pair formation between sulfate and divalent metal ions can account for 

 around 10% of the concentrations of the alkaline earth metals (excluding 

 beryllium) as well as of nickel, copper, cobalt and zinc (Goldberg and Arrhenius, 

 1958). Of particular interest in a later section is the very strong carbonate 

 complex of uranium, U02(CC>3)3 4 ~, which makes this element rather un- 

 reactive in the marine environment. 



The lack of available data makes prediction upon bromide, phosphate, 

 carbonate, etc. complexes impossible. Further, the existence of dissolved fatty 

 acids in microgram concentrations (Williams, 1960), as well as of other organic 

 compounds, can give rise to strong complexes with metal ions. For example, 

 Laevestu and Thompson (1958) postulate that the markedly high iron abund- 

 ances in coastal waters are due to the formation of ferric-organo complexes. 

 However, firm conclusions as to the importance of such organic complexes in 

 open ocean waters cannot be reached inasmuch as the character of the bulk of 

 the organic compounds, whose concentrations attain values of 2-3 mg of carbon 

 per liter of sea-water, has as yet not been defined. 



Although most of the dissolved constituents of sea-water have had many 

 thousands of years to reach equilibrium, nonetheless, thermodynamically un- 

 stable species, such as manganous, iodide and arsenite ions, persist. Their 

 occurrence can be attributed to a lack of reaction sites where equilibrium might 

 be obtained, i.e. the water-mass containing ions of this type does not encounter 

 a surface of the lithosphere, biosphere or atmosphere at which the energetically 

 possible reactions can proceed. 



