This question about solubility between Cu2+ and Zn2+

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There are solution containing Cu2+ 0.01 M and Zn2+ 0.01 M. We would like to precipitate Cu2+ almost 99.9% using H2S. So, how minimum pH used to precipate it so there will be separation between Cu2+ and Zn2+. Thank you

You haven't said how much H2S to use.  I am thinking you mean some amount of H2S and then add a strong base to adjust pH?  Or do you mean to ask what amount of H2S to add (without adding strong base to adjust pH)?  Sorry I am just confused a bit.  But here is what I have thought of so far.

http://bilbo.chm.uri.edu/CHM112/tables/KspTable.htm

 

found Ksp values above.

 

CuS + H2O <—-> Cu2+ + OH- + HS-

 

Ksp = 6. x10 ^-37 = [Cu2+] x [OH-] x [HS-]

 

ZnS + H2O <—> Zn2+ + OH- + HS-

 

Ksp = 2 x 10 ^-25 = [Zn2+] [OH-] [HS-]

 

And my problem is that if it is an arbitrary amount H2S and ALSO an arbitrary amount of strong base to adjust pH that with 2 arbitrary amounts it is hard to arrive at a direction (for me) to solving.  I am tempted to just propose some amounts H2S and pH and see if I stab at one that precipitates the copper out to 99.9%