Its importance increases after the equivalence point is passed in titration, and if present during neutralization, it counteracts the change in the value modeled by the central ratio of the Henderson-Hasselbach equation, functioning as a buffer.  
In nucleophilic aliphatic substitution reactions, this mechanism depresses the reaction rate by making the first step of SN-1 reactions reversible, and it slows DDT solvolysis in saline solution.  Its importance increases after the equivalence point is passed in titration, and if present during neutralization, it counteracts the change in the value modeled by the central ratio of the Henderson-Hasselbach equation, functioning as a buffer.  An application of Le Chatelier's principle, FTP, name this effect that alters the molar solubility of a compound by addition of species identical to a pre-existing product of dissolution.
In nucleophilic aliphatic substitution reactions, this mechanism depresses the reaction rate by making the first step of SN-1 reactions reversible, and it slows DDT solvolysis in saline solution.  
the common ion effect