In the Classroom
tested demonstrations
Burning Phosphorus under Water Safely Submitted by:
Larry C. Taylor 5241 Tarbell Rd., Edinboro, PA 16412
Checked by:
Cole McWherter and George Gilbert Department of Chemistry, Denison University, Granville, OH 43023
I have developed a new procedure for the lecture/laboratory demonstration for burning phosphorus under water. For years, among high school and college chemistry instructors, there has been a general dissatisfaction with methods for performing this demonstration. Thus, there is a need for change—and unlike many changes, this one is nonthreatening. Before discussing my new procedure, I would like to briefly illustrate some of the hazards and problems that exist with two of the procedures commonly employed. One undesirable method is simply to pass oxygen gas through glass tubing into a beaker of hot water, which contains white phosphorus. The oxygen generator utilizes potassium chlorate, which in itself poses a potential explosion hazard. Because the oxygen generator and delivery tube are usually secured by a ring stand, the beaker has to be continually moved around by hand in order that the oxygen continues to come into direct contact with the moving phosphorus. As the oxygen bubbles from the glass tubing, sometimes violently, the hot “water” (which has become phosphoric acid), as well as the phosphorus itself, sometimes “bumps” out of the open beaker, presenting other hazards. Because the beaker is a completely open system, the smell of phosphorus is present in the air. Attempts to “chase” the phosphorus around by means of the oxygen generator tube or by moving the beaker around are not very practical or successful. At the same time, oxygen production is diminishing and the water containing phosphorus is cooling, both being deterrents for a spectacular display. This results in disappointment and lack of interest for chemistry instructors and students alike. Although the second method I will illustrate is a functional improvement over the previous one, its hazards may be equal or greater. Thus it, too, is unsatisfactory. Oxygen gas is passed through glass tubing into a test tube containing very hot water and white phosphorus. The test tube is supported and stands vertically in a beaker waterbath, which is heated to boiling and sustained at boiling temperature by a burner. Oxygen is generated in the same potentially hazardous fashion as in method one (using potassium chlorate), and is delivered via a glass tube running to the bottom of the open test tube, which is half full of water con-
Editor’s Note Caution: White phosphorus is extremely hazardous. Ingestion of even small amounts may produce severe gastrointestinal irritation, bloody diarrhea, liver damage, skin eruptions, circulatory collapse, coma, convulsions. Approximately fatal dose: 50 to 100 mg. External contact may cause severe burns. Chronic poisoning may result from ingestion or inhalation and causes deterioration of bones (especially the jawbone), spontaneous fractures, anemia, and weight loss. Keep white phosphorus under water and handle only with forceps.
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Figure 1. Setup for lecture demonstration. Each flask should be supported by a ring stand and clamp. Pass oxygen directly into the melted phosphorus. A: 3% aqueous hydrogen peroxide. B: Oxygen is passed directly into the melted white phosphorus in the bottom of the test tube. C: Manganese dioxide. D: Water in flask is heated to boiling and the boiling is sustained. E: Test tube should be loosely set into the flask to prevent buildup of vapor pressure. Depending on the duration of the demonstration, you may or may not need to add additional water to the Erlenmeyer flask. A round-bottomed flask could be used; this might retard the rate of water loss. F: Water. G: Do not apply suction. At the conclusion of the demonstration, when all or most of the phosphorus has reacted, test the water in the suction flask with litmus paper. An acid reaction will be found, due to phosphorus pentoxide dissolving in the water to form phosphoric acid.
taining phosphorus. The open test tube presents the same hazards as the open beaker in method one. The test tube is suspended by a piece of cardboard or thin sheet metal, containing a central hole, which is supported by the rim of the beaker waterbath. In the procedure that I have developed (Fig. 1), a much safer 3% hydrogen peroxide solution is used to generate oxygen, rather than the potentially explosive potassium chlorate. Oxygen is released more gently from the hydrogen peroxide solution and oxygen generation can be manually controlled by means of the stopcock on the separatory funnel. The test tube in the Erlenmeyer flask is sealed with a twohole stopper, which does not permit phosphoric acid or phosphorus to escape if bumping occurs. In the two previous methods, contents could escape. With the new procedure the smell of phosphorus is barely noticed, if at all. It is kept to an absolute minimum because phosphorus pentoxide does not escape into the air directly. With the other methods, the smell is a problem. Not only does this procedure afford a most spectacular display, but it can last an hour or so, if desired. The other methods last only minutes.
Journal of Chemical Education • Vol. 74 No. 9 September 1997
