[Platinum]

Excuse me if this question is lack of knowledge, but I have seen cases where a salt reverses back from sultafe to chloride. According to what I can see in this reaction, HCl gas is produced and won't remain (long time) in solution - but I'm still not sure if I end up with ONLY MgSO4. If I add an great excess of H2SO4, I maybe face som problems to get rid of this excess.

So, to the real question: Will this reaction produce only MgSO4? If I use great excess H2SO4 in aq-solution, should I go to pH 3, then dry away the water until only oily (from H2SO4) MgSO4 remains, then wash this several times with EtOH/IPA?

[AWK]

Compare boiling points of HCl and H2SO4 solution. What happens during boiling this mixture?

[Platinum]

Compare boiling points of HCl and H2SO4 solution. What happens during boiling this mixture?

Of course I know this, but maybe the HCl reverse to MgSO4 before it evaporate - even though it has a very low boiling point. Like if I would have NH3 in H2O solution, NH3 would remain there for a significant time - even though NH3 boils at -33 C. I coud just measure the time the H2O smells ammonia, and realize that the very last part of NH3 remains surprisingly long.

[Arkcon]

You're missing something here.  You know that excess H₂SO₄ remains after water evaporates.  But what if there  is excess HCl, or like your example, NH₃, when the water evaporates?

[Platinum]

You're missing something here.  You know that excess H₂SO₄ remains after water evaporates.  But what if there  is excess HCl, or like your example, NH₃, when the water evaporates?

Probably not. I can just guessing, that it's very likely that when the water has evaporated, all HCl is gone. The important thing is that ALL the chloride converts to sulfate. And I believe it does if H2SO4 is in excess, because however long HCl will remain in the water, H2SO4 will remain much longer.

Still, I want to be sure that I end up with ONLY MgSO4, not a trace of chloride. Please tell me how accurate this suggested method of preparing MgSO4 is, when it has to be free from chloride which in case will poison later catalyst.

I can keep on playing that I know for sure that I get rid of all chloride, because everything speaks for that. But I have so many surprises in chemistry, sometimes out of logic.

[AWK]

For absolutely pure MgSO₄ use MgO or MgCO3 for reaction. Both are cheap and easy accessible. Very pure MgSO4 can be buy at any drugstore without prescription.

[Arkcon]

Let me offer you some more useful tips for pure MgSO₄ ...

1).  Purchase it from a chemical dealer.  This isn't a controlled substance, so if you have a reasonable need for it, you can purchase it.

2).  Use a different starting material.  The insoluble MgO can be added to excess HCl and filtered from the solubles.

3).  Recrystallization is a common way to purify a slightly impure solid.

[Platinum]

For absolutely pure MgSO₄ use MgO or MgCO3 for reaction. Both are cheap and easy accessible. Very pure MgSO4 can be buy at any drugstore without prescription.

Ok, it was as I believed difficult to ensure chloride-free sulfate from MgCl2. I know I can buy here MgSO4 at the drugstore, but it's very expensive (100 g more than 20 USD). I actually have Mg(OH)2, made from MgCl2 recently. The idea was to go that way to ensure chloride-free sulfate. This Mg(OH)2 is now in water suspension, and I will was it several times with distilled water. Then I was thinking of adding some H2SO4 to this later, and wash the solids with EtOH.

Is the route through Mg(OH)2 as good as MgO and MgCO3 (MgCO3 is here very cheap, MgO very expensive).

However, thanks for the answers.

[Platinum]

Let me offer you some more useful tips for pure MgSO₄ ...

3).  Recrystallization is a common way to purify a slightly impure solid.

Recrystallization was my thought also later, what solution is appropriate for MgSO4? Slightly soluble in EtOH, which means 1 to 10 grams per liter (if I remember correct). Saturated solution from cooled water?

[Arkcon]

Saturated solution from cooled water?

This is the type of recrystallization I meant.  This is why pure reagents are expensive -- you take some, apply a simple physical situation, and discard much of what you want, and most of what you don't, to have a small amount of very pure substance.  Is there a trace of chloride in even the very purest MgSO₄?  Very likely.  If you spent tons of effort, would there still be one chloride atom per kilo?  That is also likely, given that much of the Earth's surface is an aqueous chloride solution.  Also, human body fluids are chloride rich, so don't permit anyone to sneeze.  The real question is, what purity do you need for your application.

[Platinum]

Saturated solution from cooled water?

This is the type of recrystallization I meant.  This is why pure reagents are expensive -- you take some, apply a simple physical situation, and discard much of what you want, and most of what you don't, to have a small amount of very pure substance.  Is there a trace of chloride in even the very purest MgSO₄?  Very likely.  If you spent tons of effort, would there still be one chloride atom per kilo?  That is also likely, given that much of the Earth's surface is an aqueous chloride solution.  Also, human body fluids are chloride rich, so don't permit anyone to sneeze.  The real question is, what purity do you need for your application.

Ok, I don't need more chloride-free than lab-grade. I have realized that many chemicals that are unpure are dirt cheap, because the cost seems to be the purification in many cases.

Lucky for me that I have ridiculous cheap MgCl2 (25 kg for 15 USD) so I can process and be satisfied to have 10% left, or less.