Article pubs.acs.org/jchemeduc
Whoosh Bottle Safety, Again: What About What Is Inside? Robert B. Gregory*,† and Matthew Lauber‡ †
Department of Natural and Applied Sciences, University of Dubuque, Dubuque Iowa 52742, United States Department of Chemistry, Indiana University, Bloomington, Indiana 47405, United States
‡
ABSTRACT: Studies regarding the whoosh bottle combustion experiment have largely focused on the detonation hazard of the demonstration, particularly with regards to fuel and container choice. Previous work has suggested that the fuel should be 2-propanol owing to its relatively cool flame characteristics. The current study has found that the combustion of 2-propanol in such fuel-rich environments creates significant levels of polycyclic aromatic hydrocarbons and other complex organic compounds. These materials pose a potential risk to the presenter and should be disposed of as hazardous waste.
KEYWORDS: First-Year Undergraduate/General, Second-Year Undergraduate, Analytical Chemistry, Demonstrations, Organic Chemistry, Safety/Hazards, Alcohols, Aromatic Compounds, Gas Chromatography, Mass Spectrometry
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in vessels constructed of glass, polypropylene, polyvinyl chloride, and polyethylene. These vessels ranged in size from 4 to 20 L. Liquid residue from the combustion reaction was analyzed after filtration through a Pasteur pipet containing glass wool and Celite. The filtered mixture was injected into an Agilent 6890 gas chromatograph with flame ionization detection for qualitative analysis of the reaction products. The mixtures were also analyzed using a Finnegan INCOS 50 mass spectrometer connected to a Hewlett-Packard 5890A gas chromatograph for the identification and quantification of the organic components. In both cases, the separation was performed on a Supelco Equity-5 (5% methylsilicone) bonded capillary column with the dimensions of 0.32 mm × 0.23 μm × 30 m. Run times were 32 min, with a split ratio of 20:1. The temperature program consisted of a 5 min hold at 50 °C, a temperature gradient of 10°/min to 220 °C, followed by a 10 min hold at that temperature.
he whoosh bottle is one of the most enduring demonstrations in chemical education and is used in several contexts in general chemistry alone. It is relatively safe, generates a very impressive sound and light show, and provides both entertainment and a teaching moment. Three articles have been published in this Journal,1−3 including a review of the safety of the demonstration that concludes that 2-propanol is the most appropriate fuel. There are YouTube videos of a plethora of colored flames emanating from all sorts of containers, including several frightening examples that end in detonations. Of the articles that have been published, most of the attention regarding the safety of the demonstration has been focused on the integrity of the bottle in the reaction. Indeed, most of the lay press coverage of the whoosh bottle has been for accidents that have happened while the demonstration was being performed. But for all the investigations into the risk for detonation, little attention has been paid to the actual reaction chemistry. The reactions are assumed to proceed via complete combustion, producing essentially water and carbon dioxide, with a little obvious soot and some residual fuel left in the bottle. In fact, the chemistry of the materials inside the bottle is quite complex. This study has found relatively large fused-ring compounds in the combustion of 2-propanol within the context of the whoosh bottle demonstration. The organic compound created in the highest concentration is naphthalene, at approximately 90 μg/mL in one product mixture. Compounds with up to four fused rings are easily identified in these mixtures.
Procedures
Apparatus
The procedure for the combustion reaction has been described elsewhere in the literature.1−3 The combustion reaction can be performed in vessels of a wide variety of sizes, but a 20 L polycarbonate water-cooler bottle provides a safe and impressive demonstration, with sufficient reaction product volume for analysis. The vessel was prepared by pouring approximately 50 mL of the alcohol (Burdick and Jackson, HPLC-Grade, Muskegon, MI) into the vessel, and then distributing the liquid alcohol over all of the interior surfaces by rotating and tilting the bottle repeatedly. The vessel was then inverted to allow as much liquid fuel as possible to drain into a waste container. In a
The primary reaction vessel was a 20 L polycarbonate bottle with a 75 mm diameter mouth. Reactions were also performed
Published: February 7, 2012
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EXPERIMENT
© 2012 American Chemical Society and Division of Chemical Education, Inc.
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Figure 1. Typical GC−MS total ion chromatogram from the combustion of 2-propanol. The peaks labeled in red are trace components in 2-propanol starting material and the peaks labeled in blue originate in the GC−MS instrumentation.
