1. Experiment 7 Preparation and Properties of Buffers
Purposes
1. Grasp the properties of buffer solutions and effect factors of buffer capacity
2. Learn the preparation of buffer solution and operation of the measuring pipet
3. Learn to determine the pH with colorimetry and pH-meter

2. change in pH when a small amount of acid or base is
Principles
Buffer solution is defined as a solution that resists a
change in pH when a small amount of acid or base is
added to it or when the solution is diluted. The buffer
solution consists of a weak acid (HB) and its conjugate
base (B-).The pH value of the buffer solution can be
calculated by the following equation:
Ka is the acid ionization constant
From the equation above we can see that the pH value of
the buffer solution depends upon the value of the ionization
constant of conjugate acid and the concentration ratio of
the conjugate acid to its conjugate base at equilibrium

3. When the buffer solution is prepared by mixing
equal concentrations of weak acid and its conjugate
base, the equation can be expressed as

4. Buffer capacity is a quantitative measure of the
ability of the buffer solution to resist changes in pH,
it depends on concentrations and the ratio of [B-
]/[HB]

5. pH value of a sample solution can be measured by
pH-meter. There are two electrodes on a pH-meter,
one is reference electrode which electrode potential
is a constant in some conditions; Another electrode
is sensitive to [H+]. When they are immersed in a
solution, the potential difference between the two
electrodes is related to pH of the solution. The pH
meter is ordinarily used when an accurate
determination of pH is needed.

6. 3. Procedure (1) Preparation of a buffer solution
Calculate the volumes of 0.1mol·L-1 HAc solution and
0.1mol·L-1 NaAc solution (pKa=4.75) required for preparing
30mL of a pH=4 buffer solution.
According to the results of calculation, add the HAc
and NaAc solution with buret into a small beaker, mix the
solution well. Measure its pH value with pH indicator paper.

7. (2) Properties of buffer solutions
Measure out the solutions according to Table, determine its pH value after adding acid, base and distilled water, account for the reasons.

8. 0.1mol·L-1 HCl 4 drops 0.1mol·L-1 NaOH 4 drops distilled water 5 mL
adding
reagent
the
Universal
pH indicator
0.1mol·L-1 HCl 4 drops
0.1mol·L-1 NaOH
4 drops
distilled water
5 mL
1# Buffer solution pH=4
HCl pH=4
Explaination

9. (3) Buffer capacity The relationship between buffer capacity
　 and the ratio
According to Table, preparation of 3# and 4# buffer solution
with 0.1 mol·L-1 HAc and 0.1 mol·L-1 NaAc solution into small
beaker respectively. Measure its pH value by pH meter.Record
the data. Add 2.00mL of 0.1 mol·L-1 NaOH solution, mix the
solutions thoroughly. Measure the pH of the solution again.
Explain.