Homogeneous Reactions 



A complete understanding of the distribution of trace elements in 

 the oceans requires a knowledge of the forms— free and combined — in 

 which these elements occur in seawater. The speciation of trace ele- 

 ments in seawater is poorly known, and trace elements are often char- 

 acterized as either "soluble" or "particulate." 



Present knowledge of trace-element speciation is based on a variety 

 of experimental techniques. The distribution of trace metals between 

 free and complexed forms has been calculated from thermodynamic 

 data (Goldberg, 1965; Stumm and Morgan, 1970). These calculations 

 serve to identify the most important complexes, but their accuracy is 

 often poor because of lack of knowledge of the activity coefficients 

 of individual ions in seawater, and because of errors in the thermo- 

 dynamic association constants themselves. Polarographic techniques 

 such as anodic stripping voltrammetry have shown that only small 

 fractions of zinc, cadmium, copper, and lead are present as free ions 

 at the normal pH of seawater (Zirino, et al., 1972; Zirino and Healy, 

 1972). This suggests that these metals are complexed with pH-dependent 

 species such as bicarbonate, carbonate, and hydroxide. Separation 

 techniques such as solvent extraction (Slowey and Hood, 1971) have 

 shown that trace metals in seawater exist in both free and complexed 

 forms. Association constants for iron-hydroxide complexes in seawater 

 have been determined by potentiometric methods (Kester and Byrne, 

 1972). 



Solving problems such as the behavior of radioactive tracers in 

 seawater requires a better knowledge of the chemical speciation of 

 these tracers. The use of radioactive tracers usually requires the as- 

 sumption that the radioactive and stable isotopes of the tracer behave 

 similarly. However, it is known that artificially produced zinc-65 does 

 not behave in the same manner as does the naturally occurring zinc 

 in seawater (Piro, Bernhard, and Verzi, 1972). This difference in be- 

 havior is because artificially introduced zinc-65 occurs in different 

 forms than natural zinc. Similar differences in behavior may occur 

 with other isotopes. This consideration is important in undertaking 

 research involving radioactive isotopes such as that discussed in Session 

 C and for tracer equilibration in analysis. 



The speciation of trace elements in the sea must be studied using 

 both field and laboratory techniques. Laboratory studies of the associa- 

 tion of trace elements with naturally occurring inorganic and organic 

 ligands in seawater will yield a knowledge of equihbrium speciation. 

 Naturally occurring trace elements may be present in compounds that 



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