RE: Isopentane Substitute /making N2 slush

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From:jim <> (by way of Histonet)
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I used demonstrate the making N2 slush to school kids. Sit a 500ml beaker half
filled with liq N2 on a bit of insulating material (bit of cardboard) within
the belljar of an evaporator and pump using the two stage rotary pump (2 cubic
m/h or larger). The N2 will boil furiously. Boiling cools, hence boiling water
remains at 100 degrees despite extra heat input. Since we are not adding
substantial heat to the liq N2, the pumped on liquid becomes colder and more
viscous and suddenly turns solid. When the belljar is vented the solid
nitrogen, which looks like ice slush, will soon become liquid again.
If an insulated container (a stainless vacuum cup would be good) was used, the
N2 would remain solid for longer and could be used for snap freezing of
Liq N2 does not freeze rapidly because any object inserted is surrounded by N2
gas, which is formed from the liq N2. This gas envelopes slows the snap
freezing process and its known as "Leidenfrost". Leidenfrost is the reason
one can insert for a second or two a finger into liq N2, without harm, but
cannot touch a cold metal block!
A specimen inserted into N2 slush would cause the surrounding slush to quickly
turn to liq N2, but this would be well below the boiling point of N2. The
advantage of using slush is not so much the lower temperature of solid N2, but
the faster heat exchange.
Jim Darley
ProSciTech                 Microscopy PLUS
PO Box 111, Thuringowa  QLD  4817  Australia
Ph +61 7 4774 0370  Fax:+61 7 4789 2313
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On Tuesday, May 16, 2000 9:21 PM, Philip Oshel [] wrote:
> Slush nitrogen. Best way I can think of to make this is get some from the
> UW Chem or Physics departments. You should be able to make it from regular
> LN2 by pulling a vacuum on it, so far I've failed with a little 28-30" of
> Hg type of pump.
> Phil
> >I was contacted by a researcher looking for a non-flammable substitute
> >for Isopentane for snap freezing tissue. Is there anything?
> >
> >Victor Tobias
> >Histology Supervisor
> >University of Washington Medical Center
> >Seattle, WA
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> Philip Oshel
> Technical Editor, Microscopy Today
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> or

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