In the Classroom

JChemEd.chem.wisc.edu • Vol. 76 No. 12 December 1999 • Journal of Chemical Education 1657

Ultrasound is defined as sound above the human limit ofdetection of about 16 kHz (1). Since Alfred Loomis’s studiesin the 1920s, ultrasonic waves of various frequencies havefound uses in areas such as medical imaging, geological im-aging (SONAR), industrial cleaning and welding, solutionmixing, and even jewelry cleaning (2). With the developmentof modern equipment, the use of ultrasound and sonicationdevices in biological and chemical research is increasing.

When compared to more traditional forms of chemistrysuch as wet, photo-, thermo-, and electrochemistry, sono-chemistry remains almost completely ignored at the highschool and collegiate levels, although one paper suggestingsome experiments has appeared (3). As the use of ultrasoundcontinues to expand into medicine, industry, and chemistry,the need to teach students about the application and theoryof ultrasound grows.

The following is a simple yet effective way to demon-strate the usefulness of sonication devices in environmentalstudies. Chlorinated hydrocarbons are a class of compoundsthat create a problem for water treatment facilities. Thesecompounds are used as solvents in manufacturing processes.In the environment, these compounds are resistant to degra-dation and are difficult and expensive to remove at watertreatment facilities. Sonication could be a possible solutionto this problem. Carbon tetrachloride, one of these environ-mental pollutants, has been shown to degrade and produceHCl and HOCl upon sonication in aqueous solutions.Carbon tetrachloride is degraded initially via the followingmechanism (4, 5):

CCl4 → ?CCl3 + ?Cl ; ?CCl3 → CCl2 + ?Cl

Then, the products react with the aqueous environment asfollows:

?CCl3 + O2 + H2O → COCl2 + HOCl

COCl2 + H2O → CO2 + 2HCl

CCl2 + H2O → CO + 2HCl

2 ?Cl + H2O → HCl + HOCl

Trichloroethylene, another chlorinated hydrocarbon, alsodegrades by a mechanism similar to that of carbon tetrachlo-ride to produce hydrochloric acid as follows (6 ):

Cl2C=CHCl → ?ClC=CHCl + ?Cl

Cl2C=CHCl + ?Cl → ?ClC=CCl2 + HCl

Cl2C=CHCl → ClC≡CCl + HCl

The degradation products from these chlorinated hydro-carbons cause a lowering of pH, which can be observed visuallywhen indicators are also present in solution.

MaterialsMethanolCarbon tetrachloride or trichloroethyleneDistilled water100-mL volumetric flask16-mL vial with lidPipets: 25 µL and 10 mLIndicators: chlorophenol red, methyl red, or bromocresol purplepH paper (must cover pH 4–7) or pH meterSonicator bath, 40–50 kHz1

ProcedurePrepare a stock solution of either carbon tetrachloride

(CCl4) or trichloroethylene (Cl2C=CHCl) by adding 25.0 µLof CCl4 (one drop) to 10 mL of methanol or by adding27.1 µL (one drop) of Cl2C=CHCl to 10 mL of methanol,producing a 5000 ppm solution. To prepare the 500 ppmsolution for sonication, dilute the 10.0 mL of stock solutionto 100 mL with distilled water. (The 500 ppm solution shouldhave a pH of 6.1–6.3). Store all solutions in a chemical refrig-erator until use to prevent loss of the CCl4 or C2HCl3.

CAUTION: Carbon tetrachloride and trichloroethylene aretoxic and are suspected carcinogens. Methanol is also toxicand flammable.

Pipet 10 mL of the 500 ppm solution into the 16-mLvial and cap tightly. Add 4 drops of one of the indicators tothe vial. Immerse the vial in the bath and sonicate until thecolor changes, approximately 5–15 min. If indicator solutionsare not available, the pH of the solution may be measuredbefore and after sonication using pH paper or a pH meter.

CAUTION: Do not place hands in the sonication bathduring operation. Use only nonflammable liquids, such aswater, in the sonicator.

Note1. Sonication baths can be obtained from any major scientific

equipment supply company, such as Fisher (1-800/766-7000), Sigma(1-800/325-3010), or Cole Parmer (1-800/323-4340). They generallyoperate at 40–50 kHz. The cost is approximately $300.

Literature Cited1. Suslick, K. S. Sci. Am. 1989, 260(2), 80–86.2. Mason, T. J.; Lorimer, J. P. Endeavor 1989, No. 3, 123–128.3. Goh, N. K.; Teoh, A. C. C.; Chia, L. S.; Teo, K. C. Ultrasonics

Sonochem. 1996, 3, S207–S214.4. Francony, A.; Pétrier, C. Ultrasonics Sonochem. 1996, 3, S77–S82.5. Orzechowska, G. E.; Poziomek, E. J.; Hodge, V. F.; Engelman,

W. H. Environ. Sci. Technol. 1995, 29, 1373–1377.6. Drijvers, D.; De Baets, R.; De Wisscher, A.; Van Langerhaove,

H. Utrasonics Sonochem. 1996, 3, S83–S90.

Environmental Chemistry Using Ultrasoundsubmitted by: Belinda K. Wilmer, Edward J. Poziomek,* and Grazyna E. Orzechowska

Department of Chemistry and Biochemistry, Old Dominion University, Norfolk, VA 23529;*epoziome@odu.edu

checked by: Melvyn M. MosherDepartment of Chemistry, Missouri Southern State College, Joplin, MO 64801-1595

Tested Demonstrationsedited byEd Vitz

Kutztown UniversityKutztown, PA 19530