[mehc]

I want to convert magnesium sulphate heptahydrate to anhydrous form. According to literature, it requires heating for one hour at 250 ^oC.
But the melting point of heptahydrate form is 150^o C. Will not it melt when heated at 250^oC???

Thanks

[Borek]

My bet is that if you will heat it slowly before you will get to the melting point it won't be heptahydrate any longer.

[OC pro]

Who told you that the melting point is only about 150°C? That is complete nonsense. I mean it is an inorganic salt. These compounds have in general high melting points.
The melting point of magnesium sulfate heptahydrate is in fact 1124°C!

I would heat to 150°C under vacuum to remove the water.

[Borek]

This is not a complete nonsense. Some of highly hydrated salts, if heated in closed vessel so that they are not allowed to dry, simply dissolve in their own hydration water - which is technically indistinguishable from melting.

1124 is a melting point of anhydrous salt, which lost its hydration water during heating - so it is hard to classify as melting point of heptahydrate.

[mehc]

Ok, I'll try to do it near 100⁰C for longer period. But will there be any surety that heptahydrate form has completely converted to anhydrous form???
I mean can we test it in any way?

[Borek]

What's the problem with the salt "melting" and then recrystallizing while losing water? I would not worry. Besides, could be it will lose water fast enough so that you will be not able to observe the liquid phase.

[mehc]

Yes, OC pro, 1124^oC is the melting point of anhydrous form.

Thanks a lot to both of you for your help.

[OC pro]

I have dried salts like lithium chloride and other stuff for use in moisture sensitive reactions very often. Simple heating to 100°C will effect nothing. You have to apply vacuum as I have mentioned above.