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piranha solution not powerful enough?

piranha solution not powerful enough? - Chemistry Forum

piranha solution not powerful enough? - Chemistry Forum. Discuss chemical reactions, chemistry.


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  #1  
Old 07-22-2003, 11:19 AM
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Default piranha solution not powerful enough?



In our lab we use piranha solution (30% H2O2, 70% H2SO4) for cleaning
glassware that has to be really clean. (it's about self assembled
monolayers on silicon and gold). At the moment, we're out of substrates
and I want to recycle some used ones, but regular piranha seems not to
be powerful enough. Is it an option to increase the concentration of
hydrogen peroxide (literature references usually mention 30% H2O2 in
water)? I know this stuff has the tendency to blow in your face so I
wonder if someone is using a more powerful variant of piranha with
success.
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  #2  
Old 07-22-2003, 07:58 PM
Bruce Hamilton
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Default piranha solution not powerful enough?

On Tue, 22 Jul 2003 13:19:31 +0200, <[Only registered users see links. ]> wrote:


OK just to clarify - piranha solutions are extremely dangerous, and
should always be handled only after full training and formal safety
reviews. As they have to be hot and fresh to be effective, and are
usually used to clean up after experiements ( when people are in a
hurry and often have organic solvents lying around as well), many
laboratories have banned them. Don't use them without full disclosure
to, and full approval of, laboratory mangers and safety officers.

Piranha solutions ( both acid and base ) react vigorously with organic
compounds, especially when hot. If you provide sufficient reactants
(eg photoresist or solvents such as propan-2-ol and acetone ), they
generate enormous quantities of heat and gas and will rapidly
disassemble your glassware, glove box, fumehood, and maybe you.

I assume the poster's lab has a full SOP on the properties and
handling of piranha solutions, so I'm a little surprised by the
following question.


Once the H202 concentration hits about 50%, the probability of an
explosion increases substantially. The solutions are only effective
fresh and hot, and are used to remove small residues of photoresist (
which is usually a strong base ) and acetone, not usually the bulk
materials.

The base piranha solution requires some heating to about 60C to
initiate the cleaning reaction, whereas the acid one is self-starting
- due to the heat of dilution between the water in the peroxide
solution and the sulfuric acid.

I think you should consult with users in the semiconductor industry
about how they clean up recycled components. They may use a preclean
or some other cleaner to remove the bulk contaminants before a final
clean ( which may involve HF instead of piranha ). You may not be
removing some inorganic or inert material, and increasing the peroxide
concentration will probably not help.

The normal acid piranha solution is usually a 3:1 vol/vol mixture of
concentrated sulfuric acid with 30% hydrogen peroxide solution,
although I've seen formulations that use a 2:1 ratio there has been no
evidence they are more effective, and may be less of the temperature
is lower.

The base piranha solution is usually a 3:1 vol/vol mixture of ammonium
hydroxide solution with 30% hydrogen peroxide solution. Both cleaners
are extremely reactive and dangerous when above 60C.

When preparing acid piranha solution, you use very pure (
semiconductor ) grades of acid and peroxide and always add the
peroxide to the acid. The H2O2 is added immediately before the
cleaning because it immediately produces a violent, exothermic,
reaction with vigorous gas generation. The acid piranha solution is
potentially explosive, and will become hot ( 100C + ), apparently
depending on the quality of the reagents used ( refer below ).

Once used, the solutions should be disposed of, as they are only
effective fresh and hot. Never store hot piranha solutions. Warm or
hot piranha solutions stored in a closed container probably will
explode.

If readers haven't got the message, let me be more explicit, piranha
solutions have a very specialised use and are routinely used in the
semiconductor industry - with authority, training and full protective
equipment. Use them elsewhere without that knowledge, expertise, and
equipment, and you risk serious injury.

They have been discussed at length on sci.chem, along with some of the
other semiconductor cleaners - such as hot fuming nitric and very hot
( 300C ) concentrated sulfuric acid. These are all cleaners that
general labs routinely do not need or cater for. Below is some
additional comment on the acid piranha solution.

Bruce Hamilton

[ begin old post ]
From: Sean O'Brien <[Only registered users see links. ]>
Newsgroups: sci.chem
Subject: Re: Piranha Solution
Date: Tue, 08 Apr 1997 12:43:17 -0500

There are some interesting aspects to this extremely dangerous
mixture.

First is the temperature spike. As you mix acid and water of course
the temperature will increase. Since 30% H2O2 is 70% H2O this is a
key effect, however if you are using ultrapure H2SO4 and H2O2 the
temperature spike is much lower. With laboratory grade reagents it
is quite easy to achieve a 170oC solution within 1-2 minutes after
mixing. However using ultrapure top quality semiconductor grade
reagents the temperature spike is closer to ~90oC. While this
still is dangerous I would think the cooler solution would be much
less dangerous, the extra 80oC would accelerate reactions from
moderate to explosive.

