Ocean Acidification

From A Geological and Chemical Perspective

Timeframe
1 Fifty minute class period
Description

Target Audience This lesson plan emphasizes balance between natural geologic process-
High school chemistry or earth science es and human activities affecting ocean seawater acidity, as measured
students (grades 9-12) by pH, and discusses how human activities are altering these processes
Materials to cause a rapid decline in ocean pH (termed ocean acidification).
- PowerPoint presentation
Objectives
For each group:
- 1 Student Data Sheet (at back of
• Students will develop a basic understanding of the logarithmic pH
lesson) scale and how an increase in CO2 in the ocean will lead to a de-
- 4 Erlenmeyer Flasks or other clear crease in pH (increase in acidity).
containers (500mL capacity)
- 4 types of water (see “Activity” sec- • Students will be able to identify how differences in ocean alkalinity
tion for further instruction on making
affect the rate at which ocean acidification occurs.
waters):
1. Fresh water (tap is ok, store-bought
• Students will gain experience developing a testable hypothesis and
distilled (DI) water is best)
2. Cretaceous seawater (fresh water
testing that hypothesis with an experiment.
with 5g baking soda added per 10L
or seawater with 2.5g baking soda Teacher Background
added per 10L)
Carbon is an element that is the chemical backbone of all known life.
3. Modern seawater (fresh water with
What does that mean? All known life forms rely on carbon. Our Earth has a
~2.5g baking soda added per 10L,
seawater, or artificial seawater) carbon cycle. That means carbon is always on the move. Plants use
4. Estuary water (approximated by carbon dioxide (CO2) and sunlight to generate energy and grow. The
filling the flask with half fresh water carbon then becomes part of the plant. When we eat plants (or eat
and half modern seawater)
things that eat plants), some of the carbon becomes part of us, some
- Bromothymol blue pH indicator dye
is exhaled in our breath. When plants die theysometimes are buried
- Straws or tubes for blowing into water
and turn into fossil fuels like coal and oil over millions of years. . In fact,
- Bromothymol blue pH color reference
scale (on student data sheet)
during the Carboniferous Period in the distant past, roughly 360 to 300
- Stopwatch million years ago, nearly all of the fossil fuels we use today were begin-
Contact: ning to be formed. When we burn fossil fuels much of this long-stored
SMILE Program
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 Teacher Background (Continued)
carbon quickly (re)enters the atmosphere as carbon dioxide. This is in
contrast to the carbon dioxide we exhale, which was only obtained by
plants in the food we eat within the past couple of years.
One of the most critical jobs the oceans have is to store carbon.
The oceans contain the largest reactive pool of carbon on the planet,
and without the benefit of carbon storage in the oceans, the current Key Vocabulary
atmospheric carbon dioxide levels would be at least 25% higher than pH
they are today. Carbon in the form of carbon dioxide can be removed
CO2
from the atmosphere when absorbed by the ocean through a physical
Ocean acidification
process called diffusion. Diffusion occurs when there is more of some-
Alkalinity
thing in one area relative to another, and that material moves to the
Corrosivity
area with less. Prior to the combustion of fossil fuels the oceans were
actually a small source of CO2 to the atmosphere, but they have be- Buffering capacity

come a large “sink” for CO2. In fact, in the last 150 years, 25-35% of all
CO2 from the burning of fossil fuels has ended up in the ocean, and it
will stay there for a long time.
The oceans are naturally equipped to absorb large amounts of
carbon dioxide from the atmosphere with little change to seawater
chemistry. This is possible due to the natural buffering system of the
oceans. The same way antacids buffer a sour stomach, the ocean’s
buffering capacity slows changes in the ocean chemistry. To explain
the ability of the oceans to store carbon and the buffering capacity of
seawater we need to understand what happens when carbon dioxide
enters the ocean.
Several chemical reactions occur when the oceans absorb car-
bon dioxide. The first is that carbon dioxide (CO2) reacts with seawater
(H2O) to form carbonic acid (H2CO3). The carbon from the carbon diox-
ide has been colored orange to illustrate the storage effect.
CO2 + H20 ↔ H2CO3 (1)
Carbonic acid is a weak acid and separates (or more exactly,
dissociates) to form one bicarbonate ion (HCO3-), and one hydrogen ion
(H+) is released into the seawater.
CO2 + H2O ↔ H2CO3 ↔ HCO3- + H+ (2)
Bicarbonate is the primary form of inorganic carbon in the
ocean and is one of the two “products” shown in the above reaction
(the other is the hydrogen ion). Consider that we have only shown one

