In the Classroom Tested Demonstrations
A Simplified Determination of Percent Oxygen in Air submitted by:
Chin-Hsiang Fang Department of Chemistry, National Kaohsiung Normal University, Kaohsiung, Taiwan, R.O.C.
checked by:
Cole McWherter and George L. Gilbert Department of Chemistry, Denison University, Granville, OH 43023
Abstract This paper presents a simplified determination of the percent oxygen in air with simple and special designed apparatus. The method is based on a consumption of oxygen resulting in reduced pressure and causing water to be drawn in to the inverted flask. This experiment should be very safe, rapid, and simple. The experiment, the combustion of ethyl alcohol with oxygen in air, can either be demonstrated by chemistry teachers or be performed easily and exactly (overall average 20.77 to 20.81 % O2) by students in the student laboratory. In this way, a fountain will be produced, utilizing the siphoning effect of reducing the pressure in the inverted flask. The maximum volume of water, which is drawn into the inverted flask from the Erlenmeyer flask below by creating an interesting fountain, is reached in 5–10 minutes. Keywords Demonstrations Chemical Education Research Laboratory Instruction Gases Qualitative Analysis Quantitative Analysis Supplementary Materials No supplementary material available.
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Journal of Chemical Education • Vol. 75 No. 1 January 1998 • JChemEd.chem.wisc.edu
In the Classroom Tested Demonstrations
A Simplified Determination of Percent Oxygen in Air submitted by:
Chin-Hsiang Fang Department of Chemistry, National Kaohsiung Normal University, Kaohsiung, Taiwan, R.O.C.
checked by:
Cole McWherter and George L. Gilbert Department of Chemistry, Denison University, Granville, OH 43023
The unit on the oxygen content of air is one of the major topics in junior high school chemistry in Taiwan, and the experiment to determine the percentage by volume of oxygen in air appears in the student chemistry laboratory manuals. But it is difficult to determine the percent oxygen by burning a candle with oxygen in a jar that is inverted in a 500-mL dish two-thirds full of water, as described in general chemistry laboratory manuals. It is necessary to design a simple quantitative experiment to measure the percent oxygen in air for junior high school chemistry teachers. Birk and co-workers (1) use alkaline pyrogallol, white phosphorus, and copper in ammoniacal ammonium chloride or steel wool as the oxygen-absorption reagent to determine the percentage by volume of oxygen in air. The experiment requires 4–6 h to remove the oxygen completely from a 25-mL air sample. Martins (2) used steel wool with the test tube assembly, which is inverted in a 400- or 600-mL beaker twothirds full of water. The reaction requires 30 min. These methods are based on a chemical absorption of O2 resulting in a rise in liquid level. Both methods gave an average of 20% oxygen in air. None of the published methods involves the combustion of alcohol. This paper presents a simplified determination of the percent oxygen in air. The method is based on the consumption of oxygen resulting in reduced pressure, causing water to be drawn into the inverted flask. This ex-
periment should be very safe, rapid, and simple. Moreover, it is accurate and interesting for use in junior high school laboratories in Taiwan. The experiment, the combustion of ethyl alcohol (ethanol) with oxygen in air, is interesting to students and can either be demonstrated by the chemistry teacher or be performed easily, accurately, and enjoyably by students in the student laboratory. A “fountain” will be produced, utilizing the siphoning effect of reducing the pressure in the inverted flask (as in the ammonia fountain [3, 4]). Experimental Procedures
Materials 150-, 200-, 250-, and 300-mL round-bottom flasks 500-mL Erlenmeyer flaskClamp with strong rubber band and iron (ring) stand Rubber stopper (size # 7) Stainless steel injection needle (0.63 × 60 mm) Plastic hose (30 cm long) with a plastic valve Glass tubing with a Teflon valve A piece of cotton Ethyl alcohol (ethanol)
Apparatus The simple, specially designed apparatus for determining the percent oxygen in air is set up with the following materials (Fig. 1). The reaction vessel is an inverted roundbottom flask whose opening is stopped by a small rubber stopper (size #7). A stainless steel injection needle (0.63 × 60 mm) is inserted in the middle of the rubber stopper. The injection needle must be joined with a plastic valve to a plastic hose 30 cm long. Procedure Quantitative determination of the percent oxygen in air can be carried out easily. The general procedure is as follows. 1. Support a small piece of cotton on the head of the injection needle, which is inserted in the rubber stopper. 2. Add 1 or 2 drops of ethyl alcohol (ethanol) to the cotton. (CAUTION : Ethanol is extremely flammable. Make sure not to have excess ethanol near the open flame.) 3. Close the valve on the plastic hose. 4. Invert the round-bottom flask (150, 200, 250 or 300 mL) and clamp or secure it with a strong rubber band at the iron (ring) stand. 5. Ignite the ethyl alcohol on the cotton with a match and place the rubber stopper firmly and quickly into the inverted round-bottom flask as shown in Figure 1. The flame goes out after several seconds when the oxygen is consumed completely, reducing the pressure in the flask.
