Lecture 9: Ocean Carbonate Chemistry:

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Lecture 9 Ocean Carbonate Chemistry
Carbonate Reactions Reactions Solutions nume
rical graphical K versus K
What can you measure?
Theme 1 (continuation) Interior Ocean Carbon
Cycle Theme 2 Ocean Acidification (mans altera
tion of the ocean)
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Sarmiento and Gruber (2002) Sinks for
Anthropogenic Carbon Physics Today August 2002 30
-36
1Pg 1015g
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CO2 rocks HCO3- clays
CO2
River Flux
Gas Exchange
Atm
Ocn
CO2 ? H2CO3 ? HCO3- ? CO32-
Upwelling/ Mixing
H2O CH2O O2
Ca2 CaCO3
CO2
BorgC
BCaCO3
Biological Pump
Controls pH of ocean Sediment diagenesis
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Weathering and River Flux
Atmospheric CO2 is converted to HCO3- in rivers
and transported to the ocean
Examples Weathering of CaCO3 CaCO3(s) CO
2(g) H2O Ca2 2 HCO3-
1 2 Weathering of alumino-sil
icate minerals to clay minerals.
silicate minerals CO2(g) H2O clay miner
als HCO3- 2 H4SiO4? cation
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1 2 A specific example
of orthoclase to kaolinite KAlSi3O8(s) CO2(
g) 11/2H2O
1/2 Al2Si2O5(OH)4(s) K HCO3-
2H4SiO4?
Example Global River Flux River Flow x global
average HCO3 concentration Global River Flux 3.
7 x 1016 l y-1 x 0.9 mM 33.3 x 1012 mol y-1
2.8 x 1012 g y-1 2.8 x 10-3 Pg y-1
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CO2 reacts with H2O to make H2CO3

CO2 (g) H2O H2CO3 KH
H2CO3 is a weak acid H2CO3 H HCO3- K1

HCO3- H CO32- K2
H2O is also a weak acid H2O H OH- KW

H2CO3 carbonic acid HCO3- bicarbonate CO32-
carbonate H proton or hydrogen ion OH-
hydroxyl
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Equilibrium Constants 4 equilibrium constants in
seawater K f (S,T,P)
These are expressed as K'. 1. CO2(g) H2O
H2CO3 (Henry's Law) KH H2CO3 / PCO2
(note that gas concentrations are given as
partial pressure e.g. atmospheric PCO2 10-3.5
) 2. H2CO3 H HCO3- K1 HCO3-(H
) / H2CO3 3. HCO3- H CO32- K2 (H
)CO32- / HCO3- 4. H2O H OH- Kw
(H)(OH-)
( ) Activity effective concentration
Concentration
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Values of K
The values here are for S 35, T 25?C and P
1 atm. Constant Apparent Seawater Constant (
K') KH 10-1.53 K1 10-6.00 K2 10-9.10 Kw
10-13.9
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H from pH -log H at pH 6 H 10-6
OH- from OH- KW / H at pH 6 OH- 10-8
Total CO2 (SCO2 or CT ) Dissolved Inorganic
carbon (DIC) DIC H2CO3 HCO3- CO3
2-
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Example If you add reactions what is the K for
the new reaction? H2CO3 H HCO3- K1 10
-6.0 plus HCO3- H CO32- K2 10-9.1 -
-----------------------------------------------
H2CO3 2H CO32- K12 10-15.1
Example Say we want the K for the reaction
CO32- H2CO3 2 HCO3-
Then we have to reverse one of the reactions. It
s K will change sign as well!!
So H2CO3 H HCO3- K 10-6.0 H C
O32- HCO3- K 109.1 -----------------------
---------------------------------------------
H2CO3 CO32- 2HCO3- K 103.1 ?
