soma
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drying with MgSO4
I read that you should use gravity filtration to remove MgSO4 from a solvent that's been dried. Is that because vacuum filtration would cause water to
be sucked from the MgSO4 also?
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Oscilllator
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No. Water will not be sucked from the MgSO4.
There is no reason I can think of that gravity filtration would be preferably to vacuum filtration in this case. In fact, using gravity filtration
would expose your solvent to water in the atmosphere for longer, and so vacuum filtration would be preferred.
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Sulaiman
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I think that using a vacuum has some possible negatives;
. particles get embedded in the pores of the filter and block it
. many solvents evaporate under vacuum
. the MgSO4 would absorb a lot of atmospheric water from the large volume of air pulled through it
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Loptr
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Quote: Originally posted by Sulaiman  | I think that using a vacuum has some possible negatives;
. particles get embedded in the pores of the filter and block it
. many solvents evaporate under vacuum
. the MgSO4 would absorb a lot of atmospheric water from the large volume of air pulled through it |
I am sorry, but these don't really apply to vacuum filtration.
1. Blocked pores in a filter will have an even greater effect on gravity filtration. The vacuum allows you to overcome the additional force required
to pull the solution through the filter.
2. Solvents evaporate under vacuum, but what do you have under vacuum in a vacuum filtration? The flask will be under vacuum, not the filtrate yet to
go through the filter.
3. It doesn't matter. The water of hydration can only easily be removed by heating MgSO4 or through the use of a dehydrating agent, such as H2SO4.
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veganalchemist
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I work at a place of higher education and we always use gravity filtration, with a fluted filter paper, to remove magnesium sulfate for the solution
we want.
Büchner filtration is used when it's the solid you want to keep.
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veganalchemist
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That should read
"to remove magnesium sulfate FROM the solution we want."
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Sulaiman
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| Quote: Originally posted by Loptr | | Quote: Originally posted by Sulaiman | I think that using a vacuum has some possible negatives;
. particles get embedded in the pores of the filter and block it
. many solvents evaporate under vacuum
. the MgSO4 would absorb a lot of atmospheric water from the large volume of air pulled through it |
I am sorry, but these don't really apply to vacuum filtration.
1. Blocked pores in a filter will have an even greater effect on gravity filtration. The vacuum allows you to overcome the additional force required
to pull the solution through the filter.
2. Solvents evaporate under vacuum, but what do you have under vacuum in a vacuum filtration? The flask will be under vacuum, not the filtrate yet to
go through the filter.
3. It doesn't matter. The water of hydration can only easily be removed by heating MgSO4 or through the use of a dehydrating agent, such as H2SO4.
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1 I was just repeating what I read in Vogel, 'a text book of practical organic chemistry', page 154.
2 I assumed that if the pores of the filter paper do get clogged then there may be sufficient vacuum in the receiving vessel to cause some solvents to
boil ... wrong again?
3 yes, I see now that it's the solvent that is required, the MgSO4 dessicant can easily be dehydrated, d'oh !
I claim immunity due to noobishness 
at least I have 'earned' my rank of 'Hazard to Others' !
[Edited on 5-6-2015 by Sulaiman]
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soma
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Maybe it's because some of the MgSO4 could dissolve in the water and is less likely to go through filter paper in a gravity filtration?
I recently dried THF with MgSO4 using vacuum filtration and noticed that a fair amount of MgSO4 had gone into the frit and was clogging it.
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Mesa
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Unless the text specifically instructs you to use gravity filtration and not to use vac filtration, It's probably a courtesy/protocol thing rather
than having a legitimate practical basis.
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Chemosynthesis
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I have had very fine powders get pulled through filters or clog Frits under relqtively high vacuum, which was not as bad when gravity filtering, but
this could be precluded with crystal/pore size. Not sure if that is a risk in this instance.
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smaerd
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I don't know about you all, but when my solvent is dried by say MgSO4 I can decant 95+% of the solvent without any trouble at all. The filter is just
a fail safe incase I get sloppy. Then I rinse the dessicant again with fresh solvent, etc. I don't think its really preferred to have the solid end up
in the filter. Could be wrong but that's how I've been doing it for years.
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