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What is Ocean Acidification? (K-MS)

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Submitted by Danny on Mon, 08/26/2019 - 03:00

Climate change is at the forefront of environmental concerns and it often revolves around carbon dioxide in the atmosphere and its effect on average temperature. However, carbon dioxide is not only increasing in the atmosphere but in the ocean as well. The source of this carbon dioxide is the much talked about atmospheric carbon dioxide, as carbon dioxide is soluble in water. Of the 37 Gt of carbon dioxide we release globally, 9.25 Gt ends up dissolved in the ocean—which is a whopping 25%. While this means less carbon dioxide, is in the atmosphere and reduces heating, it has also led to another problem. Carbon dioxide, when dissolved in water, reacts to form carbonic acid, which is causing widespread acidification of our oceans.

If you are teaching in HS-Secondary check out this more in-depth version of the experiment

Ocean and Atmosphere in a Box

Many experiments demonstrate water acidifying from the addition of carbon dioxide or a common acid such as vinegar to water with a pH indicator. However, this experiment is more accurate in that it demonstrates atmospheric carbon dioxide entering the water to decrease its pH.

Air to Water

The carbon dioxide is produced by Alka-Seltzer tablets in a minimal amount of water—causing a drastic increase in concentration in the air. The carbon dioxide in the air will slowly dissolve in the water and change the pH of the water sample. Note that this change is only noticeable after a couple of hours but will intensify over half a day or more.

Materials

  • PocketLab Air
  • Sealable container (suggest sterilite gasket boxes as they are airtight)
  • Tap water or distilled water
  • 400 ml beaker or translucent container
  • 100 ml beaker or translucent container
  • Bromothymol blue or Bogens universal indicator
  • 2 Alka-Seltzer tablets

Experiment

  • Fill a large beaker or translucent container with 200-300 ml of water
  • Add a couple drops of Bogens universal indicator or bromothymol blue until a strong color is observed; note the color and then chill in the fridge until the solution is cold throughout
  • Place the chilled beaker inside a large sealable container
  • Fill your small beaker/container with water and then place this beside your larger beaker inside the sealable container
    • The diameter of beaker/container and volume of water should be just enough to fit 2 Alka-Seltzer tablets
    • A taller container is best for this to ensure the bubbles do not spill out into the larger container
  • Setup and add your PocketLab Air to the container and start recording
  • Quickly add 2 Alka-Seltzer tablets to the small beaker/container and then seal the large container immediately
  • There should be an immediate spike in carbon dioxide which will likely max out the carbon dioxide sensor at 5000 ppm and over time the indicator color should slowly change
    • Change is noticeable after ~2 hours, but giving a day will lead to a more noticeable change in color

Water to Air

This second experiment is commonly used for hands-on ocean acidification experiments and is faster than the first experiment. However, it is less representative of real-world ocean acidification. This time, the experiment is going backward. Carbon dioxide is being both produced and captured by the water causing a change in indicator in color, and the excess is entering the air.

Materials

  • PocketLab Air
  • Sealable container (suggest sterilite gasket boxes as they are airtight)
  • Tap water or distilled water
  • 400 ml beaker or translucent container
  • 100 ml beaker or translucent container
  • Bromothymol blue or Bogens universal indicator
  • Alka-Seltzer tablet

Experiment

  • Fill a large beaker or translucent container with 200-300 mL of water
  • Add a couple drops of Bogens universal indicator or bromothymol blue until a strong color is observed; note this color
  • Place this beaker/container inside a large sealable container
  • Setup and add your PocketLab Air to the container and start recording
  • Add an Alka-Seltzer tablet to the beaker/container and then seal the large container immediately
  • The indicator should quickly show that the water is acidic, and following soon after, there should be a sharp spike in carbon dioxide within the air of the container which depending on container volume may max out the carbon dioxide sensor at 5000 ppm

 

Results

Indicator

Type of water

Initial color

Change in color

Bogens universal indicator

Tap water

green to yellow

yellow to orange-red

Distilled water

bright yellow

deeper yellow to orange-red

Bromothymol blue

Tap water

light blue

light yellow

Distilled water

light yellow

lighter yellow to almost colorless

 

A picture containing indoor, table, food, cup

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A picture containing cup, indoor, table, sitting

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Bogens universal indicator in tap water before (right) and after (left) experiment

Bogens universal indicator in distilled water before (right) and after (left) experiment

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A picture containing indoor, food, beverage, cup

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Bromothymol blue in tap water before (right) and after (left) experiment

Bromothymol blue in distilled water before (right) and after (left) experiment

 

Some notes on the options for indicators:

  • The most noticeable change occurs when using tap water.
  • Using distilled water shows students that it is water interacting with carbon dioxide and not things dissolved in the water.
  • Showing students the change in indicator is easiest if you have the beaker from the container beside a new beaker that you have added water and indicator to.
    • If you prepare this second beaker at the same time as the first it shows students that the change in indicator color does not happen outside of the container.

Background

The oceans have changed from a pH of 8.16 before the industrial revolution (1850) to 8.07 in 2014. While this change may seem minor, it is about a quarter increase in the amount of acid; which is a drastic change in less than 200 years. This increase in acidity is due to increasing atmospheric carbon dioxide, which dissolves into the ocean—becoming carbonic acid. With the predicted increase in atmospheric carbon dioxide concentration, it is modeled that pH will drop to 7.96 if we follow the Paris agreement and 7.64 if we continue with our current emissions. This is a 190% and 230% increase in the amount of acid. Lots of common sea life have hard structures (skeletons or endoskeletons) made of limestone like coral, crab, lobster, mollusk, sea urchin, starfish, and sand dollars. All of these organisms require the oceans to be quite basic in order to form these structures. With our current emissions of atmospheric carbon dioxide, by 2095 all of the arctic and southern oceans and parts of the North Pacific Ocean will be acidic enough to make most of these organisms unable to form these structures.

Shell dissolving over 45 days in water that is lacking in  a more soluble form of calcium carbonate (aragonite) (1)
Shell dissolving over 45 days in water that is lacking in  a more soluble form of calcium carbonate (aragonite) (1)

This will cause extreme disruptions in the oceans—especially when it comes to corals, which act as homes and the foundation for their entire ecosystem. Currently, this additional carbon dioxide is concentrated in the surface water, but between the next 300 to 1,000 years, this will start to be cycled throughout the deeper water of the oceans—affecting even more ocean communities. These disrupted ecosystems mean a loss of food, tourism, and ecosystem services for coastal communities globally.

To dig deeper into this topic check out the HS-Secondary version

 

(1) Liittschwager, D. Pteropod Dissolution. http://liittschwager.com/Pteropod_dissolution_stack_to_grid_of_9_larger_text-photo.html (accessed Aug 14, 2019).

pteropod dissolution