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Experiment 7 Preparation and Properties of Buffers

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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

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

When the buffer solution is prepared by mixing

equal concentrations of weak acid and its conjugate

base, the equation can be expressed as

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]

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.

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.

(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.

(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-1NaOH solution, mix the

solutions thoroughly. Measure the pH of the solution again.

Explain.