Geology Reference
In-Depth Information
Box 4.5 The extent of ion-pairing in seawater
An ordinary chemical analysis of seawater, like that in
Table 4.3, does not identify the actual species (ion pairs,
complexes or free ions) in which the element occurs. For
example, magnesium could be present in seawater in any
of several alternative forms.
has a dissociation (equilibrium) constant given by:
aa
a
⋅
2
+
−
Mg
HCO
K
=
(4.5.3)
3
+
MgHCO
3
+
MgHCO
3
Laboratory experiment shows that:
2
+
0
0
+
Mg
MgSO
MgCO
MgHCO
4
3
3
−
.
116
K
MgHCO
+
=
10
at
25
°
C
free ion
←
ion pairs complexes
/
→
3
Figure 4.5.1 shows the molal proportions of principal cati-
ons and anions in seawater. The detached segments show
the proportion of each ion involved in ion pairing (ignoring
less significant pairings such as CaHCO
3
+
). The remaining
part of each segment represents the proportion left as
free ions.
It is clear that a high proportion of certain anions like
SO
4
2−
are bound up in ion pairs. More than 40% of the
sulfate ion in seawater appears to be paired - in more or
less equal proportion - with Na
+
and Mg
2+
; only 55% of the
sulfate present exists as free ions. No more than 10% of
the CO
3
2−
in seawater appears to be present as free ions.
As one would expect, pairing is more prevalent among
divalent ions.
(The superscript
0
signifies the zero charge on a neutral
dissolved species.) Table 4.3 gives no clue as to how
important each of these species might actually be, but
clearly it must be true that:
(
)
=
m
total
m
+
mmm
+
+
(4.5.1)
Mg
2
+
0
0
+
Mg
MgSO
MgCO
MgHCO
4
3
3
53 10
3
−
−
1
=×
molkg
This is an example of a
mass balance equation
.
The stability of an ion pair or complex is measurable in
terms of its
dissociation constant
. For example, the disso-
ciation reaction
2
MgHCO g CO
3
+
+
+
−
(4.5.2)
3
Figure 4.5.1
Pie charts showing the molal proportions of principal cations, anions and ion pairs in seawater.
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