Acid Rain in a Micro-Environment

Introduction

Create a micro-environment in a standard aquarium and generate sulfur and nitrogen oxides. Simulate an acid “rainstorm,” which will carry airborne pollutants to the pond and countryside below. The effects are immediate, obvious and unforgettable!

Concepts

• Acid rain

• Pollution
• pH

Background

Acid rain. The term is familiar to anyone who has watched television or read a newspaper in recent years. It has been a popular rallying cry for environmentalists and a frequent topic of discussion in political campaigns. To be able to critically evaluate what is being said and written, a fundamental understanding of acid rain is necessary.

Briefly defined, acid rain is precipitation that has absorbed and reacted with compounds in the atmosphere to form acids. In more concrete terms, acid rain is precipitation with a pH significantly less than 5.6. A pH of 5.6 is generally considered to be the pH of “normal” rainwater. The term “acid rain” dates back to mid-19th century England. Following a long period of deforestation, homes and businesses gradually converted to burning coal as a primary source of fuel. It was noted by scientists and others that over this period the pH of rain falling in England and nations to the east was becoming more and more acidic. Eventually the connection between the increasing acidity and the combustion of coal was made. Since that time numerous studies have been done, environmental effects documented and the connection made much clearer.

The pH of pure water is theoretically 7.0, a value considered to be neutral on the scale of 0 to 14. Carbon dioxide gas (CO₂), naturally present in the atmosphere, dissolves in and reacts with water by the following equation:

The free hydrogen ions on the right side are the cause of the moderate acidification and the resulting pH of 5.6. With other minor contributors, the pH of normal precipitation may on occasion range down as low as 5.0.

The chemicals primarily responsible for acid rain fall into two basic classes: sulfur oxides (SO_x) and nitrogen oxides (NO_x). It is important to understand that acid rain is created both naturally and by manmade sources. The primary natural sources for SO_x are volcanoes, fires, wetlands and other systems with significant concentrations of anaerobic bacteria. Manmade sources for SO_x are the burning of coal, oil and gas (fossil-fuels) and ore smelting and other industrial processes. Natural sources for NO_x include fires (high temperature combustion) and lightning. The most significant man-made source of NO_x is automobile emissions. In industrial regions, human generated sources of both SO_x and NO_x greatly outweigh contributions from natural sources.

Sulfur is present as a contaminant in fossil fuels. Most notably in coal and oil, and to a much lesser extent in natural gas. The combustion of these fuels results in the production of sulfur dioxide (SO₂). Compounds naturally present in the atmosphere are capable of further oxidizing the SO₂ to form sulfuric acid (H₂SO₄). These oxidants include hydroxyl radicals, hydrogen peroxides, dissolved oxygen and ozone.

Nitrogen oxides are formed by the combination of nitrogen and oxygen. Since our atmosphere is approximately 78% nitrogen (N₂) and 21% oxygen (O₂) the reactants are certainly abundant. However, the reaction to create acid rain will only take place when these reactants are involved in a high temperature combustion. Truck and automobile engines are ideal environments for this reaction and are by far the greatest sources of NO_x emissions. Oxygen and nitrogen combine to form nitric oxide (NO) which further reacts with oxygen to form nitrogen dioxide (NO₂). Ultimately nitrogen dioxide reacts with hydroxyl radicals and ozone to form nitric acid.

The sulfuric and nitric acids formed are eventually deposited on the Earth’s surface by one of two methods—wet deposition or dry deposition. Wet deposition arrives at the surface as the acids are “washed” out of the atmosphere by rain, snow, dew, fog, frost, hail and so on. Dry deposition includes particulates and gases which either simply settle out or are directly taken up by plants. Generally, dry deposition occurs in greatest concentration nearest the source of emission while wet deposition may disperse over hundreds or even thousands of miles.

The most visible effects of acid rain, can be seen in the damage that has been done to our lakes and forests. Large-scale dieoff of trees in forests and the acidification of lakes to the point where they can no longer support life are dramatic and alarming. Just what are the mechanisms by which they occur?

Terrestrial systems are affected primarily by the leaching of nutrients from leaves, roots and soils. Potassium, calcium, and magnesium compounds all react readily with acid compounds with the end result being that these nutrient elements are either removed from the soil and transported away, or simply made unavailable to the plants which require them. The incoming acids also dissolve and mobilize a number of toxic metals including mercury, manganese, lead, zinc and aluminum. These metals can rise to concentrations sufficient to kill microfauna (soil bacteria and other microorganisms) disrupting natural processes of decomposition and nutrient cycling. These effects may kill trees and plants directly, or may weaken them, increasing their vulnerability to disease and temperature extremes.

Aquatic systems are also affected in a variety of ways. Directly, increased acidity can disrupt or disable physiological processes within the organism, with juvenile and larval forms frequently being the most susceptible. Indirectly, toxic metals (previously listed) can be released from lake sediments as pH decreases and may also flow in from surrounding terrestrial systems. Both increased hydrogen ion (H⁺) concentration and decreased levels of calcium ions can interfere with ion transport mechanisms into and out of cells. Lake damage can also include population and species reduction among planktonic organisms and other species low on the “food chain.” The subsequent and dramatic loss in diversity means that affected lakes may or may not eventually recover chemically, but will almost certainly never fully recover biologically.

