Both Fe2+ and Cd2+ are precipitated by sulfide. However, they can be quantitatively separated through pH adjustment. You are given a solution containing 0.086 M Fe2+ and 0.062 M Cd2+. Calculate the [H+] range for a saturated H2S solution (ie, [H2S]=0.10 M) in which at least 99.99% of the least soluble ion can be precipitated while the other remains entirely in solution. The relevent equilibria are:
FeS(s) ¡ê Fe2+ + S2‾ Ksp = 4.9 10-18
CdS(s) ¡ê Cd2+ + S2‾ Ksp = 3.6 10-29
H2S(aq) ¡ê 2 H+ + S2‾ Ka12 = 1.3 10-20
What is the lower limit [H+] of the separation range at which the most soluble ion will begin to precipitate?
What is the upper limit [H+] of the separation range at which ¡Ã99.99% of the least soluble ion will remain precipitated?
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