The pH of a buffer solution is determined by the equilibrium between the acid and its conjugate base.
The equilibrium for the reaction of acetic acid with water is:
CH3COONa + H2O <==> CH3COONa + H3O+
By definition, a buffer has a pH of 7. A buffer will not change the pH at which it is buffering until all of the buffer has been consumed; that is, until all of the acid has reacted with its conjugate base.
To make a buffer solution from acetic acid, we need to react acetic acid with water until enough has been consumed so that there is no more free CH3COOH remaining in solution. This means adding enough water so that there isn’t any more CH3COOH remaining in solution after mixing completely.
Last modified: September 11, 2022