RogueRose
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Temperature effect on drying solvents with salts - does temp effect speed or efficiency?
I'm trying to find out if drying a solvent such as ethanol, methanol or acetone with a salt like MgSO4, CuSO4 or other salts works better if the
solvent is at a higher temp or lower temp and if there is significant difference in speed at different temps. It seems that in most reactions there
is some heat generated when the water is absorbed so IDK if that means a higher temp will help with the process or not.
[Edited on 30-1-2017 by RogueRose]
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Praxichys
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There are several factors that need to be considered. Higher temperatures increase brownian diffusion of water through the solvent and therefore
increase the speed at which water might come into contact with the sequestering agent/trapped in a sieve. On the other hand, desiccants are driven
kinetically, gaining water at low temperatures and releasing it at high temperatures. This temperature-moisture equilibrium plays a part in how dry a
desiccant can get a solvent, with colder being more suitable. Further, the solvent and the desiccant compete with each other for the water. The
desiccant mostly wins because by nature it has a high affinity for water, but there is always a small equilibrium that exists. Most desiccants also
have various hydration levels, with the higher levels having less affinity for water than the lower, which leads to nonlinearity in the equilibrium,
favoring an infinite amount of desiccant, which is obviously impractical. What is needed to attain maximum dryness is really related to how much you
can add before mechanical losses become unacceptable, and how cold you can get it before the time taken to desiccate is impractically long.
This is why reactive desiccation (i.e. refluxing with Mg, Na, CaO, etc.) is used to get things truly dry, since conventional hydration reactions
usually don't have enough reverse enthalpy to drive the dehydration to a useful level for water-sensitive applications. Even then, it can take hours
of refluxing over sodium to achieve a few tens of ppm H2O.
I hate to leave you with a steaming mess of conceptual chemistry, but the answer is that a number of these factors will contribute to a set of
conditions where a minimum water content can be achieved for each particular desiccant/solvent system. However, I wouldn't worry about it too much.
Most common desiccants can easily achieve <1% H2O in modest amounts at room temperature in under an hour with stirring, which is fine for the vast
majority of applications. In the practical laboratory, use of a particular desiccant is mainly related to chemical compatibility rather than drying
power, for everything other than water-sensitive reactions.
[Edited on 30-1-2017 by Praxichys]
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Sulaiman
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I think that buried in the above is a very important tip;
"Most common desiccants can easily achieve <1% H2O in modest amounts at room temperature in under an hour with stirring"
.... with stirring.
I recently learned how slow diffusion can be.
CAUTION : Hobby Chemist, not Professional or even Amateur
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Praxichys
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Indeed. Absorption of water changes the morphology of crystalline compounds, generally increasing their volume. Because of this, desiccant
particulates without agitation tend to both passivate and agglomerate, both processes drastically decreasing the active surface area, destroying
desiccating efficiency.
Don't let it become that large, unreactive brick at the bottom of the flask!
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