In the Laboratory
edited by
filtrates & residues
James O. Schreck University of Northern Colorado Greeley, CO 80639
Oxygen from Hydrogen Peroxide: An Experimental Modification James H. Burness Department of Chemistry, The Pennsylvania State University–York Campus, 1031 Edgecomb Ave., York, PA 17403-3398
A very common experiment performed at the high school and college levels is the generation of a gas to explore molar mass and molar volume relationships. A relatively recent paper (1) describes the biodecomposition of 3% hydrogen peroxide solution by dry yeast to produce oxygen. This experiment is very inexpensive, easy to set up, and gives quantitative results. The oxygen generation system consists of a test tube containing the hydrogen peroxide solution and a pencil, coated with petroleum jelly and subsequently sprinkled with dry yeast, which is brought into contact with the H2O2 solution after the system has been closed. The volume of oxygen produced is measured by displacement of water from an attached 50-mL buret. The purpose of this paper is to describe a modification to the gas generation portion of this experiment. This modification maintains the desirable qualities of the experiment (quantitative results, inexpensive, easy to set up) but offers the additional advantages of a more secure seal between the stopper and the test tube, faster reaction rate, easier cleanup, and a smaller likelihood that the yeast will fall into the hydrogen peroxide solution prematurely. Modification of Experiment [NOTE : this section describes the equipment needed to replace only the gas generation system shown in the figure of reference (1). The gas collection system remains unchanged.]
Equipment needed (one of each item per setup): 1. 20 × 150 mm test tube with sidearm 2. 12 × 75 mm culture tube (either plastic or glass) 3. Beral disposable plastic pipet, cut approximately. 3/4 in. from the closed end so that a small plastic cup is formed. 4. #2 solid green neoprene stopper 5. Fresh, dry active yeast powder Gas generation system setup Weigh the H2O2 solution (approximately 4.00 g) directly into the culture tube, supporting the culture tube in a small beaker on the balance pan. Add yeast to the plastic cup so that the yeast level is at the top of the curved section (i.e., at the point where the cup’s cross section becomes cylindrical rather than conical). Holding the sidearm test tube upside down, assemble the system as shown in Figure 1. (The culture tube can be held inside the test tube with one of the fingers of the hand that is holding the test tube, while the neoprene stopper is inserted with the other hand.) Make sure that the rubber stopper is inserted snugly by twisting it as it is being pushed into the test tube. It is convenient to support the buret in one side of a buret clamp and the assembled sidearm test tube (still upside down) in the other side.
Figure 1. Modified gas generation system (not drawn to scale).
The entire system can be checked for leaks by lowering the leveling bulb so that the water level is near the bottom of the buret, then connecting the tubing from the gas collection apparatus to the sidearm. Increase the pressure inside the system by lifting the leveling bulb to a position near the top of the buret. This increase in pressure will cause the water level in the buret to rise slowly if there are leaks. After correcting any leaks, disconnect the tubing from the sidearm and position the leveling bulb so that the initial level of the water in the buret is at the zero-milliliter mark. Reconnect the tubing to the sidearm, adjust the leveling bulb so that its water level matches the water level in the buret, and take the initial volume reading. To initiate the reaction, simply invert the test tube carefully to mix the yeast with the hydrogen peroxide solution. The sidearm test tube can be gently shaken or tapped periodically until the reaction is complete. Discussion and Conclusion Elimination of the petroleum jelly–yeast combination not only prevents falling pieces of yeast from prematurely starting the reaction, but at the same time makes the reaction faster and simplifies cleanup. The fit between the neoprene stopper and the top of the test tube is tighter, as there are no protruding pencils or tubing (which can be additional sources of leaks). This design also reduces the likelihood of cuts from broken tubing. Literature Cited 1. Bedenbaugh, J. H.; Bedenbaugh, A. O.; Heard, T. S. J. Chem. Educ. 1988, 65, 455.
Vol. 73 No. 9 September 1996 • Journal of Chemical Education
851