Ocean Acidification From a Geological & Chemical Perspective Oregon State University | SMILE Program| 2
 Teacher Background (Continued)
reaction from one molecule of carbon dioxide. There are actually MANY
of these reactions taking place, one for each molecule of carbon di-
oxide, and after these many reactions occur, some hydrogen ions will
remain free floating in the seawater while others will react with an-
other carbon ion in seawater (detailed in reaction (4) below). The more
hydrogen ions that float freely in seawater, the more acidic the sea- How Do We Measure
water becomes. Looking back to our first equation, this means when
Acidity?
CO2 increases the seawater will become more acidic (more CO2 being
The pH scale is used to measure the
absorbed by the oceans ultimately leads to more hydrogen ions - see acidity of a solution, like seawater, and
pH equation to the right). This process, the uptake of carbon dioxide is based on the negative logarithmic
by the oceans and concurrent change in chemistry, has been named concentration of hydrogen ions. It is
important to note that the pH is ef-
ocean acidification.
fectively an indicator of the acid-base
Note that a product of the above reaction includes bicarbon- balance of any solution. In seawater
ate, which is how the ocean so effectively stores carbon, because the pH reflects the relative abundance of
carbonic acid and associated (inorgan-
carbon in the CO2 gas initially absorbed into seawater is now in bicar-
ic) carbon molecules.
bonate form. After this happens, more room will have been created for
pH = -log10[H+] (3)
even more CO2 gas form the atmosphere to be absorbed by the oceans.
As the concentration of hydrogen ions
We are steadily increasing the concentration of CO2 in the atmospherei
goes up, pH goes down, and acidity
by burning fossil fuels primarily, but also because of loss of vegetation increases. A 10-fold increase in the
(land use changes). The increase in atmospheric CO2 is directly respon- concentration of hydrogen ions means
a decrease of 1 unit in the pH scale.
sible for increasing mean global temperature through the amplification
The mean global pH of seawater is
of a natural process, called the greenhouse effect. The increase in glob- 8.1. However, in the last 150 years,
al temperature caused by the intensification of the greenhouse effect is the pH of the ocean has decreased
called climate change (also, global warming). by 0.1 units. That does not sound like
much of a change, but because pH is
Remember that free-floating hydrogen ion shown in reaction measured on a logarithmic scale, this
(2)? Instead of being free-floating, some of these ions will combine equals an approximate 30% increase
with another important ion in seawater, carbonate (CO₃²-), to form into in acidity!

another bicarbonate ion.

CO2 + H2O ↔ H2CO3 ↔ HCO3- + H+ ↔ CO3²- ↔ 2H+ (4)
In the above reaction you can now see all components of the
dissolved inorganic carbon system in seawater. The formation of an-
other bicarbonate ion, resulting from the initial absorption of atmo-
spheric CO2 by the oceans, is the primary way that the ocean naturally
slows the increase in acidity on immediate time-scales. We call the
ability of the ocean to take up (or “sequester”) hydrogen ions and
prevent the pH from dropping its buffering capacity. We talk about the
buffering capacity of seawater because it is linked to the amount of

