Properties of Buffers
Introduction
Buffers resist changes in pH when acids or bases are added to them. An effective buffer
system contains significant quantities of a specific weak acid and its conjugate base. There
are two common methods used to prepared a buffer. One method is to combine
approximately equal quantities of an acid and its conjugate base. The other method is to
convert about half of the weak acid in a solution or half of the weak base in a solution to its
conjugate form by adding a strong base or strong acid respectively. This process is called
partial neutralization. The [H+] is calculated from equation (1) where HA is a weak acid and
A-is the conjugate base.
[H+] = Ka[HA]/[A–](1)
pH may be calculated from the Henderson–Hasselbalch equation. This is the logarithmic
form of equation (1). Two equivalent forms of the equation are:
(2)
In this experiment you will prepare a buffer and measure its pH with a pH-meter. The weak
acid of the buffer will be acetic acid (HC2H3O2) and its conjugate base, the acetate
ion(C2H3O2–). The buffer solution will be prepared using both methods described above.
You will measure the pH as a strong acid and base are added to the buffer solution, and
compare the results with measurements made when a strong acid and base are added to
plain water and to dilute buffer solutions.
Note: The symbol HOAc is often used to represent acetic acid and OAc–to represent the
acetate ion.
Procedure:
Special equipment:
pH-meter
magnetic stirrer and small stirring bar
two 50-mL burets
Note: If performing this experiment in the studio classroom use 25-mL burets.
