In the Laboratory
An n-Bottle Lab Exercise with No Hazardous Waste Claire R. Olander Appalachian State University, Boone, NC 28608
n-Bottle-type laboratory exercises have enjoyed popularity in introductory chemistry labs since the first version was published in this Journal in 1940 (1). They expose students to some characteristic reactions of ionic compounds, they encourage deductive reasoning, accurate measurement, and careful observation, and they are fun to do. Students learn some chemistry and enjoy solving the puzzle of “What’s in the bottle?” Traditionally, n-bottle exercises have relied heavily on precipitation reactions of heavy metal ions. The waste was disposed down the drain without much thought to environmental contamination. Today’s laboratory climate is different. Strict regulations for the disposal of hazardous waste have discouraged many educators from scheduling labs for which the waste must be specially treated. The exercise described here employs the same reaction types as the traditional n-bottle problem but does not produce hazardous waste. This lab was designed as part of a course for non-science-majors but can easily be modified in form and difficulty to challenge students who have more extensive chemical knowledge. The substances used are inexpensive and the solutions are easy to prepare. Sodium thiosulfate and sodium carbonate solutions should be freshly prepared for each term, but the other solutions may be stored for long periods. Outline of the Experiment Each student or student pair is provided with a set of eight bottles, each bottle containing a colorless solution. Five of the bottles are “unknowns” and are labeled with letters. Students are informed that these bottles contain 0.1 M solutions of AlCl3, CaCl2, MnCl2, NaCl, and Na2S2O3, one solution per bottle. The remaining three bottles contain 0.2 M solutions of Na2CO3, NaOH, and H2SO4 and are so labeled. These are called “reagents”. Students are instructed to combine exactly 1 mL of each unknown with 1 mL of each reagent in a 13 × 100 mm test tube and mix thoroughly. (Calibrated plastic Berol pipets are convenient for these measurements and transfers.) They must observe the mixture Presented at the 13th Biennial Conference on Chemical Education, Bucknell University, August 1994.
carefully for 15–30 seconds and record their observations. After completing all 15 of the mixtures, they must observe each mixture again to see if any subsequent changes have occurred. Students successfully identify the unknowns by combining careful observations with necessary information that has been provided to them. Sloppy measurements and/or failure to make careful observations result in incorrect identifications. Necessary information provided to students is: 1. All compounds containing sodium ion are soluble. 2. The carbonates of Ca2+ and Mn 2+ are insoluble. When AlCl3 reacts with Na2CO3, both a precipitate of Al(OH)3 and CO2 gas are formed. The CO2 appears as tiny bubbles that rise to the surface of the reaction mixture shortly after the precipitate appears.1 3. The hydroxides of Al3+ , Ca2+ , and Mn2+ are insoluble but not to the same degree. Calcium hydroxide is somewhat soluble and may appear as a faint cloudiness. Aluminum hydroxide forms a fluffy, translucent precipitate. Manganese hydroxide appears gold-colored. Part of it may decompose to form black manganese oxide. 4. Aluminum, calcium, and manganese sulfates are soluble. 5. When a solution of sodium thiosulfate becomes acidic, the thiosulfate ion chemically changes into elemental sulfur and hydrogen sulfite ion – (HSO3 ). The hydrogen sulfite ion remains in solution and the sulfur slowly comes out of solution (after several minutes) as a pale yellow precipitate. 1 Aluminum carbonate does not exist in aqueous solution. Hydrolysis of aluminum ion produces hydronium ions, which react with the carbonate ions. As with any carbonate ion–containing solution, there are several equilibria occurring simultaneously. This reaction mixture could serve as a springboard for discussion of competing equilibrium reactions. This mixture also serves as a check on a student’s care in measuring volumes. If a student uses too much sodium carbonate solution, the carbon dioxide bubbles do not form.
Vol. 73 No. 9 September 1996 • Journal of Chemical Education
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In the Laboratory
The equation for the reaction is: –
Table 1. Summary of Reactionsa
–
HS2O3 (aq) → HSO3 (aq) + S(s) The report sheet for the lab includes the reactant side of chemical equations for all the fifteen possible combinations. Students are required to complete and balance only those equations for which they observed a reaction.
