Ocean Acidification in a Cup

1. Put on your safety goggles.
2. Pour 1 1/2 fluid ounces (40–50 mL) of acid-base indicator solution into each of the two clear plastic cups. 
3. Add 1/2 teaspoon (2 grams) of baking soda to the paper cup.
4. Tape the paper cup inside one of the clear plastic cups containing the indicator solution so that the top of the paper cup is about 1/2 inch (roughly 1 centimeter) below the top of the plastic cup. Make sure the bottom of the paper cup is not touching the surface of the liquid in the plastic cup—you don’t want the paper cut to get wet. The second plastic cup containing indicator solution will be your control.
5. Place both clear plastic cups onto a sheet of white paper and arrange another piece of white paper behind the cups as a backdrop (this makes it easier to see the change).
6. Carefully add 1 teaspoon (about 5-6 mL) of white vinegar to the paper cup containing the baking soda (image below). Be very careful not to spill any vinegar into the indicator solution. Immediately place a Petri dish over the top of each plastic cup. 
   

Position yourself so you are at eye level with the surface of the indicator solution and look closely. What do you see? Where is the color change taking place? 

After a few minutes have passed, you should notice a distinct color change at the surface of the liquid. As you continue to observe the reaction taking place, the liquid in other parts of the cup will also begin to change color.

This activity illustrates how the diffusion of a gas into a liquid can cause ocean acidification. It also models part of the short-term carbon cycle—specifically the interaction between our atmosphere and the ocean’s surface.

Mixing vinegar and baking soda together in the paper cup creates carbon dioxide gas (CO₂). The CO₂ gas then diffuses into the liquid below. When CO₂ gas diffuses into water, the following chemical reaction takes place and results in carbonic acid (H₂CO₃):

CO₂ (aq) + H₂O → H₂CO₃

Carbonic acid dissociates into H⁺ and HCO₃^-. The increase in H⁺ causes the solution to become more acidic.

Carbonic acid is a weak acid. Even so, the presence of this acid affects the pH of the solution. Thus, after a short time, the surface of the indicator solution changes color: from blue to yellow if you’re using bromothymol blue or from purple to pale pink if you’re using cabbage-juice indicator. This color change indicates a pH change caused by the diffusion of CO₂ gas into the liquid.

Outside of your paper cup, on a much larger scale, atmospheric CO₂ diffuses into the oceans.¹ Oceans are the primary regulator of atmospheric CO₂. Human activities such as burning fossil fuels and changes in land use have increased the amount of carbon dioxide (CO₂) in the atmosphere from 540 gigatons of carbon (Gt C) in pre-industrial times to 800 Gt C in 2015.

Current atmospheric CO₂ levels are greater than they have been in 800,000 years, and as a result, the fast carbon cycle is no longer in balance. From 1860 to 2009, the oceans absorbed an additional 150 Gt C from the atmosphere.

The CO₂ taken up by the oceans reduces oceanic pH through a series of chemical reactions. The first of these is the reaction you just observed: the creation of carbonic acid via the diffusion of CO₂ gas into water.²

In pre-industrial times, the pH of the oceans was close to 8.2. In 2005, it was approximately 8.1.³ While the pH of the ocean is still basic, it is more acidic than it used to be. Since the pH scale is logarithmic, this means that the oceans are 30% more acidic now than they were in pre-Industrial times.⁴

Diffusion goes both ways—from the atmosphere into a liquid and from a liquid into the atmosphere. This experiment shows passive diffusion: the CO₂ gas diffuses into the liquid. What experiment might you try in order to show that diffusion also goes the other way—from a liquid back into the atmosphere?

In March 2015, the global monthly average of the atmospheric concentration of CO₂ was around 400 parts per million (ppm), or 0.04%. It is a small amount, but it is increasing by more than 2 ppm every year due to the combustion of fossil fuels such as oil, gasoline, natural gas, and coal, as well as land-use changes such as deforestation.

Increases in the concentration of atmospheric CO₂ have led to increases in the concentration of CO₂ and other carbon-containing molecules in seawater. The CO₂ added to seawater reacts with the water molecules to form carbonic acid in a process known as ocean acidification. The oceans are absorbing about 25% of the CO₂ we release into the atmosphere each year. Additionally, as more CO₂ gas enters the atmosphere, the atmosphere gets warmer, causing global temperatures to rise.

Ocean acidification is expected to impact ocean species to varying degrees. Photosynthetic algae and seagrasses may benefit from higher CO₂ conditions in the ocean, as they require CO₂ to live (just like plants on land). On the other hand, studies have shown that a more acidic ocean environment has a dramatic effect on some calcifying species including oysters, shellfish, clams, sea urchins, shallow water corals, deep sea corals, and calcareous plankton. When shelled organisms are at risk, the entire food web may also be at risk.

Prior to trying Ocean Acidification in a Cup, learners should be familiar with acid-base indicators and know that baking soda and vinegar create CO₂ gas when mixed. This lesson dovetails with lessons on surface interactions and diffusion.

Making your cabbage-juice indicator
If bromothymol blue indicator is hard to come by, or if you’d prefer not to use this chemical in your classroom, you can use cabbage-juice indicator instead. It’s easy to make: Just take a quarter of a head of purple cabbage, place it in a blender with water to cover, and blend until you get a uniform puree. Strain the resulting mixture—the purple liquid you’re left with is your cabbage-juice indicator. Dilute it with some water and proceed with the experiment, using it instead of bromothymol blue. You will need to experiment with the ratio of water to cabbage juice to see what dilution gives you good results. Note that unlike bromothymol blue, cabbage-juice indicator turns pink, not yellow, in the presence of an acid.

References:

1. Dr. Jürgen Schieber (Indiana University, Geology 1425 course notes): Chapter 8: The Importance of Carbon for Climate Regulation 
2. Mathez, E. A. (2009). Climate Change: The Science of Global Warming and Our Energy Future. Columbia University Press, New York. 
3. NOAA Ocean Acidification Program: What is Ocean Acidification (OA)?
4. NOAA PMEL Carbon Program: What is Ocean Acidification?                                           

Thanks to Chris Sabine of NOAA’s Pacific Marine Environmental Lab and to Jim Bishop of University of California at Berkeley for sharing their expertise.