edited by MIRIAM C. NAGEL Avo" High Schwl Avon. CT 06001
How Safe is That Experiment? Mlriam C. Nagel Avo" High School Avon. CT 06001
Students in a small Massachusetts hieh school will never forget a demonstration they witnessed lasiyear. But what they recall will not be chemical princi~les.Their teacher was oer~ ~ o k a n e o Fire" u s experimkntl forming the tricky when the mixture of zinc dust, ammonium nitrate, ammonium chloride, and barium nitrate ignited prematurely. The blazing mixture spewed over the teacher's hands and burned them severely. No one reported the color of the flames. The exact cause of the accident is not known, but the hvgroscopic nature of ammonium salts could have been the culprit. The intended reaction is triggered by a drop of water. and the premixed reagents may have picked up enough at: mospheric moisture to cause the accident. In Maryland two students were seriously injured while watching a "motivating" demonstration conducted outdoors to show the difference between a chemical and physical reaction. The teacher was using potassium and sulfuric acid in a metal beaker. According to the state fire investigator, "The proper (sic) chemical reactions occurred. There were puffs of smoke and it turned colors. . . for some unknown reason the metal beaker exploded."2 The fire inspect~rmay not have had a background in chemistry, but the teacher should have known the hazards of handling peroxide-prone potassium. Motivation does not require unreasonable risks. All experiments should be planned using the safest chemicals and safest reactions that will illustrate the concent. If no safe experiment is possible, audiovisual aids can often provide adequate illustration. In December 1981, a Connecticut teacher was injured in a fire so intense a steel sink split and a fire blanket ignited. The teacher was demonstrating the flame characteristics of difs one of ferent substances, including magnesium. S ~ a r kfrom the tests fell into a container ofmagnesium left open on the lab bench top. Besides the fire blanket, sand and three fire extinguishers were used to control the fire. Flame tests can he instructive. They can also be conducted safely. Wood splints soaked for several hours in a beaker half-filled with very dilute HCl can be used in place of conventional platinum or nichrome wire. A small sample of the salt to be tested is put on a watch glass. When touched by a wet splint, some of the salt crystals will adhere to it. The splint is put quickly into a Bunsen flame, and vivid colors of the salt are visible for several seconds. The only risk is in the use of the Bunsen burner. Magnesium should not he tested when flame colors are heing demonstrated. If there is any reason to demonstrate magnesium burnine, nomore than 1cm of maenesium ribbon should be used w i t h i large evaporating dish piaced under the demonstration. Nothing flammable should be nearby, least of all more magnesium. During any experiment, extra reagents should be in closed containers, properly stored before the work is begun. That should be standard practice.
A few years ago, in a large New England high school, alahoratory teacher stepped out of the room while honor student3 completed an experiment involving the preparation of lead iodide. A student, carrying 5 ml of ethanol needed in the experiment, passed another student who was using a Bunsen burner. Somehow the alcohol spilled. I t caught fire, burning the student's hand and igniting his clothing. Panic stricken, he began to run. Another student, a quick thinking athlete, grabbed him and rolled him on the floor, extinguishing the flames. The victim was treated a t the local hospital and released, fortunately suffering only minor burns. The teacher who stepped out of the room was negligent. Flammable liquids should never by used or carried near an open flame. Inherently, flammable solvents are high-risk chemicals. Students should be cautioned of their dangers. Warning labels should he on every bottle containing a flammable solvent. Students should. of course..~~ he taueht " to work safely and in a proft.ssional manner with all chemicals. 'l'his training in handline flammable solvents is oarticularlv importan; for the apGication it will have outside the laboratory. The flash point of ethanol is 13%. Thismeans that a t 13'C enough vapors will be given off to form a mixture with air, above the surface of the liquid, capable of ignition from an external source. The lower the flash ~ o i n t the . ereater the hazard. The tl.mmahlc iexplosive)limit%of ethani, expressed as percent of \~olume,are 3.3Di and 19'7. The explosive limiu are the minimum and maximum concentrations of the tlammahle vapors in air which will support combustion. The low minimum limits for most flamma& liquids, including ethanol, mean an "empty" container is a far greater explosion hazard than a full one. The vapor density of ethanol is 1.6, which means it is 1.6 times as dense as air. Gases much more dense than air tend to flow along the bench top. If they encounter an ignition source while their concentration is still within explosive limits, the flame will travel back along the path of the gas to its source, usually with dire results. A good reference for information on flammable liquids is the NFPA Guide No. 325111, "Fire Hazard Properties of Flammable Liquids, Gases and Volatile Solids."3 Besides beine unconscionable. unnecessm risks could lead to a teacher hehg held liable for negligence ha suit following a lab injury.Some states require local school boards to provide liability insurance for teachers, but it is unlikely they would afford protection should an accident be deemed the result of a wanton or reckless act. Negligence is decided in the courts. The award in a case settled recentlv was in excess of a million dollars. Two students did not follow directions in a published manual and were seriously burned when they improperly
Safety Tips is planned to be a source of safety information and pranical suggestionsto meet lhe special needs of hi* schwl chemistry teachws. H is also intended to be a forum for teachers to share meir experiences and seek solutions to safety-related problems.