Geographically, regions vary a great deal with respect to the quantity and severity of the acid precipitation they receive. Sources are, of course, not uniformly distributed and regions “downwind” (i.e., to the east) of the heaviest sources are going to be the hardest hit. Regions also vary in their vulnerability to acid rain. Regions with measurably alkaline soil and or limestone bedrock will be able to neutralize, by natural processes, much of the incoming acid. In contrast, regions with thin or acidic soil and or granite bedrock will not be able to neutralize any of the incoming acid and will be immediately susceptible.

The greatest problems and the most severe effects are seen in regions of high vulnerability that also receive heavy concentrations of acid precipitation like the northeastern United States and southeastern Canada.

The principles and problems discussed above are quite complex and difficult to elucidate. The following demonstration is an excellent means of introducing many of the steps in the formation and delivery of acid rain and its consequent effects.

Materials

Safety Precautions

Conduct this demonstration under an efficiently operating fume hood or in a very well ventilated area. If it is a nice day, you may want to do the demonstration out of doors. Wear chemical splash goggles, chemical-resistant gloves and a chemical-resistant apron. Sodium hydroxide solution and hydrochloric acid solution are corrosive liquids; skin burns are possible, very dangerous to eyes. Toxic by ingestion or inhalation. Nitric acid is severely corrosive and a strong oxidant. Toxic by inhalation. Avoid any and all contact with acetic acid and readily oxidized substances. Avoid all body contact. Nitrogen dioxide (NO₂) gas is toxic by inhalation; TLV 3 ppm in air; forms corrosive acid in contact with moisture; severely corrosive to skin, eyes and mucous membranes. Sulfur dioxide (SO₂) gas is toxic by inhalation; TLV 2 ppm in air; forms corrosive acid in contact with moisture; severely corrosive to skin, eyes and mucous membranes. Wear chemical splash goggles, chemical-resistant gloves and a chemical-resistant apron. Please review current Safety Data Sheets for additional safety, handling and disposal information.

Disposal

Please consult your current Flinn Scientific Catalog/Reference Manual for general guidelines and specific procedures, and review all federal, state and local regulations that may apply, before proceeding. “Pond” water and all rinsings from the demonstration can be flushed down the drain with large amounts of water according to Flinn Suggested Disposal Method #26b.

Procedure

1. Begin by preparing the micro-environment. Place the sand in the aquarium, wet it completely, and shape it into a contoured landscape with a hill at one end sloping down to a pond at the other. Use the baking dish as the bottom and sides of the pond. Be as creative as you like to impart realism to the demonstration.
2. Familiarize the students with the indicator solution and its various color changes. Place approximately 10 mL of 1 M sodium hydroxide solution in a test tube, add a few drops of universal indicator solution and note the color. Repeat this test with the 1 M hydrochloric acid solution and with distilled water. Save these three tubes as color standards for later reference.
3. Fill the pond with water and, following the procedure in the previous step, remove a sample for testing with the universal indicator.
4. Place the crucible and the crucible lid about 15 cm apart on top of the sand hill. Place the penny (or 3 g of elemental copper) in the crucible and place two to three grams of sulfur powder in the crucible lid. The toy cars should be placed next to the crucible and the factory building next to the crucible lid.
5. Cover the aluminum pan (with holes punched in the bottom), lip side up, with aluminum foil.
6. When all preparations have been made, add 5 ml concentrated nitric acid to the crucible containing the copper. Quickly ignite the sulfur in the crucible lid. Immediately cover the aquarium with the aluminum pan/foil lid, lip side up. Sulfur dioxide (from the burning sulfur) and nitrogen oxide (from the copper-nitric acid) are now being generated.
7. Let the setup stand until the burning sulfur is extinguished. Remove the aluminum foil covering from the roasting pan and pour water (about 0.5 gallon) into the pan to simulate “rain.” When all the rain has fallen remove the cover and remove a sample of pond water and test with the universal indicator as before. Note and discuss any observed changes.
8. Add the limestone marble chips (500 g) to the pond and let stand for 30 minutes. Retest with the indicator and note and discuss observed changes.
9. Prior to repeating the demonstration, rinse and wash out the crucible and lid and dispose of the lake water. A turkey baster can be used to remove any other excess water. The marble chips may be simply rinsed and reused. Caution: Following the demonstration the pond and ground water will be acidic. Wear proper protective clothing at all times. See Safety Precautions.

Chemical Reactions

SO₂(g) + H₂O(l) → H₂SO₃(aq)
Cu(s) + 4HNO₃(aq) → Cu(NO₃)₂(aq) + 2NO₂(g) + 2H₂O(l)
3NO₂(g) + H₂O(l) → 2HNO₃(aq) + NO(g)
2NO(g) + O₂(aq) → 2NO₂(g)
H⁺(aq) + CO₃^–2(aq) → HCO₃^–(aq)
H⁺(aq) + HCO₃^–(aq) → H₂CO₃(aq)

Student Worksheet PDF

10068_Student1.pdf

Answers to Questions

1. Describe what happened in this demonstration.

A micro-environment, containing a pond and a hill made of sand, was set up. On top of the hill was a crucible containing copper and a crucible lid containing sulfur. The sulfur was ignited just as nitric acid was added to the copper. Rain was then simulated on the micro-environment. When the pond water was tested with universal indicator, it was shown to have become very acidic. Limestone marble chips were added to the pond, and the pH was tested again. This time, the water was basic.