Solution of
Reacts with
Observations
AlCl3b
Na2CO3
White ppt. followed by bubbles
MnCl2
Other Possible Versions As with any n-bottle-type lab exercise, the pattern of reaction for each unknown with the three reagents is unique. Additionally, the pattern of reaction of any of the eight substances with the other seven is unique. Therefore, other versions of this exercise may be contrived at various levels of difficulty. Examples are: 1. Students are first provided with labeled bottles of each solution. They must, by combining each solution with the other seven, determine the pattern of reaction for each substance. They are then given any number of unknowns to identify. 2. After determining the reaction pattern as in #1, the labeled solutions are withdrawn, and students are given the eight solutions in coded containers. By judicious combination, observations, and deduction, the students can identify all eight “unknowns”. 3. For advanced students whose knowledge of ionic reactions is established or who may be expected to research the reactions on their own, the initial trial-with-knowns procedure may be eliminated. Table 1 summarizes the reactions of solutions of the eight substances already discussed and includes a ninth substance if a traditional nine-bottle problem is desired (2). Waste Disposal The waste from this exercise should be collected in a beaker and made basic with sodium hydroxide solution. The precipitates should be allowed to settle, the liquid portion decanted down the drain, and the solid disposed of in the trash (3).
NaOH
White ppt.
Na2S2O3
Pale yellow ppt. after 10 minutes
Na2CO3
White ppt.
NaOH
Gold ppt., turns black on top
CaCl2
Na2CO3
White ppt.
NaOH
Slightly cloudy
NaCl
none
Na2CO3
AlCl3
White ppt. followed by bubbles
Na2S2O3
ZrOCl2
MnCl2
White ppt.
CaCl2
White ppt.
H2SO4
Bubbles immediately
ZrOCl2
White ppt., dissolves on mixingc
AlCl3
Pale yellow ppt. after 10 minutes
H2SO4
Pale yellow ppt. after 40 seconds
ZrOCl2
Pale yellow ppt. after 15 seconds
Na2CO3
White ppt., dissolves on mixingc
NaOH
White ppt.
Na2S2O3
Pale yellow ppt. after 15 seconds
a Concentrations of Na2 CO3 , NaOH, and H2 SO4 solutions are 0.2 M. All others are 0.1 M. Combinations of reagents not listed are colorless solutions and are labeled "no reaction". One milliliter of each reagent is mixed in a 13 3 100 mm test tube. b Use aluminum chloride hexahydrate for preparation of solutions. It is hygroscopic but does not fume or react violently with water like the anhydrous salt. cIn aqueous solutions of moderate ZrO 2+ and hydrogen ion concentration, ZrO2+ ion is converted to a tetranuclear [Zr4 (OH)8 (H2O)16]8+ species in which the geometry around each Zr is a square antiprism. When Na2 CO3 solution is added, the initial precipitate may be ZrO2 . Subsequent complexation with carbonate ion resolubilizes the species.
A Ninth Unknown A 0.1 M zirconyl chloride solution was tested for inclusion in this exercise. Zirconyl chloride shows a unique pattern of reaction with the other eight solutions (see Table 1) and zirconium ions are classified as “low toxic hazard” (4). The chemistry of zirconyl ion in solution is interesting but quite complex (5). Students in introductory chemistry could identify the zirconyl chloride unknown if they had preliminary experience with labeled solutions. They should not be expected to predict zirconyl ion chemistry.
850
Literature Cited 1. MacWood, G. E.; Lassettre, E. N.; Breen, G. J. Chem. Educ. 1940, 17, 520–521. 2. Ifft, J. B.; Roberts, J. L., Jr. Qualitative Analysis of Unlabeled Solutions: The Nine-Solution Problem, Franz/Malm’s Essentials of Chemistry in the Laboratory, 3rd ed.; W. H. Freeman: San Francisco, 1975; p 323. 3. Mahn, W. J. Academic Laboratory Chemical Hazards Guidebook; Van Nostrand Reinhold: New York, 1991; Chapter 5. 4. Committee on Hazardous Substances in the Laboratory, Commission on Physical Sciences, Mathematics, and Resources, and National Research Council. Prudent Practices for Disposal of Chemicals from Laboratories; National Academy: Washington, DC, 1983; Chapter 6. 5. Cotton, F. A.; Wilkinson, G. Advanced Inorganic Chemistry, 5th ed.; Wiley: New York, 1988; Chapter 19.
Journal of Chemical Education • Vol. 73 No. 9 September 1996
