pH – Titration Curves (OCR A Level Chemistry)

• Titration or pH curves are obtained by reacting known concentrations of acid with alkalis and measuring the pH during the neutralisation
• A suitable selection to use would be 1.0 mol dm^-3 solutions of
  • hydrochloric acid
  • ethanoic acid
  • sodium hydroxide
  • ammonia solution

• A pH probe is calibrated by placing the tip of the probe in pH 4 buffer solution and adjusting until the reading is 4.0
  • The probe is washed in distilled water and then checked against pH 9 buffer
  • If is it working correctly it should read 9.0
  • This is known as a two-point calibration

• 25 cm³ of ethanoic or hydrochloric acid is measured using a volumetric pipette and filler and then transferred into a beaker
• Its pH is measured and recorded
• The alkali (either sodium hydroxide or ammonia solution) is placed in the burette
• 5 cm³ at time is added to the beaker and the pH measured after each addition, until a total of 50 cm³ has been added
• The procedure is repeated for all four combinations of acids and alkalis

Specimen Titration Curve Results Table

 

Analysis

• The four characteristic titration curves are shown below
• The more data points that are available; the easier it is to draw the shape of the curves.
  • So, it is often better to add smaller portions nearer the equivalence point

The four characteristic acid-base titration curves