Acid Rain and Chemical Buffers
Pure water has a neutral pH of 7, and the average pH of rainwater, unpolluted, is around 5.6. Any pH below 7 is considered acidic, and a pH above 7 is considered alkaline, or basic. Acid rain is defined as precipitation with a pH of less than 5.6.
Power plants, automobiles, and burning fossil fuels can introduce pollutants into the atmosphere. When water droplets in the atmosphere encounter contaminants in the air, including carbon dioxide (CO2), sulfur dioxide (SO2), and nitrogen oxides (NOx), the rainwater becomes acidic, following the simplified process outlined below:
1) Carbonic acid is formed in the atmosphere:
H2O(l) + CO2(g) = H2CO3(aq)
2) Carbonic acid ionizes in water:
H2O(l) + H2CO3(aq) = HCO3-(aq) + H3O+(aq)
When water vapor containing dissolved acid precipitates, the resulting rain can have detrimental impacts on rocks and soils on the surface, dissolving carbonate minerals. It also can kill microbes in soils and lower the pH in surface water, damaging aquatic life.
In areas where limestone is present in the subsurface, calcium carbonate (CaCO3) can act as a buffer, to help resist changes in pH caused by acid rain. The acidity is neutralized by carbonate (CO32-) and bicarbonate (HCO31-) ions.
While the interaction of acid rain with calcium carbonate can alleviate the detrimental effects of acid rain on the environment, other materials made of limestone can suffer from acid rain. Gravestones, for example, and building materials, can dissolve in acid conditions.
In this experiment, students will explore the effects of acid rain on both a buffered and a non-buffered system.
- Distilled water
- Baking soda
- 1M H2SO4
- pH meters
- Universal Indicator
- (2) 200-mL beakers
- 25-mL graduated cylinder
- 10-mL pipette
- Safety goggles
1. Create a buffer solution to simulate the presence of limestone in an acid rain environment: add .5 tsp baking soda to 1 liter distilled water.
2. Create "acid rain" by dissolving 4 mL 1M H2SO4 in 2 liters distilled water.
3. Add 25 mL distilled water to one beaker, and 25 mL buffer solution to the other beaker. Add 6 drops Universal Indicator to each beaker. Measure and record the pH of each.
4. Using the pipette, add the acid solution dropwise to the distilled water, and measure pH after each addition. Record the amount of acid necessary to turn the solution solid pink (acidic).
5. Repeat step 4 with the buffered solution.
1. What is a buffer? How does it work?
2. Write the chemical equation for the interaction of acid rain with limestone (CaCO3).
3. Which of the solutions tested took longer to become acidic? Why was there a difference?
4. How does this experiment relate to acid rain in a natural system?
Acid Rain Experiments: Cornell University, Ithaca, NY 14853 (http://ei.cornell.edu)