This question about solubility between Cu2+ and Zn2+

oktriTue, 03/07/2017 - 00:15

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.

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%

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%