models of the combustion of methanol, but they have confined themselves to the oxidation of methanol to largely carbon monoxide, carbon dioxide, and water.8−10 Thus, on first examination, the assumption of complete combustion in the demonstration would seem appropriate. The impetus for this research, however, was the strong odor associated with the bottle contents after the completion of the reaction. It seemed that there was likely more going on than simple combustion. The initial gas chromatographic (GC) analysis was surprisingly rich. Over 30 significant peaks and more than 100 minor peaks were found to elute from the mixture. Further analysis by gas chromatography−mass spectrometry (GC−MS) showed that significant organic synthesis had occurred in the reaction vessel and that the reaction had generated more than just aliphatic and simple aromatic compounds. The excess 2-propanol from charging a reaction vessel was collected prior to ignition to analyze any extractable organics from the bottle and to account for the peaks of the 2-propanol. Although the 2-propanol was of high purity, neat injection of the solvent reveals several trace components in the mixture, highlighted in red in the chromatogram in Figure 1. The remaining fuel in the bottle was then ignited as usual. The total ion chromatogram for a typical combustion product mixture from the 2-propanol used in the experiments is found in Figure 1. The peaks found in the mixture were identified by library search, which yielded 42 identifiable compounds, many of them PAHs. Solvent peaks were established from injections of neat solvent, and system peaks were obtained from a blank injection cycle where no actual injection was performed. Authentic materials were available for 29 of the identified compounds, and 100 ppm solutions of each were used to confirm the library search results by matching the retention times and mass spectra of the peaks found in the combustion mixture. One concern in the analysis of these mixtures might be water formed in the products of the reaction, as this can cause degradation of the stationary phase of the columns and in mass spectroscopy, of the electron ionization source. However, there is likely considerably less water present in the products of reaction than one would anticipate. First, most of the vaporized fuel contents are pulled from the bottle and combusted above the mouth, generating the CO2 and water vapor, the expected oxygen-rich combustion products. The 2-propanol that is left behind on the walls is the principal liquid solubilizing the
20 L bottle, approximately 25 mL of 2-propanol is retained on the walls of the container. The vessel was placed upright and allowed to stand so that the solvent partially vaporized into the atmosphere of the bottle. Although modification of the atmosphere is possible, room temperature ambient air was investigated in this study. After approximately 30 s, a flame was brought to the edge of the opening of the vessel to ignite the vapor/air mixture. After the completion of the reaction (10−15 s), the vessel was capped to allow the reaction products trapped in the liquid film on the bottle walls and the products in the vapor phase to cool, condense, and collect in the bottom of the vessel. About 15 mL of a straw-yellow liquid clouded by some solid particles of soot remained. This mixture was filtered to remove the particulate soot residue and then analyzed as described above.
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RESULTS AND DISCUSSION The whoosh bottle is a familiar and frequently used demonstration in chemistry courses. It is a convenient and memorable example of rapid oxidation, combustion reactions, and chemical reactivity. When the reactions are carried out with an appropriate attention to safety (2-propanol as fuel, nonglass container, personal protective equipment, and a blast shield to protect the students), the demonstration is simple and inexpensive to perform and is relatively safe. The chemical safety of the demonstration is rarely considered because the combustion is assumed to be essentially complete, converting the alcohol to water and carbon dioxide. An examination of the literature would seem to support such an assumption. Studies that have investigated the combustion of organic compounds in air have demonstrated repeatedly that hydrocarbons of all sorts form polycyclic aromatic hydrocarbons (PAHs) readily, but generally have found no detectable quantities of any organic product formation in the flames of alcohols.4,5 These studies have generally found that alcohols are simply too reactive to form organic products while undergoing combustion. In fact, the addition of alcohols (among other oxygenated additives) has been shown to actually reduce soot and PAH formation in diesel engines.6 One study did find PAH compounds formed in specialized laminar flame burners burning ethanol in very fuel-rich conditions,7 but such equipment seems considerably removed from the open burning fuel that the whoosh bottle appears to be. Several articles describe 621
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products. About 15 mL of the initial 25 mL of 2-propanol coated on the walls of the vessel is recovered after combustion. So, the majority of the liquid would seem to be 2-propanol. Additionally, because the analyses were run under a 20:1 split ratio, much of the 2-propanol and whatever water was generated in the reaction would be removed from the sample before entering the column. Peak shapes were reasonably symmetrical with no excessive tailing, so it is reasonable to conclude that the water had been removed in the injector. Over about 200 injections were used in the course of this project, and no degradation of either the column (via peak shapes) or the ionization source in the MS (via MS response) was encountered. One potential explanation for the complexity of the mixture might be that the plastic container is participating in the combustion reaction. The flame front does travel down the wall of the vessel, and there would be ample opportunity for interaction between the complex organic components of the plastic wall and the energetic and oxygenated fuel liquid and vapor mixture. To examine this possibility, combustion experiments were performed in containers made of polyethylene, polystyrene, polyvinyl chloride, and glass. The products from each container were filtered and analyzed by gas chromatography with flame ionization detection (GC-FID). Each of the experiments yielded qualitatively equivalent GC profiles. These results, and in particular the glass bottle data, show that any container wall involvement in the reaction chemistry is not the mechanism that produces these volatile components. The reaction chemistry seen in this study most likely results because the