Sciencemadness Discussion Board

Drying salts

cnidocyte - 28-12-2010 at 17:25

I'm making a desiccator to dry I2 crystals and I have plenty of MgSO4 but I hear that epsom salts they sell in the pharmacy are far from anhydrous so I need to remove the moisture. I'm heating it up on a frying pan and its sizzling and I can see water boiling off but will this kinda heat cause the MgSO4 to react with the O2 in the air or anything like that? Also heres what wikipedia says for the melting point of MgSO4
Quote:

1124 °C (anhydrous, decomp)
200 °C (monohydrate, decomp)
150 °C (heptahydrate, decomp)

What do they mean by decompose, do they mean that the H2O trapped in the crystalline structure gets released or do they the actual sulphate anion decomposes? I'm guessing the former because I haven't smelled any H2S and haven't felt any SO2 burning my throat yet.

The MgSO4 has coalesced and formed hard rocks which I'm gonna have to crush and powderise. Is this a bad way to dry the stuff? Are there any better (as in quicker and easier) ways to do this?

[Edited on 29-12-2010 by cnidocyte]

Sedit - 28-12-2010 at 18:11

I always just stick mine in a pyrex backing sheet anymore and pop it in the microwave for a few minutes until its done. Best to do so in periods breaking it up in between else it becomes rock hard. If you start to break it up while there is still a little moister it is like a sponge. I follow this up with a blender for just chemicals and then pop it in for a few more minutes.

entropy51 - 28-12-2010 at 18:16

Quote: Originally posted by cnidocyte  
The MgSO4 has coalesced and formed hard rocks which I'm gonna have to crush and powderise. Is this a bad way to dry the stuff? Are there any better (as in quicker and easier) ways to do this?
Yes, of course there is. Dry it spread out in a pan in an oven. Raise the oven temperature slowly by 30 C or so, holding at that temperature for about 30 minutes before raising the temperature another 30 C or so. Without hunting up my notebook, it's anhydrous at about 150 C IIRC. After the final heating it can be crushed to a fine powder. Heating it quickly as you have done causes it to melt in its water of crystallization and form a substance resembling concrete, as you discovered.

MgSO4 is mostly used for drying organic liquids. For a dessicator, CaCl2 works better and is sold OTC for drying out closets and melting ice on driveways.

[Edited on 29-12-2010 by entropy51]

cnidocyte - 28-12-2010 at 18:55

I bought a bag of CaCl2 dessicant the other day and forgot all about it. Lucky cuz I just turned the kilo of MgSO4 I had into rock.

Sedit - 28-12-2010 at 19:00

Break it up and toss the chunks into a blender it will chop it to a dust. Just run it thru a sieve after your done to clear out any left over chunks.

UnintentionalChaos - 28-12-2010 at 19:34

Having recently dehydrated some MgSO4*7H2O by heating on a hotplate with weight monitoring, I seriously doubt that by 150C, it's anhydrous. It quickly loses it's first 6 waters though. The last one comes off painfully slowly. I was heating the material in a stainless "beaker." Toward the end of the heating, the MgSO4 must have become seriously insulating. Either the bottom of the steel container or hotplate surface had become a dull red, visible when I turned the light off. :o :o :o

I cooled it off rapidly while covered with a watch glass and found that it STILL was slightly above the theoretical weight loss to make the anhydrous compound.

Given how readily I2 sublimes, a dessicator is probably not the best way to dry the crystals.

[Edited on 12-29-10 by UnintentionalChaos]

bbartlog - 29-12-2010 at 17:10

Thing is, there's not much reason to render MgSO4 absolutely anhydrous before using it as a desiccant. It's not such a great drying agent that you can render things really anhydrous anyway, and for more general drying it's not going to matter much if it retains that last molecule of H2O. Now if you need it as a reagent then I guess the residual water could goof up your weights/stoichiometry, but that's a different use.

Hexagon - 29-12-2010 at 17:57

Why don't you try to stick your iodine at the bottom of a beaker, then add a layer of a dessicant (CaCl2 or what ever, I've heard un cooked rice works too) then put at the top the beaker a RBF with water and ice, then heat the beaker to obtain reasonably pure iodine at the top?

cnidocyte - 1-1-2011 at 20:24

Quote: Originally posted by Hexagon  
Why don't you try to stick your iodine at the bottom of a beaker, then add a layer of a dessicant (CaCl2 or what ever, I've heard un cooked rice works too) then put at the top the beaker a RBF with water and ice, then heat the beaker to obtain reasonably pure iodine at the top?


I like the sounds of that. I'm gonna try this, thanks.

As for drying MgSO4, after a load of trial and error, I tried doing it in the oven, raising the temperature very slowly like some people recommended and like this I managed to obtain puffy, dry MgSO4 that I could crush into powder easily.

The WiZard is In - 2-1-2011 at 08:46

Quote: Originally posted by cnidocyte  
I'm making a desiccator to dry I2 crystals and I have plenty of MgSO4 but I hear that epsom salts they sell in the pharmacy are far from anhydrous so I need to remove the moisture. I'm heating it up on a frying pan and its sizzling and I can see water boiling off but will this kinda heat cause the MgSO4 to react with the O2 in the air or anything like that? Also heres what wikipedia says for the melting point of MgSO4



In the commerial production of iodine, it is melted under
conc. sulphuric acid, which is also a v/ good desicant.

Ref. availabe upon request. (My issues of Industrial and
Chemical Engineeing are stored outback.)

Wonder I what the numbers for a vacuum/P2O5/LN2 cold trap dobe.

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