Paul,

Pentane expands as it freezes, and if the top freezes like a cap over
the top of your dewar, the dewar could become over-pressurized and blow
up. When making LN2-pentane slushes for a water trap in vacuum line
work, we chilled the pentane in a rubber bowl and then scooped the
frozen pentane into a dewar precooled with LN2. Never had a dewar blow
up. Hope that helps.

Geoffrey Garrison, Associate Researcher
Department of Earth and Space Sciences University of Washington
63 Johnson Hall, Box 351310
Seattle, WA  98195
(206)616-8763, office
(206)543-6327, lab
[log in to unmask]


-----Original Message-----
From: Stable Isotope Geochemistry [mailto:[log in to unmask]] On
Behalf Of Paul Eby
Sent: Tuesday, July 22, 2003 4:44 PM
To: [log in to unmask]
Subject: [ISOGEOCHEM] exploding cold baths

I'm hoping someone can shed some light on some trouble we've had in our
lab.

We often use slush baths (solvents cooled with dry ice, LN2, or LAr) to
reach specific temperatures in various analytical methods. Most
recently,
we've been using iso-pentane and LN2, which lets us go down to -160C. It
worked fine for a few weeks, then we ran into trouble.

When cooling the solvent down, we could hear little "pops", almost like
popping bubble wrap. Occasionally, this would be severe enough that we
would get solvent blowing out of the dewar like a geyser.

Several changes failed to solve the problem: replacing the solvent with
a
new batch out of the bottle. Changing the dewars to other clean ones.
Trying liquid argon. Trying n-pentane. In our last test, a dewar of
iso-pentane actually exploded violently, vaporizing the solvent and
sending
shards of glass flying (in a closed fume hood, of course!).

Can anyone offer up an idea as to what is happening here?

Who says science isn't exciting!

Paul Eby

[log in to unmask]
Biogeochemistry Lab (E-Hut, room 102)
School of Earth and Ocean Science
University of Victoria

PO Box 3055
Victoria, B.C.
Canada
V8W 3P6

phone: 250-721-6183
fax:   250-472-4620