Carbon Dioxide’s Ocean Chemistry
Carbon dioxide is fundamentally important for life in the oceans (and the
rest of the Earth). About half of all photosynthesis on Earth occurs in the
oceans where phytoplankton, like the coccolithophore pictured in the
photomicrograph here, use sunlight and dissolved CO2 to produce the
compounds that feed the rest of the ocean life. The CO2 is also
incorporated into the structures of many of these phytoplankton and other
organisms. It’s this latter reactivity that we will examine in this activity.
Materials
For individual small group activity:
5 mL calcium chloride, CaCl2, solution in a clear, colorless capped plastic test tube (or
equivalent)
5 mL sodium hydrogen carbonate, Na(HOCO2), solution with acid-base indicator in a clear,
colorless capped plastic test tube (or equivalent)
For whole group activity:
25 mL calcium hydroxide, Ca(OH)2, solution with acid-base indicator in a clear, colorless
flat-bottomed plastic container. (Can be scaled up.)
source of CO2 gas
overhead projector
Background
CO2(g) from the atmosphere dissolves in the ocean and reacts with the water.
CO2(g)  CO2(aq)
(1)
CO2(aq) + H2O  HOCO2–(aq) + H+(aq)
(2)
HOCO2–(aq)  CO32–(aq) + H+(aq)
(3)
Reactions (2) and (3) are shown, for simplicity, as Arrhenius acid-base reactions, instead of
Brönsted acid-base reactions. The usual pH of seawater is 8.2. At this pH, about 90% of the
dissolved CO2 is present as the hydrogen carbonate ion, HOCO2–(aq), with most of the rest as
carbonate ion, CO32–(aq). The major cations in seawater are Na+, K+, Mg2+, and Ca2+. The
calcium ion, Ca2+(aq), is most important for the reactions involving dissolved carbon dioxide that
are the basis of these activities.
Procedures, Observations, and Questions
Individual small group activity
1. Describe the aqueous solutions of CaCl2 and Na(HOCO2).
2. Mix the two solutions by pouring the CaCl2 solution into the Na(HOCO2). solution.
Observe and describe what you observe.
3. What products of the reaction between the aqueous CaCl2 and Na(HOCO2) solutions do
you think you can identify?
4. Write and balance a net reaction equation (without spectator ions) describing the reaction.
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Whole group activity
A solution of limewater contains Ca2+(aq) and OH–(aq) ions from the dissolved lime. CO2
bubbled into the solution undergoes these reactions, the familiar limewater test for CO2.
CO2(g)  CO2(aq)
(1)
CO2(aq) + OH–(aq)  HOCO2–(aq)
(4)
HOCO2–(aq) + OH–(aq)  CO32–(aq)
+ H2O
Ca2+(aq) + CO32–(aq)  CaCO3(s)
(5)
(6)
4. Place the container of limewater solution containing acid-base indicator on the overhead
projector so the audience can observe any changes that occur in the solution. Bubble a
small amount of CO2 gas into the solution. Observe and describe what you observe.
5. Do you see evidence for the above reactions? Explain how the observations provide this
evidence.
6. Continue bubbling CO2 gas into the limewater solution. Is there evidence for any
reaction(s) not included in the above sequence? If so, what is the evidence?
7. What is(are) the reaction(s), if any, responsible for your observations in item (6)? If there
is a reaction, is it related to the one you wrote in item (4) above?
Discussion
Many phytoplankton, like the one pictured at the beginning of this write-up, create external
structures from calcium carbonate, CaCO3. They use the Ca2+(aq) and HOCO2–(aq) from the
surrounding seawater to synthesize CaCO3(s) by the reaction you wrote in item (4). In seawater
at pH 8.2, these CaCO3(s) structures are relatively insoluble. When CO2 continues to be added to
a suspension of CaCO3(s), the pH decreases and the solubility of the CaCO3(s) increases, as you
have observed. This change is occurring on Earth on a very large scale. As the amount of CO2 in
the atmosphere increases (due largely to fossil fuel burning), the amount dissolved in the oceans
increases and the pH of the seawater decreases, which makes CaCO3(s) more soluble. The results
for organisms like phytoplankton and corals that use CaCO3(s) as part of their structures are
unknown, but could be a serious disruption of life in the oceans. In some sense, an unplanned
planetary-scale experiment is going on whose outcome we cannot, at the moment, predict.
Reagent Preparation (adjust volumes for the number of groups doing the activity)
Calcium chloride, CaCl2, solution. Dissolve 12 g CaCl2 in 100 mL water. Allow the solution to
cool before using.
Sodium hydrogen carbonate, Na(HOCO2), solution. Dissolve 10 g Na(HOCO2) in 100 mL hot
water. Add 1 mL acid-base indicator and mix. Allow the solution to cool before using.
Calcium hydroxide, Ca(OH)2, solution. Stir 1 g of Ca(OH)2 with 100 mL of water overnight.
Allow any undissolved solid to settle, decant 50 mL of the clear, colorless solution, and
dilute it to 100 mL. Add 1 mL acid-base indicator and mix.
Source of CO2 gas for bubbling a controlled stream of CO2 into a sample of limewater. For the
relatively small-scale procedure described here, the CO2 cartridges available at bicycle shops
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as emergency tire inflators (together with the devices required to use the cartridges) are an
ideal source.
Acid-base indicator solution. Universal indicator is best, but bromothymol blue or other indicator
that changes color around pH 6 is satisfactory.
Reference
American Chemical Society Climate Science Toolkit (www.acs.org/climatescience). Go to
“Oceans, Ice & Rocks”  “Ocean Chemistry”
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