combustion is taking place under considerably fuelrich conditions. A simple calculation reveals that, in a 20-L reaction vessel, there is, at best, a 9-fold deficit of oxygen in the vessel, assuming all the 2-propanol reacts in some fashion, but that none of it evaporates. Much of that fuel vapor and oxygen goes toward the very rapid combustion that emerges from the mouth of the bottle, leaving less available for combustion inside the bottle. Additionally, the vapor pressure of 2-propanol is only about 5% of atmospheric pressure, meaning that only a fraction of the alcohol is available to burn at the time the reaction begins. A significant amount of the heat of the reaction inside the bottle goes toward vaporizing more of the liquid fuel on the walls of the vessel. Thus, because there is not enough oxygen to react with the fuel and because the vaporized fuel becomes available slowly over the time scale of the reaction, combustion takes place more slowly and at a lower temperature than if all of the fuel were first vaporized and ignited with an excess of oxygen. This provides time for the radicals formed in the combustion that does occur to recombine to form more complex molecules. Indeed, in 2006, Ergut et al. published work with ethanol in laminar flames under very fuel-rich conditions that found PAH compounds similar to what was found with 2-propanol.7 To quantitatively assess the level of the compounds in the mixture, pure standards of each of 29 of the known materials were injected in groups at 10 and 100 ppm levels, bracketed by injections of the combustion mixture after every 6 standard injections. Nine of the aromatic compounds were not sufficiently resolved in the chromatograms and were not quantified. Quantitative values for the remaining 19 compounds were calculated using a two-point linear calibration. Twelve runs were performed to obtain standard deviations for each quantitative value. The results are shown in Table 1, with the standard deviation for each peak identified on the total ion chromatogram in Figure 1 in parentheses. Solvent and system
Table 1. Quantification of Whoosh Bottle Reaction Products by GC−MS Compound
Concentrationa (μg/mL)
Phenylethyne Styrene Benzaldehyde Phenol 1-Methylstyrene Indan Indene 1-Methylindene 2-Methylindene Naphthalene 2-Methylnaphthalene 1-Methylnaphthalene Biphenyl Acenaphthylene Acenaphthene Fluorene Anthracene Fluoranthene Pyrene
1.6 (0.5) 11 (3) 6 (1) 12 (4) 3 (1) 0.6 (0.2) Not Quantifiedb 4 (1) 1.9 (0.5) 90 (10) 6 (2) 6 (1) 4 (2) 40 (10) 1.6 (0.4) 6 (2) 30 (10) 6 (2) 20 (10)
Values in parentheses are the standard deviations (in μg/mL) over 12 runs for each of the determinations. bThe chromatographic response for indene exhibited excessive and injection-dependent variability. This behavior suggested a chromatographic interference for the peak, rendering any quantification unreliable.
a
peaks are not included in the analyses. The data show that significant levels of complex hydrocarbons are formed in the process of the combustion of 2-propanol. Naphthalene is the largest component, at 90 μg/mL, followed by acenaphthylene at 40 μg/mL, anthracene at 30 μg/mL, and pyrene at 20 μg/mL. It is important to note that none of the quantified materials is a known carcinogen, with the exception of naphthalene, which is anticipated to be labeled as a carcinogen by the U.S. government and recognized as one in California. In addition, styrene is suspected of being a carcinogen by several agencies. But with some notable exceptions, few data are available on the carcinogenicity of most PAHs. Although PAH compounds are generally regarded as harmful to human health, several, such as fluoranthene, have, in multiple studies, shown no evidence of any carcinogenic activity. None of the levels for the compounds formed in the combustion event exceed current toxicological limits for human exposure, especially as the experiment is not a sustained exposure. But many of the reaction products may be of concern for other health reasons, and most have significant impacts on the environment. Phenol, naphthalene, acenaphthylene, acenaphthene, fluorene, anthracene, phenanthrene, fluoranthene, and pyrene are on the EPA Priority Pollutant list, many of these having significant bioaccumulation risks for marine wildlife. It is clear that disposal of the product mixture through a municipal sanitary system is inappropriate. It is also important to note that the reaction represents little toxicological risk to the observers of the demonstration, as their exposure is to products of the flame above the vessel, where the oxygen content is sufficient for complete combustion. It is only inside the vessel where the oxygen deficit exists. If the bottle is capped after the reaction is complete to prevent the hot, volatile organics from escaping, audience exposure can be avoided entirely. From a thermodynamic perspective, 2-propanol remains the safest solvent for this demonstration. But, based on this study, the end 622
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products of the reaction should be handled as one would any other hazardous material (personal protective equipment, including gloves and goggles, and preferably a fume hood or respirator to eliminate inhalation hazards). With the wide variety of local regulations regarding disposal of organic chemicals, no specific disposal guidelines can be prescribed. But the disposal of the product mixture should be performed noting that the liquid material will not be mostly water, but rather an organic solution likely to contain over 200 ppm of PAH compounds.
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CONCLUSIONS These experiments show that the use of 2-propanol in the whoosh bottle demonstration results in the formation of complex hydrocarbons due to the fuel-rich, oxygen-deficient conditions. The fact that, in such a short reaction time frame, high levels of fused-ring polyaromatics are formed is unexpected, especially to most of those people who have performed the demonstration. Many of these compounds are listed hazards to humans and the environment. These results highlight the potential for toxic exposure and, potentially, the need for attention to liquid waste disposal regulations when dealing with the disposal of the reaction products from the demonstration.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected].
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ACKNOWLEDGMENTS This work has been supported by a grant from the Purdue Research Foundation. The authors gratefully acknowledge the assistance and advice given by Brian Groh of Minnesota State University and of the University for the use of their mass spectral facilities.
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REFERENCES
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