At 8:21 am ((PST)) Sun Jan 21, 2007, mycroft2152 wrote:
>I always liked the post where the pcb maker was going to "distill'
>the H2O2 to make it stronger.
Possible, done industrially, but probably high
on the "Don't try this at home" list.
[I enjoyed the post on another list where the obviously nearly
clueless individual was going to use conc. H2SO4 as an
air drying agent in his H2O2 concentrating apparatus.
Conc. H2SO4 is not something I would like to add to a device
that is already capable of a BLEVE. (see Wikipedia,
AKA "Blast Leveling Everything Very Effectively")]
I've seen it said a few times: "peroxides kill chemists".
But if you must...
Pure H2O2 boils at 150degC, so distillation is possible.
However, it is increasingly unstable at elevated temperatures,
so perhaps you might think of distilling under reduced pressure.
This will work, unless you discover that the vapour can explode
spontaneously at around 70degC.
If you avoid those "gotchas", you might instead discover that
its vapour can form contact explosives with grease and other
organic materials, so you have to be very careful about what
you use for seals in your apparatus and also ensure there are
no residues of cleaning solvents inside.
Better not to try, I think.
Safer might be sparging with air. Some H2O2 will be discharged
with the exhaust air, but at least it will go safely to waste rather
than coming back to bite you. A cursory glance at tables 13 & 14
here
http://www.h2o2.com/intro/properties/physical.html suggests
to me this might work quite well to beyond 30%, especially at low
temperatures, but would then be progressively more wasteful.
Apart from that, there doesn't seem to be an upper limit. You might
be able to use a cold trap to collect the vented H2O2 (and the water)
and fractional crystallization (see below) to recycle the waste H2O2
- economical in H2O2 but expensive in energy
[This also explains this warning from Wikipedia: Hydrogen peroxide,
if spilled on clothing (or other flammable materials), will preferentially
evaporate water until the concentration reaches sufficient strength,
then clothing will spontaneously ignite. Leather generally contains
metal ions from the tanning process and will often catch fire almost
immediately.]
"Freezer distilling" (fractional crystallization and removal of the
excess water as ice) works up to a point and is probably safest.
Tables 10 & 11
http://www.h2o2.com/intro/properties/physical.htmlsuggest that about 25% is possible at -20degC and the maximum
is around 58% at -55degC. Actual results will be worse, depending
on how good your lab technique is for removing the ice crystals
without spoiling the cold. Perhaps someone would like to start a
cryonic centrifuge project? ;-)
In practical terms, if you have a particularly cold domestic freezer
you might be able to convert 3% (10vol) peroxide to around 20%,
suitable for CuCl etchant regeneration. If you have easy access to
dry ice then you could use a freezing mixture with acetone or ether
http://www2.uni-siegen.de/~pci/versuche/english/v105-2.htmlto approach 60% peroxide. Before you attempt this, be sure you
know how to handle it, because at this strength the whole volume
of the liquid can spontaneously decompose into hot gas - ouch!
Regards, LenW