Geoscience Reference
In-Depth Information
7.3 Speciation in solutions
An important problem in the geochemistry of solutions is the distribution of the chemi-
cal components among the various species. A simple example is that of the distribution
of carbonates as H
2
CO
3
,HCO
3
, and CO
2
3
. Calculating the speciation of a solution of
known composition involves evaluating the abundance of different chemical species under
prescribed conditions of temperature, pressure, pH, and other factors. Let us make a very
simple simulation of the deep ocean isolated from the atmosphere after the formation of
bottom waters and which receives carbonate tests of sedimented foraminifera. We intro-
duce
m
CO
2
moles of CO
2
per liter into pure water, and then a very small known quantity
of calcite, such that the molarity of calcite in water is
m
CaCO
3
, and leave the calcite to
dissolve. Let us now inquire into the abundance of the species present in the solution. The
CO
2
acidifies the water allowing the calcite to be dissolved.
We choose two components, Ca
2
+
and CO
2
. The six possible species are OH
−
,H
+
,
Ca
2
+
, and the carbonates H
2
CO
3
,HCO
3
, and CO
2
3
. The concentration of H
2
Oishigh
enough compared with the rest of the dissolved species to be invariant in the process,
and so we can eliminate it both as a species and a component. If we wish to determine
the abundances of the six species, we have two mass-balance equations, one for the sum
of CO
2
:
H
2
CO
3
CO
2
3
+
HCO
3
+
=
m
CO
2
+
m
CaCO
3
(7.18)
and the other for the sum of calcium ions:
Ca
2
+
=
m
CaCO
3
(7.19)
and one equation for electrical neutrality:
HCO
3
+
2
CO
2
3
2
Ca
2
+
+
OH
−
=
H
+
+
(7.20)
We therefore need another three equations (given in the form of mass action laws) to deter-
plus
(7.5)
for water dissociation.
This seemingly natural choice is in fact arbitrary. There is nothing to prevent us from
replacing one of these equations by a combination of them. Thus, either of the two
equations for carbonic acid dissociation could be replaced by the product of the two:
CO
2
3
H
+
2
[H
2
CO
3
]
=
K
1
K
2
(7.21)
It can be seen that, in the general case, speciation calculations involve resolving systems
that include non-linear equations and therefore call on advanced numerical techniques.
There are many software packages that perform these calculations with a high degree of
sophistication allowing for many complexes and the presence of solid or gaseous phases.
Such software is economically significant for the exploration and management of water,
oil, and geothermal resources. It can also be used for understanding weathering and the
genesis of mineral deposits.