Figure 1. Experimental setup for determining the percentage of O 2 in air.
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6. Immerse the end of the plastic hose in a 500-mL Erlenmeyer flask three-fifths full of water and then open the valve on the plastic hose.
Journal of Chemical Education • Vol. 75 No. 1 January 1998 • JChemEd.chem.wisc.edu
In the Classroom 7. Immediately create a fountain from the injection needle head until the pressures between the inner and outer flasks are equal.
Table 1. Sample Data and Calculated Results Team a
8. Measure the volume of water drawn into the inverted round-bottom flask by a graduated cylinder, or measure the change in volume of water in the Erlenmeyer flask before and after reaction. This change in water level will equal the amount of water drawn up into the round-bottom flask. 9. Determine the percent oxygen in air after completing the experiment
Results and Discussion This apparatus and procedure can yield a simple quantitative experimental measure of the percent oxygen in air, assuming that ethyl alcohol combustion consumes all the oxygen in the flask. A modification that might be useful would be to replace the injection needle with a glass tube and to measure the volume of water in the Erlenmeyer flask. Take the glass tubing, which contains a Teflon valve in the middle, and attach the cotton swab to one end using a small piece of tape. Make sure to use a large-necked round-bottom flask so as not to smother the flame when placing it into the round-bottom flask. The reduction of pressure in the round-bottom flask causes water to be simultaneously drawn into the inverted flask from the Erlenmeyer flask below, creating a fountain. The results demonstrate the proportionality between the volume of water drawn into the flask (or the difference in volume of water in the Erlenmeyer flask before and after reaction) and the volume of the round-bottom flask. The percentage calculation is made according to eq 1 or 2. vol H2O drawn into round-bottom flask + vol in hose or tubing (mL)
%O2 = ———————————————————— ×100 (1) vol round-bottom flask (mL)
∆vol H2O in Erlenmeyer flask before and after reaction (mL)
%O2 = ——————————————————— ×100 vol round-bottom flask (mL)
Ethanol Added (drops)
Vol. of RoundBottom Flask (mL)
Vol. of H2O Drawn into Flask (mL)
O2 in Air (%)
S1
1
152
32.0
21.05
T1
2
152
31.8
20.92
S2
1
154
32.8
21.30
T2
2
154
32.5
21.10
S3
1
204
42.1
20.64
T3
2
204
42.3
20.74
S4
1
206
43.2
20.97
T4
2
206
43.0
20.87
S5
1
252
52.4
20.79
T5
2
252
52.2
20.71
S6
1
254
53.0
20.87
T6
2
254
52.8
20.78
S7
1
305
62.6
20.52
T7
2
305
62.7
20.56
S8
1
310
63.1
20.35
T8
2
310
63.4
20.45
Overall average = 20.81%, SD = 0.854 for S teams; overall average = 20.77%, SD = 0.679 for T teams. a
S designates student teams and T designates teacher teams.
Acknowledgment I would like to thank the Ministry of Education, the Republic of China, for the financial support to a science education program 1991, Grant No 15. Literature Cited
(2)
Table 1 shows a summary of sample data and calculated results. Combined results for six classes of 48 students with 8 teams and four classes of 32 teachers with 8 teams gave averages of 20.81% and 20.77%, respectively, which compare favorably with the accepted value of 20.95% (5, 6). The maximum volume of water, which is drawn into the flask from the Erlenmeyer flask below by creating an interesting fountain, is reached in 5–10 minutes.
1. Birk, J. P.; McGrath, L.; Gunter, S. K. J. Chem. Educ. 1981, 58, 804. 2. Martins, G. F. J. Chem. Educ. 1987, 64, 809. 3. Shakhashiri, B. Z. Chemical Demonstrations: A Handbook for Teachers of Chemistry, Vol. 2; University of Wisconsin Press: Madison, WI, 1985; p 205. 4. Thomas, N. C. J. Chem. Educ. 1990, 67, 339. 5. Masterton, W. L.; Slowinski, E. J.; Stanitski, C. L. Chemical Principles, 6th ed.; CBS College Publishing: Taiwan, 1985; p 515. 6. Gillespie, R. J.; Humphreys, D. A.; Baird, N. C.; Robinson, E. A. Chemistry, 2nd ed.; Allyn and Bacon: Needham Heights, MA, 1989; p 100.
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