Required Practical 9 (AQA A Level Chemistry)

Required Practical 9: Investigating pH changes

The experimental set up for measuring pH changes in titrations

Steps in the procedure

• Measure out 50 cm³ of 1.0 mol dm^-3 ethanoic acid using a measuring cylinder and transfer it to a 200 cm³ beaker
• Place the beaker onto a magnetic stirrer and add a magnetic stirrer bar
• Position the pH probe so that it does not interfere with the movement of the stirrer bar
• Take an initial pH reading
• Fill the burette with 1.0 mol dm^-3 sodium hydroxide
• Add 2 cm³ of sodium hydroxide from the burette into the beaker and take the pH reading
• Continue adding 2 cm³ portions until you get to 20 cm³ then switch to 1 cm³ portions
• The pH begins to rise rapidly- when it begins to slow down then you can switch back to 2 cm³ portions
• Plot a graph of pH versus volume of base added
• The procedure can then be repeated with 1.0 mol dm^-3 hydrochloric acid and 1.0 mol dm^-3 ammonia solution

Practical tips

• 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

Specimen Results

• The pH curves for a weak acid with a strong base, and a strong acid with a weak base are shown below:

pH curves from weak acid + strong base, and strong acid + weak base

Analysis

• The pH curves show a characteristic s-shape curve and the midpoint of the inflection is called the equivalence or stochiometric point
• From the curves you can
  • Determine the pH of the acid by looking where the curve starts on the y-axis
  • Find the pH at the equivalence point
  • Find the volume of base at the equivalence point
  • Obtain the range of pH at the vertical section of the curve