13. Open
CO2 System
Closed
CO2 System
Given
(Input)
pCO2 DIC
Calculate
(Output)
pH
carb. speciation
DIC
pH
carb. speciation
pCO2
DIC = [CO2] + [HCO3
-] + [CO3
-2] pCO2 = – log KH – log {CO2}
In an open system you set pCO2; in a closed system you set DIC.
You cannot set both values independently.
15. Equilibrium Speciation at 25 °C
H2O + 1 mM DIC
open system closed system
input pCO2 = 3.408 DIC = 1 mM
pH 5.61 4.68
CO2 mM 0.0133 0.979
HCO3
- mM 0.0024 0.021
CO3
-2 mM 4.7·10-8 4.8·10-8
DIC mM 0.0157 1.000
pCO2 3.408 1.54
CO2 abbreviates the composite carbonic acid: H2CO3
* = CO2(aq) + H2CO3
= CO2 + HCO3
- + CO3
-2
H2O in contact with atmosphere
(PCO2 = 0.00039 atm = 10-pCO2)
= H2CO3
*
= – 1.47 – log CO2 (in M)
16. A closed system with 0.0157 mM DIC mimics
an open CO2 system in contact with atmosphere.
An open CO2 system with pCO2 = 1.54
mimics an closed system with 1 mM DIC.
typical for groundwater
(enhanced CO2 emerges
from degradation of organic matter)
In an open system you set pCO2; in a closed system you set DIC.
You cannot set both values independently.
However, you can outmaneuver this concept:
17. But this is only half the story, and the less important half.
The concept of open/closed systems becomes especially
relevant when the solution is attacked by acids or bases:
In a open system the CO2 (or pCO2 value) remains
constant, while in a closed system DIC remains constant
(and CO2 changes).
18. CO2 System + Acid/Base
(Titration Calculations)
Part 3
19. For a given value of DIC, say 1 mM, the pH is fixed to 4.68.
There is no chance to alter pH
unless the pure CO2 system is attacked
by HCl (to decrease pH) or NaOH (to increase pH).
H2O +
1 mM DIC
HCl
pH 1
H2O +
1 mM DIC
NaOH
pH 14