17.1 The Common Ion Effect
Metal ions or salts containing a conjugate weak acid or base can shift the pH of a solution.
If we add a conjugate acid (base) to a solution of a weak base (acid), the pH will shift. This is called the common ion effect. A common ion will shift a chemical equilibrium in such a direction as to get rid of some of the added ion.
NaCH3CO2 added to a solution of CH3CO2H will make the solution more basic.