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Calculations Graphical Approach Algebraic A
pproach
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Construct a Distribution Diagram for H2CO3
Closed System a. First specify the total CO2 (e.g
. CT 2.0 x 10-3 10-2.7 M) b. Locate CT on the
graph and draw a horizontal line for that
value. c. Locate the two system points on that li
ne where pH pK1 and pH pK2.
d. Make the crossover point, which is 0.3 log
units less than CT e. Sketch the lines for the sp
ecies
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Table of acids in seawater
pK -logK
Element Reaction mol kg-1 -logC pK'
H2O H2O H OH- 13.9 C H2CO3 HCO3- H
2.4 x 10-3 2.6 6.0 HCO3- CO32- H
9.1 B B(OH)3 H2O B(OH)4- H 4.25 x 10-
4 3.37 8.7 Mg Mg2 H2O MgOH H 5.32 x
10-2 1.27 12.5 Si H4SiO4 SiO(OH)3- H
1.5 x 10-4 3.82 9.4 P H3PO4 H2PO4- H 3
.0 x 10-6 5.52 1.6 H2PO4- HPO 2- H
6.0 HPO42- PO43- H 8.6 S(VI)
HSO4- SO42- H 2.82 x 10-2 1.55 1.5
F HF F- H 5.2 x 10-5 4.28 2.5
Ca Ca2 H2O CaOH H 1.03 x 10-2 1.99
13.0 And in anoxic systems N NH4 NH3
H 10 x 10-6 5.0 9.5 S(-II) H2S HS- H
10 x 10-6 5.0 7.0 HS- S2- H
13.4
(e.g. K 10-13.9)
Q. Which is larger? pK 6.0 or 9.1
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Carbonic Acid 6 unknowns
Carbonic acid is the classic example of a
diprotic acid and it can have a gaseous form.
It also can be expressed as open or closed to the
atmosphere (or a gas phase) There are 6 speci
es we need to solve for CO2(g) Carbon Dioxide G
as H2CO3 Carbonic Acid (H2CO3
CO2 (aq) H2CO3) HCO3- Bicarbonate CO32- Car
bonate H Proton OH- Hydroxide To solve for
six unknowns we need six equations
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What can you measure?
We can not measure these species directly. What
we can measure are pH pH is defined in terms
of the activity of H or as pH -log (H)
b) Total CO2 CT H2CO3 HCO3- CO32-
c) Alkalinity Alkalinity HCO3- 2CO32-
OH- - H B(OH)4- any other bases
present The alkalinity is defined as the amoun
t of acid necessary to titrate all the weak bases
in seawater (e.g. HCO3-, CO32-, B(OH)4-) to the
alkalinity endpoint which occurs
where (H) (HCO3-) (see graph)
d) PCO2 The PCO2 in a sample is the PCO2 that a
water would have if it were in
equilibrium with a gas phase
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The Proton Balance
The balance of species that have excess protons
to species deficient in protons
relative to a stated reference level.
For H2CO3 H HCO3- 2 CO32- OH- For H
CO3- H H2CO3 CO32- OH-
For CO32- H 2 H2CO3 HCO3- OH-
The proton conditions define three equivalence p
oints on the graph and these are used to define
6 capacity factors for the solution.
You can approach each equivalence point from eit
her the acid or base direction.
If you add strong acid (e,g, HCl ) it is represe
nted as CA Strong base (e.g. NaOH) is represented
as CB. For Example For a pure solution of H2
CO3 CB H HCO3- 2 CO32- OH- CA The
n CB CA HCO3- 2CO32- OH- - H
Alkalinity CA CB H - HCO3-
2CO32- OH- H-Acidity
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Carbonate System Calculations
pH and CT
Alkalinity and PCO2
A useful shorthand is the alpha notation, where
the alpha (a) express the fraction each carbonate
species is of the total DIC. These a values are a
function of pH only for a given set of acidity
constants. Thus H2CO3 ao CT HCO3- a1 CT
CO32- a2 CT The derivations of the equatio
ns are as follows ao H2CO3 / CT H2CO3 /
(H2CO3 HCO3 CO3) 1 / ( 1 HCO3 / H
2CO3 CO3/H2CO3) 1 / ( 1 K1/H K1K2
/H2) H2 / ( H2 HK1 K1K2) The value
s for a1 and a2 can be derived in a similar
manner. a1 HK1 / (H2 H K1 K1K2) a2 K
1K2 / ( H2 H K1 K1K2) For example Assume pH
8, CT 10-3, pK1' 6.0 and pK2' 9.0
H2CO3 10-5 mol kg-1 (note the answer is in c
oncentration because we used K')
HCO3- 10-3 mol kg-1 CO32- 10-4 mol kg-1
Alk HCO3 2 CO3 OH - H
For this problem neglect H and OH (a good
assumption ), then CT a1 2 CT a2 CT (a
1 2a2) We can use this equation if we have a cl
osed system and we know 2 of the 3 variables
(Alk, CT or pH). For an open system we can expres
s CT in terms of PCO2 as follows
We know that H2CO3 CT ao ( you can also use
this equation if you know pH and PCO2)
But H2CO3 can be expressed in terms of the
Henry's Law KH PCO2 CT ao So CT KH P
CO2 / ao Now Alk (KH PCO2 / ao ) ( a1 2a
2) Alk KH PCO2 ( (a1 2 a2 ) / ao ) Alk
KH PCO2 ( HK1 2 K1K2 / H2 )
Assume Alk 10-3 PCO2 10-3.5 pK1' 6.0
pK2' 9.0 Then pH 8.3
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CaCO3 solubility calculations CaCO3 Ca2
CO32- Ks0 (calcite) 4.26 x 10-7 10-6.37
Ks0 (aragonite) 6.46 x 10-7 10-6.19 o
r CaCO3 CO2 H2O Ca2 2 HCO3-
Ion Concentration Product ICP Ca2CO32-
Omega O ICP / Ks0 If water at equilibriu
m (saturation) O 1 If water oversaturated
O 1 CaCO3 precipitates
If water undersaturated O dissolves
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What controls the pH of seawater?
