pH – Titration Curves (OCR A Level Chemistry A): Revision Note
Exam code: H432
PAG 11.2: pH – Titration curves
pH or titration curves show how pH changes during neutralisation reactions between acids and bases
They are obtained by:
Reacting known concentrations of acid with alkalis
Measuring the pH during the neutralisation
This experiment uses 1.0 mol dm-3 solutions of:
hydrochloric acid
ethanoic acid
sodium hydroxide
ammonia solution
The experiment requires a calibrated pH probe
Wash the pH probe with distilled water
Place the tip of the probe in pH 4 buffer solution
Adjust the pH probe until the reading is 4.0
Wash the pH probe with distilled water
Place the tip of the probe in pH 9 buffer solution
If is it working correctly it should read 9.0
This is known as a two-point calibration
Method
Measure 25 cm3 of ethanoic or hydrochloric acid using a volumetric pipette
Transfer it into a beaker
Measure and record the starting pH
Fill a burette with either sodium hydroxide or ammonia solution
Add 5 cm3 portions of the base to the acid
Measure and record the pH after each addition
Continue until you have added a total of 50 cm3
Repeat the process for all four acid–base combinations
Results
The results below show how pH changes as different bases are added to acids
They provide a clear comparison between strong and weak acid–base combinations
Specimen NaOH titration curve results:
Volume of NaOH / cm3 | HCl (pH) | CH3COOH (pH) |
|---|---|---|
0 | 2.0 | 3.3 |
5 | 2.1 | 4.3 |
10 | 2.2 | 4.7 |
15 | 2.3 | 5.0 |
20 | 2.6 | 5.4 |
25 | 11.2 | 6.0 |
30 | 11.4 | 11.5 |
35 | 11.8 | 11.5 |
40 | 11.8 | 11.8 |
45 | 11.9 | 11.9 |
50 | 12.0 | 12.0 |
Specimen NH3 titration curve results:
Volume of NH3 (aq) / cm3 | HCl (pH) | CH3COOH (pH) |
|---|---|---|
0 | 2.0 | 3.3 |
5 | 2.2 | 4.4 |
10 | 2.3 | 4.8 |
15 | 2.4 | 5.0 |
20 | 2.6 | 5.4 |
25 | 7.0 | 5.8 |
30 | 8.6 | 7.9 |
35 | 9.0 | 8.9 |
40 | 9.2 | 9.2 |
45 | 9.3 | 9.2 |
50 | 9.3 | 9.3 |
Analysis
Use the tables above to plot pH curves for each acid–base combination
Plot pH on the y-axis and volume of base added on the x-axis
These graphs show the pH changes and help you pinpoint the equivalence point
The four typical titration curves appear below
To improve the curve’s detail, add smaller portions of alkali near the steep section of the curve (equivalence point)
Examiner Tips and Tricks
You should be able to:
Recognise the shapes of all four titration curves
Identify which acids and bases are strong or weak
Locate the equivalence point on a pH curve
You should also understand the difference between:
End point – the volume at which the indicator changes colour
Equivalence point – the point where stoichiometrically equal amounts of acid and alkali have reacted
This lies at the centre of the steep vertical section on a pH curve
Practical skills reminder
This practical develops key skills in measuring and analysing neutralisation reactions
It also supports:
Using a volumetric pipette, burette, and pH probe accurately, including calibration
Recording precise pH values at regular volume intervals to construct a titration curve
Analysing pH curves to identify equivalence points and compare strong/weak acids and bases
Understanding the link between experimental design and the resolution of data around the vertical section
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