Exothermic demo

John Bartmess, Charles Feigerle, George Schweitzer, Wes Fellers, Peter Smith
Department of Chemistry, University of Tennessee, Knoxville

Chemical & Engineering News (15 Jun 1998) Vol. 76, No. 24, pp. 4.

We wish to point out a safety hazard with a widely performed standard chemical demonstration that caused significant damage recently in a teaching laboratory in the chemistry department at the University of Tennessee, Knoxville.

The Belousov-Zhabotinsky oscillator reaction described in Bassam Shakhashiri's" Chemical Demonstrations: A Handbook for Teachers of Chemistry," Vol. 2, involves the cerium ammonium nitrate-catalyzed redox reaction of potassium bromate with malonic acid. This reaction was carried out in an honors general chemistry laboratory without incident during the afternoon, but a fire was discovered in the lab two hours after everyone had left. It destroyed the wooden drawers beneath a stone benchtop before being extinguished by the fire department.

Investigation revealed that the three chemicals used are stable when mixed dry, and react as expected when in dilute solution. However, when 0.5 g of each are mixed together, and between one drop and 3 mL of water added, a very exothermic reaction occurs with considerable fuming. All three chemicals are needed for this to initiate. When carried out on a piece of filter paper, ignition of the paper occurred.

We suspect that in the teaching laboratory, dry chemicals were spilled during weighing, and, rather than being disposed of in the aqueous heavy-metal waste container, were placed in a trash can. This was stored under a sink with a leaky drainpipe, and the critical amount of moisture set off the reaction after everyone had left the room.

There is a safety warning in the experimental procedure regarding bromates being strong oxidizing agents and indicating that mixtures with finely divided organic materials are easily ignited. The catalytic nature of the cerium salt on this is not indicated, however. We alert people to the hazardous nature of this particular mixture, under conditions that are not unusual.

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