Topic B. Factors Affecting Solubility
Le Châtelier's principle governs
the response of a system in equilibrium when a stress is introduced. Some “stressors”
that affect solubility equilibria are presence of common ion, pH
and presence of a complexing agent.
Subtopic B.3. Effect of pH
Quantitative Separation of Metal Sulfides
By
controlling pH or [H3O+], selective separation of metal
ions using saturated H2S can be performed. Precipitates containing
sulfide ion have very low Ksp values (10-10 to 10-90).
In order to say that precipitation has completed, the amount of solute
remaining in the solution should be very small (99.9% or more) leaving less
than 0.1% (1/1000) of the ion in solution.
Required Reading:
“The Effects of pH on Solubility”
Read on section, Selective Precipitation Using pH. This portion of the online resource shows that cadmium (II) and zinc (II) are separated using selective precipitation with sulfide. Click the title start reading the resource.
Supplementary Reading:
“Chapter 11C. Separation of ions by control of the concentration of the precipitating agent” (page 268-272 of Skoog et al., 2013)
Note in Example 11-11 how cadmium (II) and thallium (I) were separated in the solution.