Required Practical 9

• Describe how to calibrate a pH probe

  1. Rinse the pH probe thoroughly with deionised (distilled) water and shake gently to remove excess water.

  2. Place the probe in the standard pH 7.00 buffer solution provided. Record the pH reading.

  3. Repeat this process using the standard pH 4.00 and pH 9.20 buffer solutions. Record the pH readings.

  4. Plot a graph of your recorded pH reading (x-axis) against the pH of the buffer solution (y-axis).
• Describe how to measure pH

  1. Rinse a burette with 0.1 mol dm -3 solution of ethanol acid and the fill the burette with this solution. Label this burette

  2. Use the burette to transfer exactly 20.0 cm3​ ​ of ethanoic acid to a clean 100 cm3​ beaker.

  3. Rinse a second burette with 0.100 mol dm-​3​ NaOH solution and then fill this second burette with this solution. Label this burette.

  4. Rinse the pH probe with distilled (deionised) water and clamp it so that its bulb is fully immersed in the ethanoic acid solution in the beaker.

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

  6. Add 2.0 cm3​ ​ of NaOH solution from the burette at a time. Stir the solution and record the pH alongside the volume of NaOH added.

  7. When the end-point is being reached, add 0.20 cm3​ ​ of NaOH each time. Stir the solution and record the pH alongside the volume of NaOH added.

  8. After this, continue adding 2.0 cm3​ ​ NaOH until it is in excess. Stir the solution and record the pH alongside the volume of NaOH added.
• How do you analyse the data produced?

  ● Use the pH probe calibration graph to adjust the pH readings obtained in the experiment. These corrected pH values should be entered into a new column in the table of results.

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

  ● Join the points in the most appropriate way (should be a curve).
• Why are acid-base indicators used?

  To detect when a reaction reaches completion/becomes neutral, usually by the presence of a colour change.
• Why does a pH meter need to be calibrated?

  So that the pH values for each pH reading are accurate.
• Why is it important to stir the resulting solution?

  So all acid and base is thoroughly mixed, so the pH doesn't significantly vary throughout the solution as this would give an inaccurate reading.