Cryogenic vials...really necessary?

Martin Goodson bapn4 at central.susx.ac.uk
Fri Oct 10 05:58:30 EST 1997


Peter (pxpst2 at vms.cis.pitt.delet.edu) wrote:
: In article <61ihrf$rom$1 at infa.central.susx.ac.uk>,
: bapn4 at central.susx.ac.uk (Martin Goodson) wrote:

:  This ice does not then melt and 
: > evaporate because it is so cold. 

: I never said the water melts.  I said that it skips the liquid phase and
: goes from solid phase to gas phase by a process called SUBLIMATION.  The
: gradient that facilitates this is the vapor pressure of the water right
: above the frozen sample.



so, ok, I am to believe, from your previous post that

:(air at -20C does not have the capability of holding moisture in a
: liquid state)

and yet also

: Cold air is capable of holding water vapor but not
: much.  

now could you explain how the direct phase transition from vapour to solid
occurs (sublimation) at atmospheric pressure?


: > This as true for the ice in a glycerol
: > as it is for the ice sheet in the freezer. There is no reason why water
: > would be drawn out of a tube and onto the freezer wall since they are
: > both in the same environment (ie very cold).
: > 
: But they are not the same enviornment.  The coils are colder than the air
: in the freezer and thus the moisture gets deposited on them hence the two
: enviornments are not equivilant.  The air in the freezer is dryer than the
: the air in the tube. And like all reactions they head for equilibrium(Le
: Chatelier's princible).

this is true: there will be a slight difference in humidity between the
coils and the tube but i suspect that this will so small to be
insignificant. There will not be much water vapour above ice at -80C. 

:For a review of the princible look up the definition of relative
: Humidity.  Then find out how the relative Humidity is solved for.   

: I suggest that you review your general chemistry. 
: Peter

is this really necessary?

martin



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