MgSO4
MgSO4 (anhydrous) is a very useful chemical; most notably for its use as a highly effective drying agent. MgSO4 is used particularly to dry organic
liquids such as ethanol and other solvents, this is because when anhydrous MgSO4 is hygroscopic. The compound absorbs undesired water from the solvent
and becomes hydrated, forming MgSO4.7H2O. Below I provide instructions to produce this compound from hydrated magnesium sulphate (commonly known as
"Epsom Salt").
Equipment
50 g "Epsom salt"(easily available)
Large beaker (250ml+)
Heat source (eg. bunsen burner/gas cooker)
Spatula
Air tight container
Method
Ensure beaker is dry.
Add 50 g Epsom salt (MgSO4.7H2O).
Heat to around 150 degrees celsius.
Keep heating at this temperature for 10 minutes while stirring constantly(the magnesium sulphate may dissolve in its own water of hydration near
the beginning)
Increase temperature to around 200 degrees celsius for 2 minutes when a white powder is formed.
While hot transfer the anhydrous magnesium sulphate to the container(this will avoid it being hydrated by atmospheric water)
Seal container well.
Hazards
Anhydrous magnesium sulphate will irritate the skin as it is a strong dehydrating agent.
Be careful when heating.
Thank you very much for reading.
Oscilllator - 26-7-2015 at 16:17
I use and recommend the use of an oven for drying MgSO4 as it is much easier on the glass vessels, so you can use standard cookware.
It's worth noting that at one point during the drying process the MgSO4 cements itself to the bottom of the container in a way that makes it
impossible to remove. If this happens don't worry, just keep on heating and eventually the MgSO4 will pop free indicating it's dried.blogfast25 - 26-7-2015 at 17:07
I use and recommend the use of an oven for drying MgSO4 as it is much easier on the glass vessels, so you can use standard cookware.
I've used silicone rubber based cookware (for baking) for this purpose. Totally non-stick!AWLB - 26-7-2015 at 21:28
Thank you for commenting; I personally wanted a method that was very quick and ensured all water was removed from the MgSO4.7H2O, but using a oven
sounds like a good idea! (Especially using silicone cookware!) I did also note that the substance formed a "cement", I thought this was likely to be
it dissolving in the water that was being emitted from it (water of hydration).
Thank You.
[Edited on 27-7-2015 by AWLB]macckone - 27-7-2015 at 08:43
One note, you need to get the whole mass well above 200C for thorough dehydration.
220C will do but it has to be above 200C.
My method is bake in the oven at the highest possible temperature which for
many ovens is 230C (450F). Baking for 4 or 5 hours with hourly stirring usually gives
a nice dry product.AWLB - 27-7-2015 at 11:33
Thank you very much for commenting "macckone", I will accordingly alter my method to heating to 230 degrees after the initial dehydration at 150
degrees celsius, my temperatures were not accurately measured, and of course using only a small mass of hydrated magnesium sulphate means heating for
4 to 5 hours is not required. Perhaps I only succeeded in dehydrating it to the mono-hydrate, but the product is a good drying agent nevertheless.
Also have you noted the strange properties of anhydrous MgSO4 when adding to water (this is of course endothermic); it seems to take around 5
minutes, with stirring, to dissolve only 1g of the product in 100ml of 20C water? Does the structure of anhydrous MgSO4 mean water is incorporated
into its structure less easily perhaps?
Thanks for posting and thanks for the tip. careysub - 27-7-2015 at 17:29
You can avoid the formation of "cement" in oven drying by drying in stages at progressively higher temperatures. I don't have the drying "schedule" I
worked out handy, but (using the Fahrenheit temps U.S. oven settings provide) it is something like going up in steps from 200 F to 500 F in 100 F
increments (this gives 4 steps, but you can do it in three). You can decide when it is done at each temperature by finding when the weight has
stabilizedAWLB - 28-7-2015 at 06:47
Thanks for commenting, I also did notice a slower heating process meant the formation of the "cement" was avoidable, as did using smaller quantities
of MgSO4.7H2O.
The idea of weighing at different stages of heating (by heating in increments) is very good, because it would allow one to evaluate at which
temperature full dehydration occurs.
If you are drying one mole of hydrated magnesium sulphate(see heptahydrate)your final mass should be approximately 140g.
