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.
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.