PAG 11.2: pH – Titration Curves
- 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
Examiner Tip
You need to:
- Be familiar with the shapes of the four titration curves
- Be able to identify the type of acid and base (weak/strong)
- Know where to locate the equivalence point
You also need to know the difference between the ‘end point’ and the ‘equivalence point’
- End point = The point at which the indicator changes colour
- Equivalence point = The point at which stoichiometric (equal) amounts of acid and alkali have been added
- This is found at the midpoint of the vertical section of a pH curve
