The Common Ion Effect uses the Principle of le Chatelier to shift an equilibrium to reduce the concentration of an ion.
In the Aluminum & Barium Groups, we wish to keep the [OH-1] ion low so that Mg(OH)2 will NOT form here. (Who wants milk of magnesia in her solution?!).
Remember that [OH-] is read, "the concentration of the hydroxide ion". It's in M, moles / liter.
Because the other ions have a low Ksp (solubility) they will still precipitate even in low [OH-].
The common ion is NH4+1.
THE GROUP REAGENTS--
NH4OH to supply OH-1 to ppt. the group hydroxides.
(NH4)2S to supply S-2 to ppt. the group sulfides.
NH4Cl to prevent the precipitation. of Mg(OH)2 by
THE COMMON ION EFFECT AH, The Principle of le Chatelier.
We do not want any Mg(OH)2 forming here. It won't form unless [OH-1 ] is high,
Mg+2 + 2OH-1 ---> Mg(OH)2
The OH-1 comes from
NH4OH <----> NH4+1 + OH-1
which is added to precipitate the other hydroxides in the group.
So we add NH4Cl to increase the [ NH4+1 ]
NH4Cl ----> NH4+1 + Cl-1
Hence: the NH4+1 (The Common Ion) shifts the equilibrium of the NH4OH equation to the left decreasing the [ OH- ] so that Mg(OH)2 can't form.
It is evident that if the concentration of NH4+ ions is increased by addition of the soluble salt NH4Cl, the equilibrium point will be shifted to the left. As a result of this equilibrium shift the concentration of NH40H molecules will be increased while the concentration of OH- ions will be decreased. When the concentration of NH4+1 ions increases the concentration of OH- ions decreases and vice versa.