Sciencemadness Discussion Board

drying with MgSO4

soma - 4-6-2015 at 16:57

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?

Oscilllator - 4-6-2015 at 18:47

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.

Sulaiman - 5-6-2015 at 04:14

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

Loptr - 5-6-2015 at 04:19

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.

veganalchemist - 5-6-2015 at 11:34

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.

B├╝chner filtration is used when it's the solid you want to keep.

veganalchemist - 5-6-2015 at 11:39

That should read

"to remove magnesium sulfate FROM the solution we want."

Sulaiman - 5-6-2015 at 12:00

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.


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 :D
at least I have 'earned' my rank of 'Hazard to Others' !

[Edited on 5-6-2015 by Sulaiman]

soma - 9-6-2015 at 00:23

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.

Mesa - 9-6-2015 at 02:25

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.

Chemosynthesis - 9-6-2015 at 07:58

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.

smaerd - 9-6-2015 at 08:06

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.