Ocean Acidification From a Geological & Chemical Perspective Oregon State University | SMILE Program| 3
 Teacher Background (Continued)
ions that can react with hydrogen ions, and that is what we call alka-
linity. Alkalinity in the oceans is approximately 97% due to the total
amount of bicarbonate and carbonate ions.
We know that the concentration of CO2 is changing on times-
cales of years, but how fast does alkalinity change? The main process
adding alkalinity compounds (like bicarbonate and carbonate ions) Can We Just Add Biocar-
to the oceans is the erosion and weathering of rocks on land through bonate?
the hydrologic cycle. In very small amounts over long periods of time, In the ocean, pH is effectively a result
rivers carry “alkalinity” to the oceans and leave it there, once water of the ratio of two main components
of the dissolved inorganic carbon sys-
evaporates. Over very long time scales, 100,000’s of years, the weath-
tem: the combined CO2 and H2CO3 in
ering of continental rocks increases the alkalinity of seawater and proportion to CO3²-. It is important to
increases the ability of seawater to offset pH decreases. While it may note that while alkalinity or buffering
capacity helps prevent changes in pH,
seem like a viable solution to the ocean acidification problem to simply
simply adding bicarbonate to normal
dump “alkalinity” into the sea, a large mismatch between time-scales seawater will actually initially lower
exists. Therefore, on the time scales important to humans (tens to pH, because a solution of bicarbon-
hundreds of years), these processes are not fast enough to significantly ate only has equal parts of CO2 and
CO3²- and will have a pH of about 7.7.
buffer against ocean acidification. In other words, the buffering capac-
Typically that solution will have excess
ity of the planet can’t keep up with the amount of CO2 being put into CO2, and if that outgasses (more CO2 in
the atmosphere, and ultimately the oceans. This is essentially because the solution than in the atmosphere),
the pH will rise to a value closer to
the normal changes in CO2 in the geologic past have occurred on much
modern ocean pH (depending on how
longer time scales, more similar to rates of weathering. much bicarbonate is initially added).
We don’t yet know how ocean acidification impacts all marine
ecosystems, but we do know that some organisms that build shells
(mussels, oysters, clams) or other body-parts (corals, sea urchins)
made out of calcium carbonate (CaCO3) are affected negatively. Why is
that?
Remember that the more hydrogen ions free-floating in sea-
water from CO2-enrichment (more and more CO2 being absorbed by
the oceans), the more carbonate ions react with hydrogen ions to form
bicarbonate. This means that the amount of carbonate ions in the sea-
water will decrease. The more carbonate ions in seawater, the easier
it is to make their calcium + carbonate shells, through a process called
calcification. A term called “corrosivity” is determined by how many
carbonate ions are in seawater. Similar to a glass of water that salt
is added to, initially the water is corrosive to the salt and dissolves it.
After adding more and more salt, at some point the glass of salty water
will not dissolve any more salt. At this time the solution is saturated

Ocean Acidification From a Geological & Chemical Perspective Oregon State University | SMILE Program| 4
 Teacher Background (Continued)
with respect to salt. Fortunately for marine organisms that build shells,
the oceans are saturated in most places with respect to calcium
carbonate, but the decrease in carbonate ions from ocean acidifica-
tion lowers the saturation state and makes it harder for organisms to
make shells, and in some cases may make waters corrosive to exist-
ing calcium carbonate structures with enough CO2.
We know that ocean pH was lower in the Earth’s past (pa-
leo-oceanographers can determine the acidity of the ocean back
as far as 400 million years ago), although it hasn’t been as acidic in
the last 1 million years. More importantly, ocean acidity has never
changed as fast as it is changing today, even though during some
periods in Earth’s history, such as the Cretaceous, the levels of CO2
Each group will need: 4 cups or flasks,
were up to four times greater than today. Still, during that time a lot of
bromothymol blue (with dropper), bro-
shell building organisms evolved and thrived! What do you think was mothymol blue pH color reference, and 4
happening? The alkalinity of the ocean was twice what it is today! straws. Not shown: Student Data Sheet,
water samples, pen for labling, and stop-
Preparation watcb

