Geology Reference
In-Depth Information
Table 4.1
Solubility products of various
salts
(tabulated as p
K
(= − log
10
K
) for 25 °C).
Names of corresponding minerals are given in brackets
Halides
Carbonates*
PbCl
2
4.8
(i.e.
K
= 10
−4.8
)
CaCO
3
8.3
(calcite)
BaF
2
5.8
BaCO
3
8.3
CuCl
6.7
FeCO
3
10.7
AgCl
9.7
MgCO
3
6.5
CaF
2
10.4
(fluorite)
Sulfides
Sulfates
PbS
27.5
(galena)
BaSO
4
10.0
(barite)
HgS
53.3
CaSO
4
4.5
(anhydrite)
ZnS
24.7
(sphalerite)
PbSO
4
7.8
SrSO
4
6.5
Phosphate
Ca
5
(PO
4
)
3
F
60.4
(fluorapatite)
*Solubility dependent on pH and H
2
CO
3
concentration.
The solubility product for BaSO
4
at 25 °C is (from
Table 4.1):
Because of the contribution from the CaSO
4
solution,
the SO
4
2−
activity is now much higher (50 times) than it
was in the BaSO
4
solution. Calculating the ion activity
product for BaSO
4
in the mixed solution gives:
10
10 0
− .
K
=⋅
a
a
=
(4.17)
BaSO
2
+
2
−
Ba
SO
4
4
−
5
−
3
a
⋅
a
2
05 10
=
.
×
×
0 505
.
×
10
The corresponding calcium salt CaSO
4
is more soluble
at 25 °C
2
+
−
Ba
SO
4
=
10
−
86
.
This is considerably greater than the solubility product
of BaSO
4
at 25 °C. In spite of the two-fold dilution of
Ba, the additional concentration of sulfate ions has
made the solution supersaturated with BaSO
4
, and we
can expect precipitation of BaSO
4
to occur until the
activity product has been reduced to the equilibrium
value 10
−10
.
This unexpected outcome has arisen because BaSO
4
and CaSO
4
share an ionic species in common, the sul-
fate ion SO
4
2−
. Had the second solution (b) consisted of
calcium chloride CaCl
2
, not CaSO
4
, no extra sulfate
ions would have been introduced, and no BaSO
4
would
have precipitated. The precipitation of BaSO
4
by the
addition of CaSO
4
is an example of the
common-ion
effect
. The same outcome could be obtained by adding
BaCl
2
solution instead of CaSO
4
(in which case Ba
2+
would have been the common ion).
Barite deposits are found on the sea floor at places
where barium-containing hydrothermal fluids (in
which sulfur is present only as sulfide species like S
2−
,
H
2
S and HS
−
, and not as sulfate) emerge into sulfate-
bearing seawater, a natural example of precipitation
due to the common-ion effect.
10
45
− .
K
CaSO
4
=
(4.18)
Suppose now we mix equal volumes of:
(a) a saturated BaSO
4
solution (BaSO
4
activity 10
−5
),
and
(b) a CaSO
4
solution with an activity of 0.001 = 10
−3
(which the reader can easily confirm is undersatu-
rated with CaSO
4
).
Mixing these solutions dilutes both BaSO
4
and CaSO
4
by a factor of two (the same amount of each salt now
dissolved in twice the volume of water). We might
therefore expect the mixed solution to be less than sat-
urated with BaSO
4
. But consider the new ion activities
of the individual ions:
−
5
a
a
a
=×
=×
=×
05 10
05 10
05 10
.
.
.
2
+
Ba
−
3
2
+
Ca
−
5
−
3
+×
05 10
.
2
+
SO
4
contribution
fro
contribution
fromCaSO
mBaSO
4
4
−
3
=
0 505 10
.
×
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