In the Laboratory
Exploration of SO2 Scrubbers: An Environmental Chemistry Project Amber L. Schilling, Phyllis A. Leber, and Claude H. Yoder* Department of Chemistry, Franklin and Marshall College, Lancaster, PA 17604; *
[email protected] Over the past twenty-five years, 17 articles devoted to the topic of acid rain have appeared in this Journal. The purpose of these articles has been to develop acid rain assays (1–3), to demonstrate the environmental consequences of acid rain (4–8), or to examine the phenomenon of natural buffers (4, 5). The present article deviates from the others by simulating various industrial SO2 scrubbers and by assessing the relative efficacy of each. Not only does this project simulate one environmental measure used by coal-burning industries to reduce the presence of acid rain in our atmosphere, but also incorporates basic concepts in chemistry, such as pH, acid–base reactions, relative acidities, neutralization reactions, and solubility. A pollutant abatement process used by coal-burning industries known as “scrubbing” involves decreasing the quantity of sulfur dioxide gas emitted into the atmosphere by forcing the coal combustion gases to react with a base such as calcium carbonate. In this way, the sulfur dioxide produced from the combustion of coal is converted into calcium sulfite:
SO2(g) CaCO3(s)
CaSO3(s) CO2(g)
(1)
To simulate this process in a setting appropriate for a first-year laboratory, the student first observes the increase in acidity that gaseous SO2 brings about in rainwater by burning a small amount of sulfur and bubbling the SO2 gas produced through distilled water. The student then performs additional experiments in which the SO2 is allowed to react with four different reagents before it is bubbled through the distilled water. The student tests the pH of the water before and after each reaction to observe the effectiveness of each reagent at neutralizing the SO2 gas.
Hazards The hazards associated with this project arise mainly from the corrosive nature of SO2 and the potassium hydroxide. Sulfur dioxide is corrosive to exposed tissues and it is extremely irritating to eyes and respiratory tract. Potassium hydroxide is corrosive and may cause serious burns. It is harmful by ingestion, inhalation, and in contact with skin. Inhalation of dust from the sulfur, calcium carbonate, and calcium sulfate could irritate the eyes and upper respiratory tract, so it is strongly advised that all work be done in a fume hood. Proper eye protection should be worn at all times. Assembly of the pieces of glass into the rubber stoppers should be done using leather protective gloves and with great care to avoid injury. Part 1: The Increase in Rainwater Acidity by SO2(g) Two side-arm test tubes are connected using rubber stoppers with bored holes, glass tubing, and Tygon tubing (Figure 1). A small amount of sulfur is placed in one side-arm test tube and a small volume of distilled water is placed in the second, which is connected to an aspirator. The sulfur is burned using direct heat from a Bunsen burner flame. The partial vacuum produced by the aspirator pulls air over the sulfur and forces the gas produced into the second test tube, where it is bubbled through distilled water. Using pH paper, the student tests the pH of the water both before and after the reaction to observe the increase in acidity of the water brought about by the reaction between SO2(g) and H2O(l) to produce sulfurous or sulfuric acid. The instructor may wish to provide a short discussion on how this reaction occurs and why an increase in acidity in the distilled water is observed. Part 2: Exploration of Four Reagents as Potential SO2 Scrubbers
tygon tubing number 2 stopper
to aspirator
sulfur
distilled water
Figure 1. Side-arm test tube apparatus for Part 1.
Using the setup illustrated in Figure 2, SO2 gas is allowed to react with calcium carbonate (CaCO3), calcium sulfate (CaSO4), sodium carbonate (Na2CO3), and potassium hydroxide (KOH) before it is bubbled through the distilled water. The student determines which reagents act effectively as SO2 scrubbers and explains the observations in terms of the specific reactions that occur. Typical student results are shown in Table 1. Reaction of SO2(g) with Calcium Carbonate Using the setup illustrated in Figure 2, a small quantity of sulfur is placed in the first test tube, and the third test tube is filled with a small volume of distilled water and connected to an aspirator. Calcium carbonate is placed in the middle test tube in one of three forms: (i) ground calcite (particle size no smaller than 1 mm); (ii) an aqueous calcium carbonate slurry; and (iii) an aqueous calcium carbonate slurry with constant stirring. The student tests the pH of the distilled
© Division of Chemical Education • www.JCE.DivCHED.org • Vol. 86 No. 2 February 2009 • Journal of Chemical Education
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In the Laboratory tygon tubing
Table 1. Typical Student Results for Scrubbing Ability of Four Reagents pH of Distilled Water Reagent
tygon tubing
glass tubing
number 2 stopper
to aspirator sulfur calcium carbonate, calcium sulfate, sodium carbonate, or potassium hydroxide
distilled water
Figure 2. Side-arm test tube apparatus for Part 2.
