91
3
Minor Elements in Seawater
3.1 Classification of Elements
The elements found in seawater (see Figure 3.1) include most of those in the periodic table.
Of these, only 14 elements (O, H, Cl, Na, Mg, S, Ca, K, Br, C, Sr, B, Si, and F) have concentrations greater than 1 ppm. Most of these elements (with the exception of Si) are generally
unreactive elements (both chemically and biologically). Many of the remaining elements,
called minor, are involved in inorganic and biological reactions in the marine environment. The biolimited elements N, P, and Si are discussed Chapter 8. The inert gases are
also discussed separately in Chapter 6.
Bruland (1983) has tabulated the range and average concentration of a number of elements in seawater (salinity S P = 35). His results are tabulated in Table 3.1. He conveniently
divided the elements into three classes based on concentration (Figure 3.1):
1. Major elements: 0.05 to 750 mM
2. Minor elements: 0.05 to 50 μM
3. Trace elements: 0.05 to 50 nM
Since many of these minor elements are metals, Goldberg (1965) divided them into three
classes based on their electronic structure (see Table 3.2). This simple classification for
metals is given in Table 3.2. Minor and trace elements in the ocean, because of their reactivity, have a wide range of concentrations (see Table 3.1 and Figure 3.2).
3.1.1 d 0 Cations
Ions of metallic elements with a rare gas configuration include the alkali metals (Li + , Na + ,
K + , Rb + , Cs + , Fr + ); the alkaline earth metals (Be 2+ , Mg 2+ , Ca 2+ , Sr 2+ , Ba 2+ , Ra 2+ ); and the lanthanide or rare earth series (La 3+ , Ce 3+ , Pr 3+ , Nd 3+ , Pm 3+ , Sm 3+ , Eu 3+ , Gd 3+ , Tb 3+ , Dy 3+ , Ho 3+ , Er 3+ ,
Tm 3+ , Yb 3+ , Lu 3+ ) plus the metals Al 3+ , Sc 3+ , Ti 3+ , and Th 4+ .
This group is characterized by the fact that its members form few complexes, mainly with
F – and ligands where oxygen is the donor atom (e.g., OH – , SO 4
– , CO 3
2– , and PO 4
3– ). There
is little or no evidence that these metal ions form complexes with the heavier halides. In
a given series, the stability of the complexes increases with increasing charge and, for
cations of the same charge, with decreasing radius. This is shown in Table 3.3 for the formation of divalent metal complexes with F – and OH – . These results indicate that the strength
of the complexes is related to electrostatic interactions (proportional to Z 2 /r, where Z is the
charge and r is the radius of an ion).
3
Minor Elements in Seawater
3.1 Classification of Elements
The elements found in seawater (see Figure 3.1) include most of those in the periodic table.
Of these, only 14 elements (O, H, Cl, Na, Mg, S, Ca, K, Br, C, Sr, B, Si, and F) have concentrations greater than 1 ppm. Most of these elements (with the exception of Si) are generally
unreactive elements (both chemically and biologically). Many of the remaining elements,
called minor, are involved in inorganic and biological reactions in the marine environment. The biolimited elements N, P, and Si are discussed Chapter 8. The inert gases are
also discussed separately in Chapter 6.
Bruland (1983) has tabulated the range and average concentration of a number of elements in seawater (salinity S P = 35). His results are tabulated in Table 3.1. He conveniently
divided the elements into three classes based on concentration (Figure 3.1):
1. Major elements: 0.05 to 750 mM
2. Minor elements: 0.05 to 50 μM
3. Trace elements: 0.05 to 50 nM
Since many of these minor elements are metals, Goldberg (1965) divided them into three
classes based on their electronic structure (see Table 3.2). This simple classification for
metals is given in Table 3.2. Minor and trace elements in the ocean, because of their reactivity, have a wide range of concentrations (see Table 3.1 and Figure 3.2).
3.1.1 d 0 Cations
Ions of metallic elements with a rare gas configuration include the alkali metals (Li + , Na + ,
K + , Rb + , Cs + , Fr + ); the alkaline earth metals (Be 2+ , Mg 2+ , Ca 2+ , Sr 2+ , Ba 2+ , Ra 2+ ); and the lanthanide or rare earth series (La 3+ , Ce 3+ , Pr 3+ , Nd 3+ , Pm 3+ , Sm 3+ , Eu 3+ , Gd 3+ , Tb 3+ , Dy 3+ , Ho 3+ , Er 3+ ,
Tm 3+ , Yb 3+ , Lu 3+ ) plus the metals Al 3+ , Sc 3+ , Ti 3+ , and Th 4+ .
This group is characterized by the fact that its members form few complexes, mainly with
F – and ligands where oxygen is the donor atom (e.g., OH – , SO 4
– , CO 3
2– , and PO 4
3– ). There
is little or no evidence that these metal ions form complexes with the heavier halides. In
a given series, the stability of the complexes increases with increasing charge and, for
cations of the same charge, with decreasing radius. This is shown in Table 3.3 for the formation of divalent metal complexes with F – and OH – . These results indicate that the strength
of the complexes is related to electrostatic interactions (proportional to Z 2 /r, where Z is the
charge and r is the radius of an ion).