It is to precipitate the metal ions Cd 2+ , Pb 2+ and Mn 2 +, present in industr

Question

It is to precipitate the metal ions Cd2+, Pb2+ and Mn2+, present in industrial wastewater concentration each 2.010-3M, such precipitation without affecting the Ca2+ and Mg2+ ions are in concentration 5.010-3 M.

a) is it possible to carry out the separation ?.

b) If it is possible what pH range such separation would be possible ?. Taking as a criterion for separation 2.010-6 M.

Facts: Ksp Ca(OH)2 = 1.310-6; Kps Mg(OH)2 = 8.910-12; Kps Cd(OH)2 = 5.810-15; Kps Mn(OH)2 = 1.910-13; Kps Pb(OH)2 = 1.210-15.

Explanation / Answer

a) Ksp of Cd(OH)2 , Mn(OH)2 and Pb(OH)2 is much less than the Ksp of Ca(OH)2 and also less than that of Mg(OH)2

So it is possible to precipitate the ions.

b) If the concentration of ions is assumed to be uniform, then to reach Ksp the maximum concentration of hydroxide ions is required for Mn(OH)2

Ksp = 1.910-13 = 2 X 10^-6 X [OH-] ^2

1.910-13 / 2 X 10^-6 = [OH-] ^2

[OH-] = 3.08 X 10^-4

pOH = 3.51

pH = 14-3.51 = 10.59

So at this pH all will ppt out

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