Calcium Sulfate question

[speakerguy]

Hey Randy,

When I am precipitating CaSO4 in a relatively large volume of water, will I get the annydrous, dihydrate, or hemihydrate form of the CaSO4? I thinking it will be the dihydrate but I want to make sure before I pick which Ksp to use. Also, are there any other hydrated forms I should be worried about forming? Thanks!

PS - I am trying to make a magnesium supplement for folks who can't get their hands on the dead sea works magnesium by adding CaCl2 to epsom salts to precipitate out as much CaSO4 as possilbe and keep MgCl2 in solution.

 

[speakerguy]

Also I am looking at concentrations around ~1M for all species upon initial mixing before any precipitation occurs if that changes things.

 

[speakerguy]

Also, I know magnesium interferes with the precipitation of calcium carbonate. Am I going to run into anything like that with this, or can I simply work the precipitation problem like the only two ions in solution are Ca++ and SO4--? Thanks!

 

[Randy Holmes-Farley]

I do not know if magnesium slows calcium sulfate precipitation the way it does for calcium carbonate, but I would guess that you'll be so far above saturation that the calcium sulfate will precipitate rapidly anyway.

I don't know what hydration form will result, but I would guess anhydrous calcium sulfate (gypsum). That is what forms as seawater is dehydrated.

 

[Habib]

It precipitates and might require some time (say hours) depending on how it is added (fast or slow).

FWIW, that personal experience is for using concentrated solutions of MgSO4 and CaCl2.

 

[Habib]

I should also add that precipitation rate might also depend on how much is added and never went as far as what you intend to do. It might get slower as the amount of sulfate decreases.