Piranha solution explosions
Jan16

Piranha solution explosions

From the C&EN archives, but I believe still relevant today: April 23, 1990, page 2 SIR: We wish to report two violent explosions experienced with a sample of “piranha solution” used routinely in many laboratories to clean badly soiled glass frits and other surfaces. At Berkeley an explosion occurred in a bottle of this mixture that had been stored for several days. In this case, the solution was prepared by carefully mixing approximately 150 mL concentrated sulfuric acid with 150 mL 30% hydrogen peroxide with cooling over 30 minutes. No difficulty was experienced during the preparation, and no incidents were encountered after the preparation while the solution was allowed to stand (we believe only loosely capped) in a fume hood. However, one week after the solution was prepared it detonated spontaneously in the hood, destroying the glass container in which it was stored, as well as other bottles of chemicals stored in the hood. The other incident occurred several years ago at Cornell University. A graduate student was cleaning glass frits by a standard procedure of drawing small portions (about 20 mL) of a freshly prepared mixture of concentrated H2SO4 and 30% H2O2 through the frits by applying vacuum suction. This operation was eventually followed by washing with deionized water and finally acetone (keeping the different liquids separate). On this particular occasion, a violent explosion occurred, which shattered the heavy walled filter flask and caused multiple cuts in the face, chest, and forearms of the student. A partially lowered hood sash and appropriate clothing (safety glasses, lab coat, and heavy rubber gloves) provided some protection, but could not prevent the infliction of serious injuries. Subsequent questioning of the student led us to believe that inadvertent mixing of the highly oxidizing H2SO4/H2O2 mixture with an acetone residue (amounts unknown) was the cause of this accident. At Berkeley, it is possible that an oxidizable organic material was somehow added to the bottle in which the solution was stored, or that the top of the container was inadvertently tightened. However, to the best of our knowledge neither of these things occurred. Furthermore, the force of the detonation makes it seem likely to us that it was due to a chemical reaction rather than simply to the buildup of pressure in the container. In any case, we feel that the most prudent interpretation of these events is that sulfuric acid/hydrogen peroxide mixtures are susceptible to spontaneous and unpredictable chemical detonation. Warnings in the literature that these solutions be handled carefully should therefore be taken especially seriously. We recommend that this mixture not be stored for any length of time, and if possible not...

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Preparing piranha
Oct02

Preparing piranha

Piranha solutions are used to remove organic residues from substrates. Typically a 3:1 mixture of concentrated sulfuric acid to 30% hydrogen peroxide, it is highly corrosive and a powerful oxidizer. Simply mixing the solution is dangerous. And mixing piranha begs the question raises the question: Add the acid to the peroxide, or the other way around? Everyone hopefully learned in chemistry labs to “never cover an acid”–that is, when diluting, always add acid to water, not the other way around. For piranha, however, best practice is to add the peroxide to the acid. Robin Izzo, director of environmental health and safety at Princeton University, said this in an email to the ACS Division of Chemical Health & Safety e-mail list earlier this month: Around 20 years ago, I worked with two chemistry professors to develop best practices for handling piranha solutions. We tested different methods and found that (1) more often the mixture bubbled vigorously and created heat when adding the acid to the peroxide and (2) peroxide concentrations greater than 60% usually reacted violently, under 30% did not react violently and between 30 and 55% sometimes reacted violently. That was the reasoning behind keeping concentrations under 30% and not to exceed 50%. For reference, here is Princeton’s current guidance for making piranha. The University of Cambridge’s directions include a story of what can happen if you’re not careful when using piranha solutions. And the University of California, San Diego’s “A Day in the Lab” has a brief scene about making and using piranha solutions, starting at...

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