Investigating pH changes (Oxford AQA International A Level Chemistry)

Required Practical 6: Investigating pH changes

Objective

To investigate how the pH of a solution of ethanoic acid changes as sodium hydroxide solution is added

Apparatus

• two 50 cm³ burettes

• two funnels

• 100 cm³ beaker

• deionised (or distilled) water in a wash bottle

• stand and clamp

• pH meter/probe

• 0.100 mol dm^–3 sodium hydroxide solution

• 0.100 mol dm^–3 ethanoic acid solution

• standard pH buffer solutions at pH 4.00, 7.00 and 9.20

• stirring rod

• graph paper.

Method

Part 1: Calibrate the pH meter

1. Rinse the pH probe thoroughly with deionised water, and shake it gently to remove excess water

2. Place the probe in the standard pH 7.00 buffer solution provided, ensuring that the bulb is fully immersed

3. Record the pH reading in a suitable table.

4. Repeat this process using the standard pH 4.00 and 9.20 buffer solutions.

5. Rinse the pH probe thoroughly with deionised water before taking each reading.

6. Record the pH readings in your table.

7. Plot a graph of your recorded pH reading (x-axis) against the pH of the buffer solution (y-axis).

Part 2: The measurement of the pH of the mixture of sodium hydroxide and ethanoic acid

1. Use one burette to transfer exactly 20.0 cm³ of ethanoic acid to a clean 100 cm³ beaker

2. Rinse and fill a second burette with NaOH solution

3. Clamp the pH probe so that its bulb is fully immersed in the ethanoic acid solution in the beaker.

4. Use a rod to stir the solution gently and record the pH reading in a suitable table

5. Add 2.0 cm³ of the NaOH solution from the burette to the ethanoic acid. Stir the solution and record its pH

6. Continue to add NaOH solution in 2.0 cm³ portions and record the pH until 18 cm³ has been added

7. As the end point is reached, add NaOH solution in 0.20 cm³ portions until 22.0 cm³ is reached

8. Then add the NaOH solution in 2.0 cm³ portions again until 40 cm³ have been added

How to set up your equipment

Practical Tip

• You may need to wait a few seconds until the pH stabilises before you take a reading

• Make sure the stirrer is not moving too fast as a rapid spin generates bubbles in the solution which gives an unsteady pH reading

Results

Record your results in a table like this:

Evaluation

1. Use the pH probe calibration graph you obtained from part 1 to adjust the pH readings obtained in the experiment

2. Enter these pH values into a new column in the table of results

3. Plot a graph of the corrected pH values (y-axis) against volume of sodium hydroxide solution added (x-axis)

4. Join the points in the most appropriate way and comment on the shape of the curve