Because tap water can be rich in minerals (and thus have a high alka-
linity) it is better to start with distilled water. Purchase two 2.5gal jugs
of distilled water from the store. The first jug will serve as the source of
fresh water. The second jug will be made into the Cretaceous water. To Guiding Questions:
this jug, add 5g (1 teaspoon) of baking soda (the recipe for this water is Activity Introduction
5g baking soda per 10L fresh water, and 10L = 2.5gal). Label each jug!
- Gauge students' understanding
This can all be done before the activity is to start. of pH. Review pH as a logarithmic
scale:
Activity Introduction pH = -log10[H+]
One of the most critical jobs the oceans have is to store carbon. Today - If we have a pH of 7, is our material
we will do an experiment to understand how an increase in carbon basic, acidic, or neutral?
dioxide leads seawater to become more acidic, a process called ocean - If the acidity of a substance increas-
es to 10 times as much, what hap-
acidification. We will also learn that ocean acidification is partially
pens on the pH scale? It decreases
managed by the alkalinity of the ocean. by 1 unit.
• When carbon dioxide (from sources such as the burning of fossil - In the last 150 years, the pH of the
fuels) is added to the ocean, seawater becomes more acidic. This ocean has decreased from 8.2 to
8.1. What percentage of increase in
hurts the ocean's ecosystem and ocean-related industries.
acidity does this equal? 30%!
• Differences in Alkalinity, or the carbonate chemistry of the ocean,
controls the ocean's ability to buffer the increase in CO2 and slow
down acidification.

Ocean Acidification From a Geological & Chemical Perspective Oregon State University | SMILE Program| 5
 • Unfortunately, changes in alkalinity occur across geologic times-
cales (caused by the erosion of rocks on land and deposition of
erosive materials in the ocean), and cannot keep up with changes in
CO2 levels which increase on a seemingly daily basis! For example,
in the Cretaceous, CO2 levels were up to 4 times greater than today,
but the alkalinity of the ocean was twice what it is today, so shells
and other marine life flourished.
• The air we breathe out is 100 times more concentrated in CO2 than
Testing the CO2 levels of human breath
our modern atmosphere. An exhaled human breath is about 3.7%
CO2ii while the modern atmosphere is ~0.04% CO2iii. We can use
the fact that our breath is high in CO2 to investigate what happens
when CO2 is added to waters with different alkalinities or buffering
capacities. Using baking soda (sodium bicarbonate) we can create
different types of water masses.

Activity:
Instructions written for 4 students groups. These can be expanded for
more groups by starting with more water in the Preparation. Activity Set-up: 4 cups labeled with
different water types, bromothymol blue
1. Ask students to label their flasks (or clear cups) and fill them with
color pH reference, straws, dropper
the water to create one of each of four types of water. This will
be an exercise in how to create dilutions:
1. Fresh water
2. Cretaceous seawater Guiding Questions:
Pre-Experiment!
3. Modern seawater - The students will have to dilute the
- Ask students about the characteris-
Cretaceous water to make this type. This could be done tics of the four kinds of water. What
by adding ½ Cretaceous water and ½ fresh water to their do they think are the differences
container. If students are measuring out amounts of water between the four types of water
with regard to pH, CO2 content, and
(this is ideal), have them add 250mL Cretaceous water and
alkalinity? Which has the highest
250mL fresh water to their container (500mL total). alkalinity? The lowest?
4. Estuary water – The students will have to dilute the Creta- - What is the difference between
Cretaceous Seawater and Modern
ceous water to make this type. This could be done by adding
seawater? Why?
¼ Cretaceous water and ¾ fresh water to their container. If
- What is an estuary? Why did we mix
students are measuring out amounts of water (this is ideal), the Cretaceous seawater with fresh
have them add 125mL Cretaceous water and 375mL fresh water to make Estuary water?
water to their container (500mL total). - What properties of the water affect
its alkalinity?
2. Have students add a straw or length of tubing to each flask for
blowing. Ask students about the characteristics of the different
types of water using the Guiding Questions to the right.

