Titration Curves (AQA A-Level Chemistry): Revision Notes

8.1.9 Titration Curves

Objective:

To obtain titration curves by measuring the pH during the neutralisation of an acid by an alkali. The aim is to understand how pH changes throughout the course of a titration and to observe the characteristic titration curves for different combinations of acids and alkalis.

Apparatus & Chemicals:

• 1.0 mol dm⁻³ solutions of:
  • Hydrochloric acid ($HCl$)
  • Ethanoic acid ($CH₃COOH$)
  • Sodium hydroxide ($NaOH$)
  • Ammonia solution ($NH₃$)
• pH metre or calibrated pH probe
• Burette
• Volumetric pipette (25 cm³)
• Beakers
• Distilled water

Key Theory:

Titration curves represent the pH changes when acids and bases react during titration. The shape of the curve depends on the strengths of the acid and the base. Strong acid-strong base titrations have a steep pH rise at the equivalence point, while weak acid-strong base titrations exhibit a more gradual rise.

At the equivalence point, the amount of acid equals the amount of base, leading to complete neutralisation. This is visually represented on the titration curve by a sharp change in pH.

Procedure:

1. pH Metre Calibration:

• Begin by calibrating the pH probe using standard buffer solutions.
• Perform a two-point calibration: Place the pH probe in a buffer solution of pH 4, adjust the metre to read 4.0, then rinse the probe with distilled water and place it in a buffer of pH 9.
• Ensure the metre reads 9.0.

2. Preparation of Acid Solutions:

• Using a volumetric pipette, measure 25 cm³ of either hydrochloric acid or ethanoic acid and transfer it into a clean beaker.
• Place the calibrated pH probe in the solution and record the initial pH.

3. Titration with Alkali:

• Fill a burette with 1.0 mol dm⁻³ sodium hydroxide (or ammonia solution) and note the starting volume.
• Add the alkali to the acid solution in 5 cm³ increments.
• After each addition, gently stir the solution (or use a magnetic stirrer) and record the pH.
• Continue adding alkali until a total of 50 cm³ has been added, ensuring to measure the pH after each addition.

4. Repeat the Process for All Acid-Alkali Combinations:

• Perform the titration four times to obtain curves for:
• Hydrochloric acid with sodium hydroxide ($HCl$+ $NaOH$)
• Ethanoic acid with sodium hydroxide ($CH₃COOH$ + $NaOH$)
• Hydrochloric acid with ammonia solution ($HCl$+ $NH₃$)
• Ethanoic acid with ammonia solution ($CH₃COOH$ + $NH₃$)

5. Data Collection:

• Record the pH values after each 5 cm³ addition of alkali, and gather enough data points to ensure a smooth titration curve, especially around the equivalence point, where pH changes rapidly.

Specimen Results:

Titration with Sodium Hydroxide:

Volume of $NaOH$ (cm³)	pH ($HCl$)	pH ($CH₃COOH$)
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	11.7	11.5
35	11.8	11.5
40	11.8	11.8
45	11.9	11.9
50	12.0	12.0

Titration with Ammonia Solution:

Volume of $NH₃$ (cm³)	pH ($HCl$)	pH ($CH₃COOH$)
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:

Titration Curve Characteristics:

• The equivalence point occurs when the number of moles of acid equals the number of moles of base, indicated by a steep rise in pH for strong acid-strong base titrations (e.g. $HCl$ + $NaOH$).
• In weak acid-strong base titrations (e.g., $CH₃COOH$ + $NaOH$), the pH increases more gradually around the equivalence point due to the formation of a buffer solution.
• For strong acid-weak base titrations (e.g., $HCl$ + $NH₃$), the curve levels off at a lower pH after the equivalence point due to the weak base not fully neutralising the acid.
• Weak acid-weak base titrations (e.g., $CH₃COOH$ + $NH₃$) exhibit a gentle slope throughout, with no sharp pH changes, and have an equivalence point at a pH below 7.

Importance of Data Points:

• The more data points collected (especially near the equivalence point), the more accurate the titration curve will be.
• This is particularly important in weak acid-strong base titrations, where pH changes occur more slowly.

Understanding the Curves:

• Strong acid-strong base ($HCl$+ $NaOH$): A sharp rise in pH around equivalence, indicating complete neutralisation.
• Weak acid-strong base ($CH₃COOH$ + $NaOH$): A more gradual pH change due to buffer formation, with an equivalence point at pH > 7.
• Strong acid-weak base ($HCl$+ $NH₃$): The pH rises, but not as sharply, and the final pH remains lower.
• Weak acid-weak base ($CH₃COOH$ + $NH₃$): A very gradual change in pH, with no sharp jump.

Conclusion:

By performing these titrations and plotting the pH against the volume of alkali added, it is possible to observe the distinct pH changes for different combinations of acids and bases.

• Strong acids and bases produce a sharp equivalence point, while weak acids or bases exhibit more gradual pH changes due to buffering effects. Understanding these curves is essential for predicting the behaviour of acid-base reactions, especially when determining the pKa of a weak acid or designing buffer systems.

Practical Tips for Success:

• Accurate Calibration of the pH metre is crucial for obtaining reliable data. Always recalibrate between experiments if necessary.
• Stirring the solution consistently ensures an even mixing of the acid and alkali, providing more accurate pH readings.
• Careful Addition of Alkali near the equivalence point will provide more data points, improving the accuracy of the titration curve, especially in weak acid-strong base systems.