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Ion separation

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Johatan3WE:
I'm kinda desperate here, I could really use some help.

Magnesium oxalate has a Ksp of 8.6x10-5, and the calcium one has a Ksp of 2.3x 10-9. What pH must be maintained to achieve the maximum separation of the calcium ion from the magnesium ion, if both have a concentration of 0.1 mol/L, and the concentration of oxalic acid is maintained at 0.1 mol/L?

Data: For oxalic acid: Ka1 = 6.5x 10-2, Ka2 = 6.1x10 -5

Well, I have tried to calculate the molar concentration of C2O42- using the ksp equation of MgC2O4, let´s name it y.

h2c2o4--->2h+ + c2o42-   K=Ka1.Ka2
?                0          Y
-X              +2X     +X
0,1            2X        Y+X

I'm not sure if what I did gives the maximum separation, any suggestions?

Borek:
The highest separation will be when you precipitate as much calcium oxalate as possible, but without precipitating magnesium oxalate.

Johatan3WE:
 So what I did is correct? Because if the concentration of C2O4^2- is greater than 'y,' magnesium oxalate will begin to precipitate.

Borek:

--- Quote from: Johatan3WE on December 09, 2023, 05:28:54 PM --- So what I did is correct? Because if the concentration of C2O4^2- is greater than 'y,' magnesium oxalate will begin to precipitate.

--- End quote ---

That's a good starting point, but as far as I can tell your approach ignores fact some of the oxalate is protonated only partially.

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