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Allan Adler 01-23-2004 03:42 PM

swirling and drying
 

On p.63 of Mayo, Pike and Butcher, Microscale Organic Laboratory, 1st ed,
they explain how one dries wet solvents by adding inorganic anhydrous
salts such as sodium sulfate. At the end of the paragraph, they write:
"Swirling the contents of the container increases the rate of drying since
it aids in establishment of the equilibrium of hydration."

If they had ended the sentence after the word "drying", I would have no
problem with it and would simply have assumed that they were drawing on
experience. If they had said something about how swirling distributes the
drying agent more homogeneously, I would also have been content. But that
isn't what they say and I don't understand what they say. How does swirling
help to establishment of equilibrium of hydration and how do they know
that it helps to establish the equilibrium?

Ignorantly,
Allan Adler
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Marvin Margoshes 01-23-2004 05:40 PM

swirling and drying
 

"Allan Adler" <[Only registered and activated users can see links. Click Here To Register...].mit.edu> wrote in message
news:[Only registered and activated users can see links. Click Here To Register...].mit.edu...
swirling

It speeds up reaching equilibrium. You may have discovered a textbook
error, by thinking as you read. You can't believe everything you read, even
in a good textbook.



Bruce Hamilton 01-24-2004 06:14 PM

swirling and drying
 
Allan Adler <[Only registered and activated users can see links. Click Here To Register...].mit.edu> wrote:


It's the added salt material that is hydrating, and it's usually added
as a fine powder that forms sticky surfaces that clump together and
prevent all the particles from interacting immediately with the solvent,
as the water then has to pass through the clump surface. Larger
particles or lumps can also be used, but the drying rate is much slower,
unless more dessicant is added.

Swirling ensures those clumps don't form, so all the particles can
interact with the water in the solvent. the swirling also means that
the water doesn't have to diffuse through the solvent and wet
desiccant layers to react all the available drying surfaces.

Molecular sieves don't have that problem, and are added as beads,
but if the water content is high, it's often better to pass the
solvent slowly through a column of molecular sieve beads
( columns of salts tend to block with very wet solvents ).

It's also worth noting that molecular sieves can usually dry solvents
quicker and better than salts, and are about the same cost for labs
( as much less is usually needed - they hold approx 22% water ),
and are my dessicant of choice for solvents these days.

Bruce Hamilton

Ron Jones 01-24-2004 10:49 PM

swirling and drying
 
Bruce Hamilton wrote:

I prefer magnesium sulphate myself. If it's still powdery after 30 mins
stir then it's dry. One 25kg bag in a 600L reactor is often good enough to
dry most non aqueous solvents, after a water separation (depending on how
good the separation was, of course!)

--
Ron Jones

Don't repeat history, see unreported near misses in chemical lab/plant
at [Only registered and activated users can see links. Click Here To Register...]




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