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%.
Robin M. Izzo, M.S.
Environmental Health and Safety
I not only use all the brains that I have, but all that I can borrow. ~ Woodrow Wilson
I recall that this is the common practice (see Princeton's:http://web.princeton.edu/sites/ehs/labsafetymanual/cheminfo/piranha.htm). My logical reasoning is : If decomposition of H2O2 occurs upon contact with acid then less acid will be splashed compare with the opposite direction. I think that most people see the H2O in H2O2 and therefore thinks that they need to be treated the same. I don't know if someone actually took the time to do a controlled experiment.
Haim Weizman, Ph.D.
Department of Chemistry and Biochemistry 0303
University of California, San Diego
La Jolla, CA, 92093-0303
YouTube Channel: http://www.youtube.com/user/ChemUCSD
Chemorphesis Project: http://chemorphesis.ucsd.edu/
On Aug 20, 2014, at 7:23 AM, Kenneth Smith <ken.smith**At_Symbol_Here**ucop.edu>
Hi Wayne –
I came across this question previously. I too initially thought add A->W just like I was taught undergraduate chemistry. But turns out that this is not necessarily the preferred method in forming piranha solution.
If you take a t look at https://www.youtube.com/watch?v=aA8mC5RIj5k&list=UUMrurXFYJyXaXVRWXK4xAOg Time mark 6:17 or so you can see the instruction is to add peroxide to acid.
From what I recall from Dr. Haim Weizman this quick scene in the video generated a lot of questions. Reviewers of the video commented and questioned this as it seemed counter intuitive to them as well. If I recall there were some technical reasons, degree of reactivity, heat capacity of acid vs peroxide solution to do it in this sequence. The exact reason I cannot recall. I did remember that the degree of preference was only slightly better in this direction over the other and the take home message was hazard of heat generation was rapid and high so the reaction should always be done very carefully with good thermal cooling controls.
I’ve cc’ed Dr.. Weizman as he might be able to contribute more.
Ken Smith, CHP CIH RRPT
University of California
Interim Director of EH&S
Laboratory Safety Manager
voice (510) 882-3499
Wayne, I say pour conc. acid into the peroxide allowing the acid to dissolve into the peroxide, thus allowing the ionizing acid to dilute into the entire volume of peroxide on contact. Pouring peroxide into the conc.acid would cause the peroxide to "spit" off the surface of the acid, as there will be pockets of conc. acid ionizing with smaller volumes of peroxide, without any medium to absorb the heat of solution.
Acid into water is what I was always taught for the reasoning above.
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When preparing piranha solution, do we add acid to the peroxide or the peroxide to the acid. I see different opinions on this. What is the standard practice?
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