Since virtually nobody has access to such ultrapure chemicals piranha
should be considered deadly explosive. However, it is used with
virtually no ill effects throughout the semiconductor industry. An
interesting alternative is to bubble O3 into H2SO4 and heat it to
about 140oC (since there is no heat of mixing). This solution will
last for days while a true piranha is dead within 1-2 hours. While it
may still be as dangerous the requirement of a heater makes a huge
difference in controlling it.

Sean
[ end old post ]
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  #3  
Old 07-22-2003, 08:38 PM
Dave Muzuno
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Default piranha solution not powerful enough?

"Bruce Hamilton" <[Only registered users see links. ].nz> wrote in message
news:[Only registered users see links. ].nz...

How about the cleaning solution made from KOH and H2O2? I used this in
graduate school after hearing of how dangerous the sulfuric acid/H2O2
mixture is. Is it any safer?

I would dissolve ~100 g of KOH into 300 mL of water (~6 M), then throw in
~100 mL of 30% H2O2. It would get really hot from the dissolving KOH and
after you added the H2O2 you would see moderate gas evolution. It was an
excellent cleaning agent, you could fill a vacuum manifold with it and it
would slowly bubble away and remove most everything. It worked more quickly
than just the normal base bath.

I know from your post that the basic piranha solution using ammonia was
dangerous, but ammonia can be oxidized unlike KOH. Does that make a
difference?

Dave


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  #4  
Old 07-23-2003, 06:20 AM
Bruce Hamilton
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Default piranha solution not powerful enough?

On Tue, 22 Jul 2003 16:38:37 -0400, "Dave Muzuno" <[Only registered users see links. ]>
wrote:


I'm not familiar with this cleaning solution, but I'm very wary of
mixing 30% hydrogen peroxide solutions with anything. I haven't
checked the literature but, IIRC, some metals and their salts can
trigger explosive decomposition of hydrogen peroxide solutions
with varying induction periods.

My instincts aren't a good reason for you to abandon what works for
you, but when I recall that hydrogen peroxide has been used as a
propellant, it reminds me there is a lot of energy available, and I
prefer it to only appear on request. I'd probably want somebody to
conduct a short risk assessment before using such cleaners.

I think I would stick with the standard alcoholic KOH cleaning
solution for glassware. Sorry I can't be more helpful, but it's not my
field....

Bruce Hamilton
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  #5  
Old 07-23-2003, 09:37 AM
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Default piranha solution not powerful enough?

The substrates used are SAMs of no more than C20 length so there should
be a minimum of oxidizable material on them. Dust will be more present
than adsorbents. Indeed I have planned to use 50% H2O2 with concentrated
H2SO4. Instead of making a bucket full of piranha I want to perform
these experiments only in very small quantities (for example 20 ml).
Would this help? I have fortunately never witnessed piranha mishaps and
assume perhaps wrongly that working with low quantities will avoid them.

When altering the composition of piranha I will certainly ask someone
who routinely uses it to assist, but no one here has worked with
anything but the regular acid piranha.


Bruce Hamilton wrote:


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  #6  
Old 07-23-2003, 06:34 PM
Steve Turner
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Default piranha solution not powerful enough?

[Only registered users see links. ] (Bruce Hamilton) wrote:


I misread the OP, which did indeed specify that it was *glassware*
which was to be cleaned. In that case, I agree with Bruce -- it's
hard to beat a base bath for a combination of effectiveness and
relative safety (though it must be assiduously prevented from reaching
skin and eyes, of course). Use a 10-30% solution of NaOH or KOH in
25-50% ethanol or methanol and allow the items to soak overnight. I
have very rarely had this fail.

For the very few deposits that base bath will not remove, aqueous HF
will often do the trick, because it dissolves a film of glass from
beneath the contaminant. Use only on borosilicate. Aqueous HF is
very, very nasty stuff, so it's probably better to discard the item if
base bath doesn't work.

Steve Turner

Real address contains worldnet instead of spamnet
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  #7  
Old 07-23-2003, 07:03 PM
William David Thweatt
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Default piranha solution not powerful enough?

[Only registered users see links. ] wrote:
: In our lab we use piranha solution (30% H2O2, 70% H2SO4) for cleaning
: glassware that has to be really clean. (it's about self assembled
: monolayers on silicon and gold). At the moment, we're out of substrates
: and I want to recycle some used ones, but regular piranha seems not to
: be powerful enough. Is it an option to increase the concentration of
: hydrogen peroxide (literature references usually mention 30% H2O2 in
: water)? I know this stuff has the tendency to blow in your face so I
: wonder if someone is using a more powerful variant of piranha with
: success.

Yet another reason I went into theory as a grad student...

--
--
William "Dave" Thweatt
Robert E. Welsh Postdoctoral Fellow
Chemistry Department
Rice University
Houston, TX
[Only registered users see links. ]
[Only registered users see links. ]
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