Ocean Acidification From a Geological & Chemical Perspective Oregon State University | SMILE Program| 6
 Activity Continued
3. Have students write hypotheses about what will happen to
the different kinds of water when CO2 is introduced via blow-
ing.
4. Ask students to add bromothymol blue to each water sample
(start with 2 drops but keep adding until they see a notice-
able color change) to measure the initial pH. Record onto data
sheet at Time 0.
5. Conduct the experiment! Begin blowing and every 30 sec-
onds, use the bromothymol blue indicator chart on the data
sheet to record the change in pH in relation to the addition of Using a dropper to add bromothymol blue
to each water type
CO2 for each water sample.

Activity Wrap Up:

1. What did you notice about the level of pH changes
between the four types of water? Are the colors the
same in all types of water? Why or why not?

2. How long did it take to change each type of water?

Which type took the longest to change? Which type
changed most quickly?

3. Do your observations match what you hypothe-

sized?
The water samples with bromothymol
4. Let’s review how the alkalinity differed between each type of blue - which water type is the most
water. How do you think the alkalinity of water plays a role in acidic?

the ability for increased CO2 to cause ocean acidification?

It should take the most time to change the Cretaceous seawater and
the least time to change the fresh water. The end point will look
different in each treatment too – the fresh water will turn all the
way yellow almost immediately, while students will have to huff
and puff for a long time to get the Cretaceous seawater to turn a
muddy green.

Water with higher alkalinity is more resistant to changes in pH

when an acid (in this case H2CO3, or carbonic acid) is added. This
example demonstrates one of the more commonly used defini-
tions of alkalinity — which can be simplified to: “its quantitative
ability to titrate an acid”iv. After addition of CO2, note color change!

Ocean Acidification From a Geological & Chemical Perspective Oregon State University | SMILE Program| 7
 Next Generation Science Standards THANKS TO THE
DISCIPLINARY CORE IDEAS: PRACTICES: FOLLOWING
ESS2.D: Weather and Climate • Developing and Using Models CONTRIBUTORS:
• Planning and Carrying Out
PERFORMANCE EXPECTATIONS: Investigations Elizabeth Brunner
HS-ESS2-4: Use a model to • Analyzing and Interpreting Iria Gimenez
describe how variations in the flow Data Annaliese Hettinger
of energy into and out of Earth’s Rebecca Mabardy
systems result in changes in CROSSCUTTING CONCEPTS:
Stephanie Smith
climate. • Cause and Effect
• Stability and change George Waldbusser
HS-ESS2-5: Plan and conduct an
investigation of the properties • Scale, Proportion, & Quantity
This project is supported by
of water and its effects on Earth
Dr. George Waldbusser's re-
materials and surface processes.
search group in the College
of Earth, Ocean, and Atmo-
spheric Sciences at Oregon
State University.

POTENTIAL
EXTENSIONAL
ACTIVITY:
Think about how
ocean acidification
affects Oregon’s coast-
al industries: https://
www.youtube.com/
SMILE Teachers blowing into their water samples at the 2016 Winter Teach- watch?v=7h08ok3hFSs
ers' Workshop!

Resources:
ihttp://www.ipcc.ch/

iihttp://cdiac.esd.ornl.gov/pns/faq_othr.html

iii Dr. Pieter Tans, NOAA/ESRL http://www.esrl.noaa.gov/gmd/ccgg/trends

ivhttp://en.wikipedia.org/wiki/Alkalinity

For a more comprehensive treatment of alkalinity, see Zeebe and Wolf-Gladrow, “CO2 in seawa-
ter: equilibrium, kinetics and isotopes” Elsevier Oceanography Series 65.

Ocean Acidification Resources Oregon State University | SMILE Program| 8

                                                                                                                                                                                                                                                                                                                                                                                                          
Student Data Sheet

Hypotheses:

Add the Bromothymol blue indicator and record your pH level! Then blow into your water
samples!

Time pH pH pH pH Observations
(sec.) Fresh Estuary Modern Cretaceous
water water seawater seawater

Ocean Acidification Handouts Oregon State University | SMILE Program | 9