Bailey, P. S., Bailey, C. A.. Andersen. J., Kashi, P. G., and Rechsteiner. C.. J. CmM. EDW.. 52, 524 (1975). Bristol, Conn.lValley Press, May 6. 1980, p. 13. National Fire Protection Association, 470 Atlantic Ave.. Boston, MA
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handled methanol near an open flame. Both the publisher and the teacher were held liahle.4 Some experiments go awry for completely unexpected reasons. I t was May, near the end of the first-year chemistry lab course. so the class was exnerienced enough to nerform an oxidation-reduction experiment involving 6 M of nitric acid. The student, a eirl. was one of the best. auiet and reliable. Perhaps her haids or the bottle were damp. At any rate, the standard reagent bottle slipped down through her hands t o the tile floor. It did not break as it hit, but the top flew off, and the contents splattered on the girl's legs and feet. Her long ruhher apron, though well splattered, effectively protected her dress. Hefore the tearher, who was helping students in the next aisle, could rearh t he scene, a boy working at the bench facing the accident had the situation under full runtml. AS he oassrd the low sink a t the end of the bench on his way to the Gictim, he turned the water on full. Without hesitating, he picked up the girl and stood her in the sink with her legs under the flowine water. After 15 minutes of thorough rinsine. the eirl was s&t barefoot to the school nurse. ~hYankst o the c$ck reactions and well-learned safetv lessons of the bov who acted so competently, the girl suffereb. no skin irritation. The only losses were her stockings and all the stitching from her shoes. Responsibility for the health, safety, and chemical education of sometimes precocious adolescents is an awesome
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charge. The potential hazards of nll the chemicals tn be used in an experiment should bt, checked in advance in reliable sourrrs such as "The Merck Indexv5 or NFI'A Guide No. 491M, "Manual of Hazardous Chemical Reactions."3 Serious consideration also has to be eiven t o the chemical wastes the experiment will generate. No matter how innocuous the end product of a reaction may seem, the anticipated waste should be checked for safe disposal before the experiment. The best advice any teacher can receive on the problem of hazardous waste is do not produce any. While that is not altoaether possible, forethought can vastlv reduce the nroblem. ~ourcehofinformation onhazardous wRstes were noied in the Occuher 1981 Safer3 ' l i ~ s .Huwrvrr, the pwblem is so volatile that any waste thatcanLot be neutralized should he discussed with the local EPA. Every experiment, whether planned for a demonstration or student experiment, should be tried beforehand by the teacher. Still. even the best nrenared . . teachers have had some unhappy surprises. It is important to share any experiences thnr micht nrvvrnt otherarcidents.'l'heeditorofSolt?t\ Tius would like to hear from teachers who have stories td.share..~f anonymity is desired, it will be respected. Those wishing recognition will be given it. ~
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'Bristol, Conn.lValley Press. September 5, 1980, p. 9. Merck a Co., Inc., Rahway. NJ 07065.
Volume 59 Number 10 October 1982
889
