Name __________________________
Are our oceans becoming acidic? Introduction Ocean’s cover 71% of the Earth’s surface and are estimated to house 50% of all Earth’s species. The processes that occur in oceans are intimately connected to all major Earth processes, including climate. Today we will explore these connections. In particular, we will examine how the Earth’s atmosphere can affect the pH (measure of acidity) of oceans, how ocean pH affects living things, and how those changes affect global atmosphere and ecosystems.
Part 1: What is pH? Most simply, pH is a measure of how acidic or alkaline (also known as basic) a waterbased (aqueous) solution is. The pH scale is shown below: Fig. 1—The pH Scale
The pH scale ranges from 0–14, with 0 being pure acid, 14 being pure alkaline, and 7 being a perfect mix (or neutral). Pure water has a pH of 7. Most chemical reactions, particularly those performed by living things, are very sensitive to pH with some reactions only occurring at very specific pH values. Because oceans make up such a large component of planet Earth, ocean pH will have large effects on the chemical reactions that drive many of Earth’s processes. pH is measured on a logarithmic scale. This means that acidity/alkalinity (depending on direction) changes by a factor of 10 for every unit on the pH scale. Therefore 8 is 10 times more alkaline than 7, and 9 is 100 times more alkaline than 7. How much more acidic is a solution with a pH of 4 than one with a pH of 7? _________ pH can be measured using many different types of pH indicators. These are compounds that have different properties depending on the pH. Today we will work with Phenol Red, which changes color depending on the pH. Phenol red is red at a
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basic pH (above pH 8.0), yellow at an acidic pH (below pH 6.6), and orange/peach at a neutral pH (between 8.0 and 6.6). The exact pH of five solutions are given below. Please indicate if these solutions are acidic, alkaline, or neutral, and what color you expect phenol red to turn in these solutions. pH = 12.1, solution is ____________, Phenol red color: ________________ pH = 2.2, solution is _____________, Phenol red color: ________________ pH = 7.4, solution is _____________, Phenol red color: ________________ pH = 8.1, solution is _____________, Phenol red color: ________________ pH = 5.0, solution is _____________, Phenol red color: ________________
Procedure 1. Examine the properties of phenol red 1.1. Add 10 drops of phenol red to 2 tubes of water. Note the color of the water. Was the water acidic or basic?__________________________________ 1.2. One drop at a time, slowly add vinegar to one test tube, and ammonia to the second tube until the color changes. Is vinegar acidic or basic? __________, Is ammonia acidic or basic? ________________. 1.3. Try to make the vinegar and ammonia change color again (from red to yellow or yellow to red). What did you need to add to the yellow solution _____________, what about the red solution _________________, Why____________________ _______________________________________________________________?
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Part 2: Carbon dioxide (CO2) and the pH of water Carbon dioxide (CO2) is a major compound on earth. CO2 is the primary gas that we remove from our bodies when we exhale, it is the compound that plants take up to make sugar through photosynthesis, and it is released when organic matter is burned. Throughout the majority of human history, CO2 made up about 0.027% of the Earth’s atmosphere. Since the industrial revolution (when humans began burning fossil fuels for energy) CO2 has risen to make up 0.039% of Earth’s atmosphere and is predicted to continue rising. CO2 is predicted to make up 0.05%–0.10% of Earth’s atmosphere by the end of the 21st century. This means that the concentration of CO2 in the atmosphere has already increased by 44% and is predicted to double or quadruple historic levels by the year 2100. Such increases in atmospheric CO2 are predicted to have numerous effects on planet Earth. We frequently hear about the potential effects of increased CO2 on global temperature, but CO2 will have other effects as well. For example, increases in CO2 affect ocean pH because of what happens when CO2 mixes with water. Examine the following equation: Carbon Dioxide + Water ↔ Carbonic Acid CO2 + H2O ↔ H2CO3 As you can see, when CO2 mixes with H2O, a weak acid called carbonic acid (H2CO3) is produced. This equation is reversible (why there is a double sided arrow). As CO2 is added to water, the solution becomes more acidic (the pH decreases). As CO 2 is removed from water, the solution becomes more alkaline (the pH increases). Now, we will explore the effects of adding CO2 to water on the water’s pH.
Procedure 1. Get an orange/peach Phenol Red tube from your instructor. What pH is the tube__________________? Using a straw, blow bubbles into the solution. By blowing bubbles, you are adding carbon dioxide (CO2) to the solution. Be careful not to suck any of the liquid into your mouth. It may take a while, but the solution will change color.
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What color did the solution turn?__________________ Did the solution become more acidic or basic?____________________ What in your breath made the solution change color?____________________
Check your understanding 1. How does the addition of CO2 affect the pH of water?
2. Is atmospheric CO2 currently changing? If so, how is it changing and why?
3. Will changes in atmospheric CO2 affect the pH of Oceans? In what way?
Part 3: How will changes in ocean pH affect living things? Your challenge is to design an experiment with your group that assesses the effect of ocean pH on shell-producing animals. Shell-producing animals are abundant in the world’s oceans and are integral components of ecosystems. These animals are also important for the economies of many countries, including the United States. They include corals, clams, snails, sea urchins, crabs, and many other animals. In recent years, many of these animals have decreased in numbers with some entire coral reefs collapsing (a phenomenon known as coral bleaching). Your experiment should explore the effects of ocean pH on the ability of these animals to produce and maintain their shells.
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Use a lab-o-matic to design your experiment. Your resources include water, beakers, acid and alkaline solutions, phenol red, digital scales, and shells from a variety of shellproducing animals (primarily clam-like animals and snails). Before setting up your experiment, complete everything before the “results” section of your lab-o-matic and have your instructor check your experimental design. Post-experiment Questions (you may wish to use a separate sheet of paper)
1. Based on your results, do acidic or basic solutions affect the shells of shelled animals? If so, describe how?
2. Given your results, would you predict that increasing atmospheric CO2 will affect the ability of shelled animals to produce their shells in the world’s oceans? If so, why?
3. Think of 1-2 ways that we could potentially reverse the acidification of the Earth’s
oceans (i.e., make them more neutral)? Do you think that either of these are doable, why?
