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

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

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.

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

(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.