water in the third test tube before and after each reaction to observe the effectiveness of CaCO3 as a scrubber and to observe how the reaction is affected by the type of CaCO3 used. The instructor should provide a brief discussion of the factors affecting heterogeneous reactions, including why little or no reaction between the SO2(g) and the ground calcite would be observed. The student should be able to explain why constant stirring of the aqueous CaCO3 slurry is beneficial in the reaction between the SO2(g) and the calcium carbonate in the aqueous slurry. Reaction of SO2(g) with Calcium Sulfate Using the same procedure as outlined above, the student observes the scrubbing ability of calcium sulfate in two different forms: (i) solid CaSO4 (Drierite) and (ii) an aqueous calcium sulfate slurry. Reaction of SO2(g) with Sodium Carbonate The scrubbing ability of sodium carbonate is observed by placing a small volume of a 1 M solution of Na2CO3 in the middle test tube and running the reaction as carried out with the two previous reagents. The student should write a net ionic equation for the reaction that takes place. Reaction of SO2(g) with Potassium Hydroxide The scrubbing ability of potassium hydroxide is observed by using KOH in two different forms: (i) KOH pellets and (ii) a 1 M KOH solution. The same procedure used for testing the other three reagents is once again employed. The student should be assisted by the instructor in writing and understanding the neutralization reaction for this reaction:
SO2(g) 2KOH(aq)
K2SO3(aq) H2O
(2)
Control with Water A control experiment is performed using water in the middle tube to determine the extent to which the medium, water, dissolves SO2 and affects the pH of the water in the last tube. 226
Before Reaction
After Reaction
Conclusion on Scrubbing Ability
CaCO3(s)
5
1–2
Ineffective
CaCO3 (aqueous slurry)
5
3–4
Ineffective
CaCO3 (aqueous slurry w/stirring)
5
5
Effective
CaSO4(s)
5
1–2
Ineffective
CaSO4 (aqueous slurry)
5
2–3
Ineffective
Na2CO3(aq)
5
5
Effective
KOH(s)
5
5
Effective
KOH(aq)
5
5
Effective
H2O
5
2–3
Control
Discussion We have included this project for the past two years in an interdisciplinary course for first- and second-year college students entitled “Applications of Chemistry to the Environment”. Of the eight laboratory modules in the course most students have expressed the greatest degree of enthusiasm for and interest in the SO2 scrubber experiment. When asked to indicate the project they enjoyed the most, at least 80% of the 30 students who have taken the course identified the SO2 scrubber experiment as their favorite laboratory project. Acknowledgment The authors are indebted to the Camille and Henry Dreyfus Foundation and the National Science Foundation for support. Literature Cited
1. 2. 3. 4.
5. 6. 7. 8.
Betterton, E. A. J. Chem. Educ. 1991, 68, 254–256. Johns, N.; Longstaff, S. J. J. Chem. Educ. 1987, 64, 449. Ophardt, C. E. J. Chem. Educ. 1985, 62, 257. Powers, D. C.; Higgs, A. T.; Obley, M. L.; Leber, P. A.; Hess, K. R.; Yoder, C. H. J. Chem. Educ. 2005, 82, 274–277. Schilling, A. L.; Hess, K. R.; Leber, P. A.; Yoder, C. H. J. Chem. Educ. 2004, 81, 246–247. Goss, L. M. J. Chem. Educ. 2003, 80, 39. Baedecker, P. A.; Reddy, M. M. J. Chem. Educ. 1993, 70, 104– 105. Epp, D. N.; Curtright, R. J. Chem. Educ. 1991, 68, 1034–1035.
Supporting JCE Online Material
http://www.jce.divched.org/Journal/Issues/2009/Feb/abs225.html Abstract and keywords Full text (PDF) Links to cited JCE articles Supplement Student handouts and instructor notes
Journal of Chemical Education • Vol. 86 No. 2 February 2009 • www.JCE.DivCHED.org • © Division of Chemical Education
