pH, Carbonate Chemistry and Ocean Acidification

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pH, Carbonate Chemistry and Ocean Acidification
Lecture 10
OEAS-306
February 19, 2009

• Outline
• Review from last lecture
• Definition of pH
• Carbonate Chemistry
• Buffering
• Ocean acidification

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Activity defined as the effective concentration
of a solute.
Concentration of ion i Activity of ion i
Generalized Equilibrium Expression
reactants
products
At equilibrium, the rate of the forward reaction
is equal to the that of the reverse.
equilibrium constant for a chemical reaction can
be defined in terms of activity
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The pH of Seawater
Water molecule dissociates into a Hydrogen ion
and Hydroxide ion
Keq for this reaction is 10-14
Remember
If
then
By definition H2O 1
Definition pH -log10 H
Example, what is meaning of pH 5.3?
pH 5.3 -log10 H
so, H 10-5.3
10-5.3 OH- 10-14
If H 10-5.3, what is OH-?
OH- 10-14/10-5.3 10-8.7
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From the definition of pH, you can see that a pH
of 7 is neutral.
pH 7 -log10 H
H OH- 10-14
so, OH- 10-7 and H 10-7
When pH is less than 7, solution is acidic
An acid is considered any chemical compound that,
when dissolved in water, gives a solution with a
hydrogen activity greater than in pure water
(i.e. pH lt 7).
When pH is greater than 7, solution is basic
An base is considered any chemical compound that,
when dissolved in water, gives a solution with a
hydrogen activity less than in pure water (i.e.
pH gt 7).
Seawater is slightly alkaline pH 8
What are the hydrogen and hydroxide activities of
seawater?, H ? and OH- ?
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Carbonate Cycle
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Carbonate Chemistry of Seawater
Step 1 Carbon dioxide gas diffuses easily into
the ocean and combines with water to form
carbonic acid.
Step 2 Carbonic acid dissociates into hydrogen
ions and bicarbonate.
Step 3 Bicarbonate dissociates into hydrogen
ions and carbonate.
Each of these reactions is in equilibrium
Total Dissolved Inorganic Carbon (DIC)
CO2H2CO3HCO3-CO32-
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Relative Proportions of Carbonate Ions is
Seawater are Related to pH
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Seawater is Buffered against changes in pH
Addition of an acid to seawater
An acid is consider any chemical compound that,
when dissolved in water, gives a solution with a
hydrogen activity greater than in pure water
(i.e. pH lt 7).
Addition of a base to seawater
An base is consider any chemical compound that,
when dissolved in water, gives a solution with a
hydrogen activity less than in pure water (i.e.
pH gt 7).
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Anthropogenic Carbon Dioxide
Most scientists believe that burning fossil fuels
has increased atmospheric carbon dioxide levels

• The global oceans are the largest natural
  reservoir for this excess carbon dioxide,
  absorbing approximately one-third of the carbon
  dioxide added to the atmosphere by human
  activities each year, and over the next
  millennium, is expected to absorb approximately
  90 of the CO2 emitted to the atmosphere.
• The uptake of anthropogenic CO2 by the ocean
  changes the chemistry of the oceans and can
  potentially have significant impacts on the
  biological systems in the upper oceans.

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Why is the ocean so effective at taking up carbon
dioxide?

• Atmospheric carbon dioxide concentrations (337
  ppm) are higher than oceanic carbon dioxide
  concentrations (64-100 ppm). This drives a
  down-gradient flux into the ocean.
• Carbon dioxides solubility is enhanced by the
  reaction with carbonate. Equilibrium constant
  for this reaction is so large that most of the
  CO2 entering the ocean is rapidly converted to
  bicarbonate (HCO3-).
• The amount of carbon dioxide absorbed by the
  ocean is greatly augmented by the sinking of
  calcium carbonate in biological materials.
• This material either is buried in the sediments
  or is transported below the pycnocline where it
  is effectively trapped and cannot escape to the
  atmosphere until it recirculates back into
  surface waters.

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Ocean Acidification
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Based on the emissions scenarios of the
Intergovernmental Panel on Climate Change and
general circulation models, we may expect a drop
in ocean pH of about 0.4 pH units by the end of
this century, and a 60 decrease in the
concentration of calcium carbonate, the basic
building block for the shells of many marine
organisms.
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Ocean acidification impacts shell formation of
planktonic organisms
Healthy shell of coccolithophorid
Increasing acidity interferes with proper shell
formation
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Aragonite is a form of Calcium Carbonate
important to coral reefs.
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• Summary
• pH is defined as the activity of hydrogen ions in
  a solution.
• By definition pH -log10H
• Seawater has a pH 8, so it is slightly
  alkaline.
• Dissolved carbon dioxide combines with water to
  form carbonic acid.
• Under normal pH conditions, carbonic acid rapidly
  dissociates into bicarbonate and hydrogen ions.
• The carbonate chemistry of buffers seawater
  against large changes in pH.
• The worlds oceans have a tremendous capacity to
  absorb carbon dioxide.
• Increasing atmospheric CO2 levels are thought to
  be increasing the acidity (decreasing pH) of the
  ocean.
• This could have significant impacts on biological
  organisms that utilize calcium carbonate, like
  certain types of phytoplankton and corals.

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There will be an Exam on the Chemical
Oceanography section on Tuesday (February 24th,
2009).

• Exam will cover all material presented in
  lectures and reading.
• Readings include chapters 6 and 7 from Textbook
  (Garrison).
• Format will be the same as last exam 50
  multiple choice questions and 5 short
  essay/problems.
• Please bring a calculator if you are not good at
  doing math in your head. No computers or phones
  can be used.
• I will be in my office Friday and Monday if you
  have questions, need help, etc ... Please dont
  hesitate to contact me.

Lectures are available at http//www.ccpo.odu.edu
/mscully/OEAS_306/