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 be prepared or used at all.
Daniel A. Dobbs
Robert G. Bergman
Department of Chemistry
University of California, Berkeley
Klaus H. Theopold
Department of Chemistry
Aug. 13, 1990, page 2
A letter by Daniel Dobbs, Robert Bergman, and Klaus Theopold (C&EN, April 23, page 2) describes two explosions associated with “piranha solution7′ (30% H2O2 and concentrated H2SO4 mixed in equal volumes). These incidents call for further comment.
One of the incidents involved the use of an H2O2/H2SO4 mixture and acetone in a cleaning procedure. It should be pointed out that even in the absence of sulfuric acid, mixtures of acetone and hydrogen peroxide can give rise to the formation of a shock-sensitive organic peroxide. A number of explosions have occurred in this way. As the world’s largest manufacturer of peroxygen chemicals, Interox strongly advises taking every precaution to prevent even inadvertent mixing of hydrogen peroxide and acetone. In this case, the presence of sulfuric acid would have tended to increase the hazard; sulfuric acid can (in effect) activate hydrogen peroxide toward reaction with most organic compounds. The described cleaning procedure is clearly hazardous and should not be used.
In the second incident, a bottle of H2O2/H2SO4 mixture exploded after standing for a week. The authors of the letter state that the bottle was, to their knowledge, loosely capped. They further state that “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.” Although it is impossible to be sure without more detailed information, the damage described (destroyed bottles) does not seem inconsistent with what might be expected if an H2O2/H2SO4 solution was inadvertently sealed. Even moderate confinement can lead to a surprisingly powerful pressure burst. We know of incidents in which stoppered glass volumetric flasks containing 1% H2O2 were shattered by the slow evolution of oxygen. We recommend the use of vented caps on all bottles containing peroxygen solutions.
Piranha solution is a powerful oxidant and should never be used casually or without a detailed understanding of the possible hazards. However, these incidents should be interpreted in light of the fact that H2O2/H2SO4 mixtures are widely (and safely) used in industry—particularly in semiconductor chip manufacturing.
C. Vance Erickson
Deer Park, Tex
I contacted Bergman and Theopold to see if there was anything they’d change or add, 25 years later. Bergman noted that while he didn’t know of more recent incidents, he thinks that the industrial use of piranha solution has probably increased substantially as companies use it to clean semiconductor surfaces, for research as well as manufacturing. If anyone from industry would like to weigh in on how they ensure piranha is handled safely, I’d welcome their contributions.
Theopold had this to say:
These documents obviously bring back very unhappy memories. Fortunately, in the intervening 25 years we have not had another incident of this sort, and we remain – naturally – very much sensitized to the danger.
Rereading those old letters, I could have wished that the response by C.V. Erickson had not sounded as if we had been deliberately using H2O2/H2SO4 mixture and acetone in some intended combination. This would indeed be the height of foolishness, and certainly was not our plan or procedure. Nevertheless, inexperience and a series of unfortunate coincidences apparently allowed the two to come into contact, with the stated consequences.
A quick google search reveals that the use of Piranha solution is very much alive, e.g. in the electronics industry (http://en.wikipedia.org/wiki/Piranha_solution). Accordingly, repeated warnings about its hazards are probably appropriate.
And if you’re preparing piranha, see this post about best practices.