Carbon Dioxide, Alkalinity and pH

36 Pages·2015·3.11 MB·English

Carbon Dioxide,
Alkalinity and pH
OCN 623 – Chemical Oceanography
12 February 2015
Reading: Libes, Chapter 15, pp. 383 – 394
(Remainder of chapter will be used with the lecture: “Biogenic production,
carbonate saturation and sediment distributions”)
© 2015 Frank Sansone
Outline
1. CO speciation
2
2. pH
3. Alkalinity
4. Calculation of composition of marine CO system
2
5. Oceanographic applications
6. Global CO
2
Why is it important to understand
the CO system?
2
• CO controls the fraction of inbound radiation that remains
2
trapped in the atmosphere (greenhouse effect), which in
turn strongly influences planetary climate
• CO is the raw material used to build organic matter
2
• CO controls the pH of the oceans
2
• Distribution of CO species affects preservation of CaCO
2 3
deposited on the sea floor
CO Speciation
2
• CO (g) has many possible transformations upon dissolution in H O
2 2
• Major dissolved forms:
CO (aqueous carbon dioxide – a dissolved gas)
2(aq)
H CO (carbonic acid – trace amount)
2 3
HCO - (bicarbonate ion)
3
CO -2 (carbonate ion)
3
• Species interconvert readily
• Perturbations to one part of CO system leads to redistribution of
2
species
• Reactions not always intuitive!
Equations for CO Speciation
2
The equilibrium of gaseous and aqueous CO :
2
CO  CO
2(g) 2(aq)
Subsequent hydration and dissociation reactions:
CO + H O  H CO  HCO - + H+
2(aq) 2 2 3 3
{H }[HCO ]
K *  3
1
[CO ]
2
HCO -  CO -2 + H+
3 3
{H }[CO 2]
K *  3
2 
Asterisk (*) indicates a “stoichiometric” constant [HCO ]
3
Hint: when pH is between 7.5 and 8.5:
CO + CO -2 + H O  2HCO -
2(aq) 3 2 3
CO
2(aq)
Seawater values shown --- freshwater curves are shifted left
CO  CO
2(g) 2(aq)
CO + H O  HCO - + H+
2(aq) 2 3
HCO -  CO -2 + H+ aob.oxfordjournals.org
3 3
How to Calculate K * and K *
1 2
pK = - log K
Millero et al. (2006) Mar. Chem. 100, 80-94
For: S = 0 – 50 g/kg, T = 0 – 50°C
At S = 35 g/kg and T = 25°C:
pK * = 5.8401 pK * = 8.9636
1 2
Effects of Pressure on K * and K *
1 2
1 atm 1000 atm
Decreased pressure {H }[HCO ]
pK * 5.89 5.55 K *  3
1 shifts reactions to
1
[CO ]
pK * 9.13 8.93 left 2
2
(K *) / (K *) (K *) / (K *)
1 d 1 1 2 d 2 1
“1” = 1 atm
As you raise a sample
from depth:
- Ks’ decrease
- Reactions shift to left CO  CO
2(g) 2(aq)
- pH increases CO + H O  HCO -+ H+
2(aq) 2 3
- CO release
2(g) HCO -  CO -2 + H+
3 3
Dissolved Inorganic Carbon
(DIC, Total CO , CO )
2 2
DIC  [CO ] + [H CO ] + [HCO -] + [CO -2]
2(aq) 2 3 3 3
• At seawater pH, >99% of CO species are HCO - and CO -2,
2 3 3
so we can simplify:
DIC  [HCO -] + [CO -2]
3 3
• Equilibrium equations can be combined to allow us to
calculate the concentration of any of the CO species
2
Partial Pressure of CO
2
(P )
CO2
In the atmosphere:
P  partial pressure of CO in the atm,
CO2 2
out of a total pressure of 1 atm (at sea level)
Measurement:
Determine fraction of atm that is CO
2
P = P · fraction
CO2 total CO2