2. Write a balanced chemical equation for each of the following reactions.

a. The reaction of copper metal in nitric acid

Cu(s) + 4HNO₃ → Cu(NO₃)₂ + 2NO₂ + 2H₂O

b. The reaction of nitrogen dioxide in water

2NO₂ + H₂O → H⁺ + NO₃^– + HNO₂

c. The reaction of sulfur dioxide in water

SO₂(g) + H₂O(l) → H₂SO₃(aq)

3. What effect did the limestone marble chips have on the “pond?”

The limestone marble chips neutralized the acid in the pond, creating a basic environment.

4. What is acid rain? Give one detrimental effect of acid rain and one way we can prevent it.

Acid rain is precipitation that has absorbed and reacted with compounds in the atmosphere and now has a pH lower than 5.6, the pH of normal rainwater. Acid rain can react with nutrients in plants, eventually seeping them away entirely. We can help prevent acid rain by converting coal and oil to cleaner natural gases to use as energy.

Discussion

Students should be relatively familiar with the pH scale to fully grasp the significance of acid rain and its consequences. A chart similar to the one on below might be used to illustrate the scale and point out the pH values for some everyday substances.

You may wish to explain that the pH scale is a measure of the hydrogen ion (H⁺) concentration present in the substance in question. More precisely it is derived from the negative log of the H⁺ concentration, given in moles per liter (M). For example, lemon juice with a pH of 2 has a H⁺ concentration of 0.01 M, or 10^–2 moles per liter. It is also important to point out that since the scale is logarithmic, each single digit change is equivalent to a 10-fold decrease or increase in acidity. As the scale above shows, the lower limit for the pH of acid rain is three or more pH units below that of normal rainfall—this is equivalent to an increase in acidity of over 1000 times!

The majority of lakes and streams will have a pH measuring in the range of 5.5 to 8.0. The biota of any one of these systems will be adapted to a relatively limited range around the average value for that system. Frequent influxes of precipitation that may be 100 or even 1000 times that average acidity will eventually overwhelm the system and the “average” pH will begin to drop. Most fish species cannot survive below a pH of 5.0, with deleterious effects on reproduction and juvenile survival being apparent much sooner.

Attempts to mitigate the damage done to lakes and neighboring terrestrial systems have included spreading lime or crushed limestone to neutralize incoming acidity. Fertilizers have also been applied to forests in efforts to replenish nutrients leached from trees and soils. These treatments have shown some success in certain areas but cannot simply be randomly applied. A consideration of the conditions prior to the onset of damage must be made—if the necessary information is available. Recall that the pH of lakes can fall within a range of values—that is to say that some lakes are naturally more or less acidic than others. If after treatment the pH of a naturally acidic lake is raised to a level significantly higher than that to which its biota are adapted, the cure may be equal to or worse than the disease.

Politically the subject of acid rain is still very controversial. The controversy stems from what are perceived to be too many unanswered questions. These questions include the complexity of atmospheric chemistry, the precise effects on biological systems and the effectiveness of proposed treatments and controls. For many policymakers these questions suggest the need for further study and observation before strategies for mitigation should be devised and implemented.

Further controversy stems from disputes over jurisdiction and responsibility. Acids and acid precursors suspended in the atmosphere have little respect for national and state boundaries. Emissions from a source in one state or country may cause damage in other states or countries making for potentially volatile situations. Frequently it seems that environmental problems are framed as environment versus economy. Although these arguments may seem naive and short-sighted they are difficult to resolve when economies are strained.

It would seem that perhaps the most practical and viable long-term solution is to investigate means of reducing emissions of acid precursors. Some suggestions might be converting from coal and oil to cleaner burning natural gas, converting from high sulfur coal to low sulfur coal, fitting smokestacks with scrubbers and converters that either remove contaminants or convert them into harmless substances, conserving energy to reduce emissions, increasing fuel efficiency of cars and trucks and conversion to alternative energy sources. The pros and cons of any of these approaches are well documented and easily researched. As with any complex problem, recognizing that something must be done is only half the battle.

References

Special thanks to David C. McMillin, the Science Department Chairman at St. Christopher’s School in Richmond, Virginia. David would like to thank his student assistants Linda Patterson and Reannon Williams.

Acid Rain—ACS Information Pamphlet; American Chemical Society: Washington, D.C. 1991.

Franck, I., and Brownstone, D., The Green Encyclopedia; Prentice Hall: New York, 1992.

Jacob, A. T., Acid Rain: The Chemistry of Acid Deposition from the Atmosphere; Institute for Chemical Education: Madison, Wisconsin 1991.