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 6:15: