Safety Hazards Posed by Ceramic-Top Hotplates

Peter De Forest, Carol Myers, and Robert Rothchild Department of Science, John Jay College of Criminal Justice. The City University of New York, New York, NY 10019

We wish to renort a ootential hazard that mav he nosed hv - . the ceramic top of a common laboratory hot plate.! A stu- dent u,ai using a Corning hot nlate- stirrer (model PC-351) to heat two water hathsuwith& a hood. The hot plate was relatively new (2-3 months old) and had appeared to he in perfect condition, was free from obvious defects, and had previously been used without incident. As we reconstructed the incident, two Pyrex heakers (600 mL), each containing several hundred milliliters of water, were being maintained a t a vigorous boil with the hot-plate heat control in the maximum HI position. The heakers were monitored and were not allowed to boil to dryness. These two heakers served as water haths for a student working on an indepen- dent nroiect who was attemntine to follow a standard foren- . - . .. sic pmctdure for fiber analysis.

The oublished pnxedure' called for a small fiher sample to he placed "in& a conical flask containing a mixtureof equal parts of glacial acetic acid and 20-volume hydrogen peroxide and heated on a water hath for 7 or 8 h." The student had employed about 0.1 g of a chemically treated jute rope sample, 20 mL of glacial acetic acid, and, in place of the "20-volume hydrogen peroxide", had erroneously used 20 mL of 30% hvdroeen oeroxide. Desienation of hvdrogen , .. . . .. peroxide concentration in thr United States is conventional- Iv stated as "vercrnt". with 3%. 3O''i. and 90q beinr: rom- - manly available commercial strengths. In Britain and Eu- roue. one freauentlv encounters designations based upon the voiumes of ~ x ~ ~ e n - ~ a s obtainable &om one volume of the solution.3 Thus, a "20-volume hvdroeen peroxide" would . . . designate a solution strength capable of evolving 20 mL of oxygen for each milliliter of hydrogen peroxide solution. In fact. 20-volume hvdroeen oeroxide denotes 6% (wlv) hvdro- ~ ~ , ~ ~~ , ~ ~ . . ~ . grn peroxide, the standard "hydrogen peroxide solutiun" of the nritish Phormoco~oeio. The reference trxt uvon which the procedure was based was published in ond din and fol- lowed the "volume" rather than the "%" terminoloev. Sub- stitution of 30% (wfv) HzOz, which corresponds to GO-vol- ume H?O? (known as "strong H.109 solution" in the 1968 - - - ~ r i t i s h ~ ~ h a r m a c o ~ o e i a ~ ) therefore represented use of a so- lution of H901 that was five times more concentrated than actually called for.

After about four hours of heating in the HI position, aloud report was heard ("like a The rather impressive explosion destroyed the reaction systems; the ceramic hot- nlite ton had blown avart with enormous force. Fortunatelv. the fumk hood windok had been safely down and there we;; no iniuries. The blast force was indicated hv suhstantial cracking and chipping of the hood window's laminated safe- tv glass. evidentlv resultina from massive ceramic fragments (that wkre recovered) impacting the inside of the window. The downward force of the hlast was seen in localized (right front corner) deformation of an aluminum plate in the hot plate; this metal plate is separated by 4 in. of thermal insulation from the heating element near the underside of the ceramic hot-plate top. This internal metal plate had been forced down about 'Iz in. into the circular magnet of the stirrer mechanism and had actually been forcibly punctured by a protruding screw inside the hot plate. Inspection of the electrical heating element and the thin "mica" sheet that

separates the element from the ceramic hot-plate underside did not show evidence of electrical shorts or scorch marks. That the hlast mav have originated in one of the heated vessels was consistent with the observation that great local- ized downward force had heen manifested in one corner of the hot plate, causing the screw puncture noted above and selectively deforming one foot of the hot plate in this same corner. On the other hand, remarkably large fragments of one of the water hath heakers and Erlenmeyer flasks were recovered, presumably the sample that did not explode. (Different types of fiher samples had been digesting in the two water baths).

Subsequent q"estioning of the student who had prepared the samples indicated that adequate water level in the water baths had been maintained to assure that the actual reaction flasks "floated," ruling out the possibility of adventitious local superheating if the flask bottom had been directly contacting the base of the heated beaker. The student re- called havine used metal crucible tones to swirl some other reaction fla& (which had been heatl'ng longer) with other (different) fiber samnles on a different hot-date setup almgside thr systrm tihrrr thr blast had occurrrh. ~ l t h u u g h the student had gripped the flask with the tongs in a manner - .- that put one of the metal jaws of the tongs wkhin the flask neck (rather than using the tongs to grab the flask neck externally), which couldbe a hazard due to metal- catalyzed decompositions, the hlast did not occur on the flasks swirled with the tongs.

The Corning hot plates that we had used bore warning stickers that read. in nart. "REPLACE TOP IMMEDIATE- LY if damaged by eichidg, scratching or chipping. A dam- aged top can break in use." No such damage had been noted prior to use of the hot plate in the present case. The analyti- cal recipe that was followed, which called for "20-volume" hydrogen peroxide, carried no warnings or cautionary notes. We feel that there may be three important morals to be learned from our experience. First, any system involving suhstantial quantities of hydrogen peroxide, organic perox- ides, peracids, or hydroperoxides, or systems likely to form such compounds should he treated with extreme care as potentially violent (and unpredictable) explosion hazards. Work with such materials should he performed on the small- est nossible scale behind laminated safetv elass or other acceptable harrier. We recommend use of a Gl&t shield with- in a hood. Fume hood windows must he kept positioned to provide protection. Eye and face protection must he worn. Extreme care should always be taken to avoid possible con- tamination by metal spatulas, tonAs, or othrr source*. Possi- ble hazards pmcd hy SVLW iiic sample anomalies must also be borne in mkd; we have not ruled out the possible role of catalytic decompositions caused by potential metal ions in particular treated fiher samples.

' A brief preliminary description of this incident was published in Chem. Eng. News 1987, 65t31). 2.

Calling. D.; Grayson, J. Identification of Vegetable Fibres; Chap- man and Hall: London, 1982: pp 6-7.

Wade. A,, Ed. Mariindale The Extra Pharmacopoeia, 27th ed.; The Pharmaceutical Press: London. 1979; pp 1199-1201.

722 Journal of Chemical Education

The second moral of this accident might be to consider seriously alternatives to a ceramic-topped hot plate. The severe fragmentation of the ceramic observed in this case suggests that this material is a potential source of hazardous hunks of shrapnel. Perhaps metal-topped hot plates or alter- native heat sources might be preferred when explosion haz- ards seem possihle.

A third point of this experience is that the cautious labora- tory worker must always treat all reaction mixtures with due respect, and prepare for "the worst possible case", including possible procedural errors. We were fortunate, indeed, that no injuries resulted. In this present case, we believe the

primary cause of the explosion to have been the use of exces- sively concentrated H202, but this is only one example of possible human error. The underlying cause of the error was the misinterpretation of a terminology that may well be unfamiliar to many workers in this country, and this speaks broadly to the need to provide procedural details that are comprehensive and hased on universally understandable measures. I t may well be advised that reagents he "redun- dantly" labeled, using synonyms or names reflecting differ- ent systems of nomenclature or concentration, to minimize the chance of ambiguity or error.

Volume 65 Number 8 August 1988 723