Geology Reference
In-Depth Information
Warm surface ocean water is saturated or oversatu-
rated with dissolved carbonate. The solubility of CO
2
and CaCO
3
increases with depth, however - partly
because deep ocean water is colder - and one can iden-
tify a
carbonate compensation depth
or
lysocline
below
which ocean water becomes undersaturated with car-
bonate. This change is encountered about 3 km down
in the Pacific and about 4.5 km down in the Atlantic. It
has been estimated that 80% or more of the calcareous
shell material precipitated near the surface is redis-
solved during or after settling to the deep ocean floor.
m
m
m
2
−
2
SO
o
+
K
=⋅
a
a
=
Ba
o
⋅
(4.24)
4
BaSO
2
+
2
−
Ba
SO
m
4
4
=×
−
10 10
10
(4.25a)
(a) purewater
:
solubilityproduct
=×
−
75 10
10
(4.25b)
(b) NaCl solution
:
solubilityproduct
Notice that substantially more BaSO
4
will dissolve in
the non-ideal saline solution (b) than in the same
amount of pure water. The reason is that the reverse
reaction responsible for restricting solubility:
Non-ideal solutions: activity coefficient
2
+
2
−
BaSO
←+
Ba
SO
4
4
solid
solution
All of the solutes considered so far have been only
slightly soluble in water (their solubility products have
been very small numbers). The discussion has in effect
been limited to very
dilute
solutions, in which the ions
are so dispersed that electrostatic attraction and repul-
sion between them (called 'ion-ion interactions') can
be ignored. The behaviour of ionic species in such
solutions can be accurately expressed, as we have seen,
in terms of equilibrium constants that involve only the
concentrations of the species of interest. Solutions suf-
ficiently dilute to comply with this simple model of
behaviour are called
ideal solutions
.
Geochemical reality is less straightforward, however.
Most natural waters are complex, multi-salt solutions,
whose properties may be far from ideal. Although
we may direct our attention to species like BaSO
4
that
occur only in low concentrations, the solutions in which
they are dissolved will typically contain larger amounts
of other more soluble salts such as chlorides or bicarbo-
nates. In these stronger solutions, all ions present, includ-
ing the dispersed Ba
2+
and SO
4
2−
ions, will experience
many ion-ion interactions, which impede their freedom
to react. In particular, Ba
2+
and SO
4
2−
ions, tangled up
electrostatically with other types of ion, will be less likely
to meet and react with each other than if dissolved at the
same concentration in pure water. This depression of
reactivity is a function of the
total salt content
of the solu-
tion. When trying to understand equilibria between
specific ionic species in such
non-ideal solutions
, account
must therefore be taken of the concentrations of all sol-
utes present, not just the species of interest.
Consider the solubility products of BaSO
4
, measured
(a) in pure water and (b) in 0.1 molal
6
NaCl solution.
is inhibited by the non-ideal, ion-ion interactions that
Ba
2+
and SO
4
2−
ions experience in the presence of abun-
dant Na
+
and Cl
−
ions.
To be dealing with equilibrium 'constants' that vary
according to the nature of the host solution is unaccep-
table. The problem can be overcome by redefining
activity so that it serves as a measure of 'effective con-
centration', incorporating the reduction of ionic reac-
tivity in stronger solutions. A complete definition of
activity is therefore:
m
m
2
+
a
=⋅
γ
Ba
o
2
+
2
+
Ba
Ba
(4.26)
m
2
−
SO
o
a
=
γ
⋅
4
2
−
2
−
SO
SO
m
4
4
γ is the Greek letter gamma, and the functions
γ
Ba
2+
and
γ
SO
4
2−
are called the
activity coefficients
for Ba
2+
and
SO
4
2−
in the saline solution. An activity coefficient is
simply a variable factor that expresses the degree of
non-ideality of the solution (for an ideal solution it
equals 1.00). Although it may look like a 'fiddle factor',
the activity coefficient has some foundation in solution
theory, and its value can in many cases be predicted
with acceptable accuracy.
Expressing the equilibrium constant in Equation
4.24 in terms of the newly defined activities:
m
m
m
2
−
2
SO
o
+
K
=⋅ =⋅
a
a
γ
γ
Ba
o
⋅
(4.27)
4
BaSO
2
+
2
−
2
+
2
−
Ba
SO
Ba
SO
m
4
4
4
This equation remains true for all circumstances.
The solubility product (which by definition relates
to the observed concentrations,
mm
i
/o, not effective
That is, a solution in which the molality of NaCl is 0.1.
6
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