subsecret
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Best Way to Dry MgSO4
I figured that the short questions thread would be better for this, but I did not want to "interrupt" the ongoing discussion by
throwing this in.
I'd like to point out that magnesium sulfate can be dried easily by spreading it in a >1cm thick layer on some aluminum foil, and baking in an oven
for up to an hour, at a relatively high temperature. I baked my sample for an hour at 200C/400F. Make sure that the salt is covered with a sheet of
foil, otherwise it will throw bits of salt all over the oven. Once it's baked, place the foil sandwich on a thick, soft object (I used a folded
kitchen rag), and beat it with a wooden spoon or other curved object. The rag allows the saltcake to flex when hit with the spoon, causing it to snap
into pieces and separate from the foil.
I shared this because I tried baking it in a glass bowl, in a thicker layer, and the whole thing melted into a cake which I could not remove from the
bowl, so the mess is soaking in the kitchen sink.
Also, leave a considerable amount of space between the sheets of foil, otherwise the salt will not dry completely. This could be solved by baking for
longer.
Fear is what you get when caution wasn't enough.
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j_sum1
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Good suggestions.
For a batch I did a week or two back I used Al trays and filled them up quite high. I did MgSO4and CaCl2 at the same time on the barbecue at 200C for
two hours. The resulting bricks were tough to grind up. Initial breaking took some force since the lumps were 4cm thick. A 1cm sandwich sounds a lot
easier.
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dermolotov
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A very good suggestion is to actually buy a cheap toaster oven. Mine I got off craigslist for $5 bucks and have even dried sodium dichromate in the
oven before.
On top of all this, I use it as a radiator to heat up the shed a tad more during the winter. Very efficient, actually. It can also work as a gentle
radiator that keeps tertiary alcohols, acetic acid, etc liquid whilst I work on something else.
Mechanical information aside, i've given up completely on magnesium sulphate and have switched to calcium chloride. I find it contains less cakey
impurities. I don't even bother with the foil anymore and just use a stainless steel tin that I can just scrap off and wash out any unscrapable layers
out. I always hated washing out the weird food-grade impurities found in OTC epson salt
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Oscilllator
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I have also tried drying it in a glass bowl, and although it did melt a little and stick to the bottom, this stage eventually passes and it can be
removed very easily. Perhaps you just didn't put it in the oven long enough?
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careysub
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Another bit of advice (I haven't tested this yet) is to dry it more gradually at increasing temperatures so that all the water of hydration is not
released at once.
First dry magnesium sulfate at 150 C/300 F until its weight stabilizes, which is the formation of the monohydrate (predominantly) from the
heptahydrate. Then the temperature is increased to 200 C/400 F, and finally 250 C/ 500 F to get anhydrous MgSO4.
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MrBlank1
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pyrex glass is the way to go. Without looking into it, iirc I avoided the use of aluminum until after baking(foil covering to prevent H2O re-uptake),
due to concerns of Al's reactivity with ?SO4's?. iirc. I was also drying Na2CO3 and CaSO4 as well, though. Always crushed and re-baked an extra half
hour, to be sure of dryness. Covered with foil at end, transferred to air-tight jar with paper towel on top upon cooling.
Use breathing protection & read Material Safety and Data Sheets. I once inhaled nasally a small amount of air-borne KOH powder through an
unrelated container-transfer process. I was very silly, and lucky. Be safe!.
EDIT : As others here have stated, increases to heat should be done in increments spaced by time. iirc, I used to start at approx 100C, with
+25C/10mins,or 50C/15-20mins as appropriate. Hold at dehydration temp+15C for full 30-60 mins. I used to have a dedicated gas oven, and a compulsion
to be over-sure, so these figures are probably excessive. Anything more than 15mm thick with MgSO4, and you may find that the bottom of the salt layer
cannot successfully discard it's H2O, resulting in a cake at the bottom in glass. Thick cake slice or egg flip to chop and flip what you can, then
another 15 mins in oven, usually makes bottom of dish salts more workable. iirc.
[Edited on 16-12-2014 by MrBlank1]
AAAAA = Australian Association Against Acronym Abuse
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Mesa
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5-10 minutes in the microwave in a non-glass container with a lid loosely covering it. It's an order of magnitude faster/more efficient than external
heating with no real drawbacks.*
*I don't suggest using any kind of glass container for this, it pops violently enough to crack glass in some cases. A tupperware container with a
loose lid works excellently.
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Oscilllator
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Mesa, wouldn't the tupperware melt?
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S.C. Wack
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Quote: Originally posted by careysub  | dry it more gradually at increasing temperatures so that all the water of hydration is not released at once.
First dry magnesium sulfate at 150 C/300 F |
You may want to start lower and keep it there for a while. A pourable product approximately retaining the original crystal size is possible if you try
to be careful.
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Artemus Gordon
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| Quote: Originally posted by Awesomeness |
I shared this because I tried baking it in a glass bowl, in a thicker layer, and the whole thing melted into a cake which I could not remove from the
bowl, so the mess is soaking in the kitchen sink.
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Just a few days ago, I poured about a 0.5 cm thick layer of Epsom salts in a 150 ml beaker, placed it on a ceramic screen and heated it with only an
alcohol lamp.
Even though Wikipedia says it takes 150 degC to decompose MgSO4 heptahydrate, and over 200 degC to get to anhydrate, I found that at temps
no higher than 90 degC it produced a lot of water and a hissing sound. I continued heating it until the water boiled away and the hissing stopped,
then I had a small amount of loose white particles and a solid white/translucent layer stuck to the bottom of the beaker. I put some of the white
particles in a test tube and added a couple of drops of water and found the tube got warm, so I did get to a less hydrated form, although possibly not
all the way to anhydrate.
I put some more particles in another tube with a CoCl2 paper that had gone slightly pink due to a poorly sealed bottle, and corked the
tube. Surprisingly, the paper did not turn blue as I expected.
As for removing the the hard layer in the bottom of my beaker, I filled it with water and let it sit overnight. This did nothing to soften or break up
the mass. However I heated it to near boiling and then the water went cloudy and then with a bit of stirring the mass broke up and dissolved, leaving
only a water-clear solution.
Later when I have more time, I will carefully weigh the before and after products to try to calculate just how much water I can remove at such a low
temperature, and I will also try a better heat source to get it to 200 degC to see how much different this low temp product is from full-on anhydrate.
[Edited on 16-12-2014 by Artemus Gordon]
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Mesa
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No, you aren't subjecting the tupperware to any exterior heat source(besides the MgSO4 losing water) but instead subjecting it to irradiation to which
the tupperware itself is inert.
The MgSO4 will rarely reach a temp more than a few degrees over that which is required to dehydrate.
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