pH in seawater is controlled by alkalinity and D
IC and can be calculated from these
two parameters as shown below.
Alk ? HCO3- 2 CO32- Alk ? CT a1 2 CT a2
Alk CT (HK1' 2 K1' K2' ) / (H2 H K1'
K1'K2') Rearranging, we can calculate pH from
Alk and CT. (H) ?-K1' (Alk-CT) (K1')2
(Alk-CT)2 - 4 Alk K1' K2' (Alk - CT) ? / 2
Alk So the question boils down to what control
s alkalinity and total CO2. Internal variations
of pH in the ocean and controlled by internal
variations in DIC and alkalinity which are contr
olled by photosynthesis, respiration
and CaCO3 dissolution and precipitation.
The long term controls on alkalinity and DIC are
the balance between the sources and sinks
and these are the weathering (sources) and burial
(sinks) of silicate and carbonate rocks
and organic matter.
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What happens to the CO2 that dissolves in water?
CO2 is taken up by ocean biology to produce a
flux of organic mater to the deep sea (BorgC)
CO2 H2O CH2O O2 Some carbon is taken up
to make a particulate flux of CaCO3 (BCaCO3)
Ca2 2HCO3- CaCO3(s) CO2 H2O The biol
ogically driven flux is called the Biological
Pump. The sediment record of BorgC and BCaCO3
are used to unravel paleoproductivity.
The flux of BorgC to sediments drives an extensi
ve set of oxidation-reduction reactions that are
part of sediment diagenesis. Carbonate chemist
ry controls the pH of seawater which is a master
Variable for many geochemical processes.
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Open System - Gas Solubility Henrys Law
The exchange or chemical equilibrium of a gas
between gaseous and liquid phases
can be written as A (g) A (aq) At eq
uilibrium we can define K A(aq) / A(g)
Henry's Law We can express the gas concentr
ation in terms of partial pressure using the
ideal gas law PV nRT so that A(g) is equ
al to the number of moles n divided by the
volume n/V A(g) PA / RT where PA i
s the partial pressure of A Then K A(
aq) / PA / RT or
A(aq) (K/RT) PA A(aq) KH
PA units for K are mol kg-1 atm-1
for PA are atm in mol
kg-1 Henry's Law states that the solubili
ty of a gas is proportional its overlying partial
pressure.
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Example Gas concentrations in equilibrium with
the atmosphere
Atmosphere Composition
Gas Mole Fraction in Dry Air (fG) (where fG
moles gas i/total moles) N2 0.78080 O2 0
.20952
Ar 9.34 x 10-3 CO2 3.3 x 10-4
Gas Pi KH (0?C , S 35) Ci (0?C, S 35 P 1
Atm N2 0.7808 0.80 x 10-3 62.4 x 10-3 mol kg-1
O2 0.2095 1.69 x 10-3 35.4 x 10-3
Ar 0.0093 1.83 x 10-3 0.017 x 10-3
CO2 0.00033 63 x 10-3 0.021 x 10-3
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Open System Distributions
Assume equilibrium with a constant composition
gas phase with PCO2 10-3.5