Our assumption to neglect the 2x in the original algebraic expression was valid. Also, one can quantitatively see how the solubility has decreased. Selective precipitation Many compounds, particularly the sulfides, have varying solubility. This difference in solubility has a practical application when attempting to separate metal ions in solution. It is never possible to get all of a given metal ion out of solution, however, a concentration which is extremely small is possible.
Selective precipitation is used in water treatment to remove heavy metal ions from the water supply.
A general chemistry student uses selective precipitation when carrying out the qualitative analysis scheme in the laboratory. This solution is adjusted to an appropriate pH which will precipitate one metal ion as the metal sulfide while leaving the other metal ion in solution. The pH adjustment controls the sulfide ion concentration. It will be easier to solve for the hydrogen ion concentration, if only one mass action expression were involved. Rearrange the above equation to solve for the hydrogen ion concentration.