letters Hot and Cold Running Methane II
To the Editor:
George Gilbert's Tested Demonstrations is the section of the Journal that I always examine first. In the September 1992 section of Tested Demonstrations, an article entitled "Hot and Cold Running Methane" presented some novel and very useful ideas. One of these, however, poses a potential hazard, which should be considered. Item three of the demonstrations suggests ?epairing" a gallon metal can which had been previously crushed by "pouring a small amount of Nz(l)into it and screwing the cap on" [1992,69, 7621. In my opinion, this poses a serious hazard. Metals tend to weaken with bending, and cans that have been crushed may behave unpredictably. If the demonstrator uses too much liquid nitrogen, a powerful explosion could result. Before performing this demonstration, I'd like a better definition of "a small amount". Also, I'd feel more comfortable if the demonstration was performed behind a safety shield. An alternative method of expanding the crushed can is to simply have the windiest student in the class blow into the can. Not even philosophy students are likely to cause the can to explode in this manner. Dan M. Sullivan University of Nebraska Omaha, Nebraska To the Editor:
Dan Sullivan has expressed a reasonable concern regarding the "repair" of the crushed can with N2(1), but the hazard associated with i t is not very great. I intended for the presenter to loosen the cap when the can had returned to rouehlv its orieinal shaoe. in which case there is no hazard wLat"soever.kven if cap becomes jammed, there are several reasons whv "a oowerful exolosion" is not likely. The weakening of the metal from bending actually reduces the possibility of a violent rupture because the can tends to crack a t the creases and leak enoueh to stoo the expansion of the can altogether. ~ r e ~ u e n t lhave y i tistop such leaks with mv" fineers to allow the exoansion of the can to continue. In response to Sullivan's letter, I tried several times to produce a worst case scenario. In each case I poured as
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much Nz(1) into the can as possible, screwed the cap on tightly, and observed the result. (It is very difficult to quantify the amount poured in because as soon as you add any Nz(l), the rush of Nzc,, out of the can interferes with adding any additional liquid, causing a lot to be spilled.) In three cases the cans developed leaks at various stages of expansion. In another case ihe seam split ahout 10cm alonfi the bottom of the can. These cases posed no hazard. In tht: last case I deliberately pumped theean down to only about half its volume to avoid creasing the metal. In this case the eutire bottom of the can blew off fairly explosively. In my experience an explosion is unlikely, but i t might he advisable to have some pliers handy as the can expands, just to guarantee the loosening of the cap. Daniel M. Stamm Frederick Douglas High School Atlanta, Georgia 30318 Combustion Toxicity and Flammability of Polymeric Materials
To The Editor: The recent note [J. Chem. Educ., 1992,69, 3891 regarding calculations of hydrogen cyanide toxicity describes a useful exercise for students. Unfortunately, by only obliquely mentioning "other gases", it could leave the impression that acrylic fiber is uniquely toxic in combustion and perhaps excessively raise student fears about the safetv of this oroduct cornoared to other fibers. both natural &d synthktic. An excellent reference not cited by the authors is The Combustion of Organic Polymers by C. F. Cullis and M. M. Hirschler [Oxford; New York, 19811. In this monograph it is pointed out that combustion of all organic materials releases carbon monoxide. The effects of the CO mav be svnereisticallv enhanced bv the oresence of hydrogen cyanide but is nevertheless ac;tely toxic by itself. Furthermore, although the cyano group is most evident in acrylonitrile, dehydration of amides produces nitriles, and combustion of wool and nylons also produces hydrogen cyanide. What is not so evident is the fact that nitrogen content correlates negatively with flammability. High nitrogen polymers are relatively effective flame retardants, and in the most realistic tests of survival times in combustion products (Cullis and Hirschler p 861, nitrogen-containing materials were significantly less toxic than
Volume 71 Number 9 Seotember 1994
809
