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Previous by Date: Subject: Re: [DCHAS-L] piranha solution Date: Wednesday, August 20, 2014 10:51:59 AM Author: Brandon S. Chance <bchance**At_Symbol_Here**PRINCETON.EDU> |
Next by Date: Subject: FW: [DCHAS-L] [DCHAS-L] piranha solution Date: Wed, 20 Aug 2014 16:47:07 +0000 Author: Kenneth Smith |
My reasoning for adding the peroxide solution to the concentrated sulfuric acid, contrary to the normally better way, was the possibility of destabilizing peroxide by tying up the water in solution by sulfuric
acid protonating it, having the peroxide about as concentrated as it is to begin with, and heating the whole thing with heat from the protonating of water with sulfuric acid. The opposite order of mixing has peroxide and water being thoroughly protonated,
(+)H2O-OH along with H3O(+), but at least the peroxides, reactive in a bimolecular reaction, are dilute.
This is a mixing in need of what chemical engineers are good for: fashioning a devise that slowly mixes the two, while effectively transferring the heat of reaction away. The rest of us just try to get away with
chilling the sulfuric and very carefully adding chilled peroxide to it.
That video is overall really good. Kudos to the professor!
From: DCHAS-L Discussion List [mailto:dchas-l**At_Symbol_Here**MED.CORNELL.EDU]
On Behalf Of Kenneth Smith
Sent: Wednesday, August 20, 2014 9:24 AM
To: DCHAS-L**At_Symbol_Here**MED.CORNELL.EDU
Subject: Re: [DCHAS-L] piranha solution
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
Ken Smith, CHP CIH RRPT
University of California
Interim Director of EH&S
Laboratory Safety Manager
ken.smith**At_Symbol_Here**ucop.edu
voice (510) 882-3499
From: DCHAS-L Discussion List [mailto:dchas-l**At_Symbol_Here**MED.CORNELL.EDU]
On Behalf Of James Saccardo
Sent: Tuesday, August 19, 2014 7:22 PM
To: DCHAS-L**At_Symbol_Here**MED.CORNELL.EDU
Subject: Re: [DCHAS-L] piranha solution
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.
Jim
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----- Reply message -----
From: "Wayne Phan" <Wayne.Phan**At_Symbol_Here**POMONA.EDU>
To: "DCHAS-L**At_Symbol_Here**MED.CORNELL.EDU" <DCHAS-L**At_Symbol_Here**MED.CORNELL.EDU>
Subject: [DCHAS-L] piranha solution
Date: Tue, Aug 19, 2014 7:09 PM
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?
Thanks,
Wayne
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