Make Your Own Darn Buffer !
In this experiment, you will be required to prepare a buffer solution that has a specific pH
value. By definition, a buffer is a solution that is resistant to changes in pH, even when small
amounts of a strong acid or base are added to the solution. This ability to maintain a constant pH
is a result of a buffer containing both components of a weak acid/base pair. When a strong acid is
added, the weak base in the buffer accepts the excess protons. When a strong base is added, the
weak acid in the buffer donates protons to neutralize the base. For this experiment, you will
prepare a buffer that contains acetic acid and its conjugate base, the acetate ions. The equilibrium
equation for the reaction is shown below:
HC2H3O2(aq) + H2O(l) <=> H+ (aq) + C2H3O2- (aq)
The equilibrium expression for this reaction, Ka, has a value of 1.8 x 10-5 at 25ºC.
Ka 
[H  ][C 2 H 3 O2  ]
Ka
[C 2 H 3 O2  ]
, where [ ] represents molarity. Re arranged,


[HC 2 H 3 O 2 ]
[H ] [HC 2 H 3 O2 ]
Since the [H  ]  10  pH ,
Ka
[C2 H 3 O2  ]

pH
[10 ] [HC 2 H 3 O2 ]
Therefore, the ratio between the molarity of the acetate ions to the molarity of the acetic acid in
your buffer must equal the ratio between the Ka value and 10- assigned pH. This ratio should be reduced
, so that either the [HC2H3O2] or [C2H3O2- ] has a concentration of 0.10 M, and the concentration
of the other component must fall within a range from 0.10 M to 1.00 M. Dimensional analysis can
be used to determine the quantities of each reactant needed to prepare 100.0 mL of your buffer
solution that has these specific concentrations. The success of your efforts will be determined by
how close the actual pH value of your buffer matches the desired pH value, and whether or not the
pH remains fairly constant when a strong acid or base is added to the buffer solution.
Procedure:
1. Obtain an assigned pH value and list of given reactants for the buffer you are to prepare.
2.
Calculate the quantities of reactants needed to prepare 100.0 mL of your buffer solution that
has a concentration of each component within the range of 0.10 M to 1.00 M. Use the lowest
possible concentrations for the acid and conjugate base in the buffer.
3.
Use volumetric pipets, a Mettler balance (+ 0.001 g), and a 100.0 mL volumetric flask to
prepare your buffer solution.
5.
Have your teacher check the pH of the buffer you prepare. Your buffer solution must match
your assigned pH value (+ 0.5), and your buffer must retain the pH upon the addition of acid
or base solution.