Precipitating a Compound from a Solution

[dubois928]

When you want to precipitate a compound from a solution, say from an experiment where magnesium chloride is obtained from the reaction of hydrochloric acid and magnesium metal, do you let the solution evaporate completely or do you wait until a significant amount of crystals have formed and then filter it?

 

Thanks,

Ken

[YT2095]

this isn`t technically a PPT reaction, it`s simple crystalisation by evaporation of the solvent to leave the solute (MgCl2 in this case).

 

if the PH of your soln is neutral or slightly on the acid side you can pour this soln out onto a flat glass evaporating dish and put it in a warm place (on top of a radiator for instance) crystals will form as the solvent evaporates, and excess HCl will be driven off also.

although if Memory serves I think MgCl2 is very hygroscopic so you may have a few problems trying to dry it enough as it will rip moisture from the air.

I know Calcium Chloride does this.

[Bluenoise]

The MgCl2 wont dry completely this way, well maybe if your humidity is close to 0%. It will likely not dry past a hexahydrate state (6 water molecules per MgCl2).

When I typically buy MgCl2 this is the state that I get it in. At this level of hydration the mass per unit of magnesium is the most stable I beleive, due to being less suseptable to change in water content under typical conditions.

 

You can probably get it alot dryer by baking it 100C for an hour or two after evaporation. (Don't take my word on it though, inorganic chem has never been my thing)

 

You might want to try Ethanol precipitation as well. However the sucess of this will be dependant on the concentration of MgCl2 and how high you can get the ethanol content. (Also how cold you freezer is too).

[dubois928]

Thanks guys. The MgCl2 scenario was just an example really. Though out of curiosity what would be the ratio of Magnesium metal to Hydrochloric acid?

 

Ken

[YT2095]

1:2 Mg to HCl (by molar mass).

 

1Mg + 2HCl = MgCl2 + H2