Transcript Carbonate Geochemistry

THE RELATIONSHIP
BETWEEN H2CO3* AND HCO3We can rearrange the expression for K1 to obtain:
aHCO 
K1
3

aH  aH 2CO3*
This equation shows that, when pH = pK1, the activities of
carbonic acid and bicarbonate are equal.
We can also rearrange the expression for K2 to obtain:
aCO 2 
K2
3

aH  aHCO 
3
This equation shows that, when pH = pK2, the activities of
bicarbonate and carbonate ion are equal.
Bjerrum plot showing the activities of inorganic carbon species as a
function of pH for a value of total inorganic carbon of 10-3 mol L-1.
-2
Common pH
range in nature
6.35
H2CO3*
-3
-
HCO3
10.33
2-
CO3
log ai
-4
OH-
-5
+
H
-6
-7
-8
0
2
4
6
8
10
12
14
pH
In most natural waters, bicarbonate is the dominant carbonate species!
THE CO2-H2O SYSTEM - I
Carbonic acid is a weak acid of great importance
in natural waters. The first step in its formation is
the dissolution of CO2(g) in water according to:
CO2(g)  CO2(aq)
At equilibrium we have:
KCO2 
aCO2
pCO2
Once in solution, CO2(aq) reacts with water to form
carbonic acid:
CO2(aq) + H2O(l)  H2CO30
THE CO2-H2O SYSTEM - II
In practice, CO2(aq) and H2CO30 are combined
and this combination is denoted as H2CO3*. It’s
formation is dictated by the reaction:
CO2(g) + H2O(l)  H2CO3*
For which the equilibrium constant at 25°C is:
KCO2 
aH 2CO3*
pCO2
 101.46
Most of the dissolved CO2 is actually present as
CO2(aq); only a small amount is actually present
as true carbonic acid H2CO30.
THE CO2-H2O SYSTEM - III
Carbonic acid (H2CO3*) is a weak acid that dissociates
according to:
H2CO3*  HCO3- + H+
For which the dissociation constant at 25°C and 1 bar is:
K1 
aHCO  aH 
3
aH 2CO3*
 106.35
Bicarbonate then dissociates according to:
HCO3-  CO32- + H+
K2 
aCO 2  aH 
3
aHCO 
3
 1010.33
Bjerrum plot showing the activities of inorganic carbon species as a
function of pH for a value of total inorganic carbon of 10-3 mol L-1.
-2
Common pH
range in nature
6.35
H2CO3*
-3
-
HCO3
10.33
2-
CO3
log ai
-4
OH-
-5
+
H
-6
-7
-8
0
2
4
6
8
10
12
14
pH
In most natural waters, bicarbonate is the dominant carbonate species!
SPECIATION IN OPEN CO2-H2O
SYSTEMS - I
• In an open system, the system is in contact with its
surroundings and components such as CO2 can migrate
in and out of the system. Therefore, the total carbonate
concentration will not be constant.
• Let us consider a natural water open to the atmosphere,
for which pCO2 = 10-3.5 atm. We can calculate the
concentration of H2CO3* directly from KCO2:
KCO2 
M H 2CO3*
M H 2CO3*  pCO2 KCO2
pCO2
log M H 2CO3*  log pCO2  log KCO2
Note that M H2CO3* is independent of pH!
SPECIATION IN OPEN CO2H2O SYSTEMS - II
• The concentration of HCO3- as a function of pH is next
calculated from K1:
K1 
M HCO  aH 
M HCO  
3
M H 2CO3*
K1M H 2CO3*
aH 
3
but we have already calculated M H2CO3*:
so
M H 2CO3*  pCO2 KCO2
M HCO  
aH 
 log K1KCO2 pCO2  pH
3
log M HCO 
3
K1KCO2 pCO2


SPECIATION IN OPEN CO2H2O SYSTEMS - III
• The concentration of CO32- as a function of pH is next
calculated from K2:
K2 
M CO 2  aH 
M CO 2 
3
M HCO 
3
3
K 2 M HCO 
3
aH 
but we have already calculated M HCO3- so:
M HCO  
3
K1KCO2 pCO2
aH 
and
M CO 2 
3
K 2 K1KCO2 pCO2
aH2 
log M CO 2  log K2 K1KCO2 pCO2   2 pH
3
SPECIATION IN OPEN CO2H2O SYSTEMS - IV
• The total concentration of carbonate CT is
obtained by summing:
CT  M H 2CO3*  M HCO   M CO 2
3
CT  pCO2 KCO2 
K1 pCO2 KCO2
aH 
3

K1K 2 pCO2 KCO2
a
2
H




K
K
K
log CT  log  pCO2 KCO2 1  1  12 2 
 a 

a


H
H


Plot of log concentrations of inorganic carbon species H+ and OH-,
for open-system conditions with a fixed pCO2 = 10-3.5 atm.
pK1
pK2
log concentration (molar)
0
CO32-
-2
CT
H+
OH-
-4
H2CO3*
-6
HCO3-
-8
2
3
4
5
6
7
pH
8
9
10
11
12
Plot of log concentrations of inorganic carbon species H+ and OH-,
for open-system conditions with a fixed pCO2 = 10-2.0 atm.
pK1
pK2
log concentration (molar)
0
H+
-2
-4
CO32CT
H2CO3*
OH-
-6
HCO3-
-8
2
3
4
5
6
7
pH
8
9
10
11
12
Calcite Solubility?
• CaCO3 -> Ca2+ + CO32• Log K=8.48
• Ca2+ in Ocean = 0.0106 m