12.2 Acid-Base Titrations Revision Notes

1. Overview

An acid-base titration is a fundamental laboratory technique used to determine the exact volume of one solution needed to react completely with a known volume of another solution. This process is essential for calculating the unknown concentration of an acid or an alkali through a neutralisation reaction.

Key Definitions

• Titration: A method of quantitative chemical analysis used to determine the concentration of an unknown solution.
• End-point: The point during a titration when the indicator changes colour, signaling that the reaction is complete.
• Indicator: A chemical that changes colour depending on the pH, used to identify the end-point.
• Burette: A long, graduated glass tube with a tap at the bottom used to deliver known, variable volumes of a liquid.
• Pipette: A piece of glassware used to measure and transfer a highly accurate, fixed volume of a liquid.
• Titre: The volume of solution added from the burette during a titration.
• Concordant results: Titres that are within 0.10 cm³ of each other.

Core Content

The Apparatus

To perform a titration, specific glassware is required to ensure high precision:

• Volumetric Pipette: Used to measure a fixed volume (usually 25.0 cm³) of the base/acid into the conical flask.
• Burette: Filled with the other reactant; it allows for the controlled addition of liquid drop-by-drop.
• Conical Flask: Used to hold the reaction mixture because its shape allows for easy swirling without splashing.
• White Tile: Placed under the flask to make the indicator colour change easier to see.

The Procedure

1. Use a pipette and pipette filler to measure exactly 25.0 cm³ of alkali into a clean conical flask.
2. Add a few drops of a suitable indicator (e.g., phenolphthalein) to the flask.
3. Fill a burette with acid and record the initial volume reading.
4. Slowly add the acid from the burette to the flask, swirling the flask constantly.
5. As the end-point approaches, add the acid drop-by-drop until the indicator undergoes a permanent colour change.
6. Record the final volume on the burette and calculate the titre (Final volume - Initial volume).
7. Repeat the experiment until you achieve concordant results (two or more results within 0.10 cm³ of each other).

Identifying the End-point

Indicators are used to show when the acid has exactly neutralised the base. Common indicators used in IGCSE include:

Indicator	Colour in Acid	Colour in Alkali	End-point Colour
Methyl orange	Red	Yellow	Orange
Phenolphthalein	Colourless	Pink	Pale Pink
Litmus	Red	Blue	Purple

Neutralisation Reactions

The chemical reaction occurring in the flask is a neutralisation reaction.

Word Equation: $\text{Acid} + \text{Alkali} \rightarrow \text{Salt} + \text{Water}$

Symbol Equation (Example: Hydrochloric acid and Sodium hydroxide): $HCl(aq) + NaOH(aq) \rightarrow NaCl(aq) + H_2O(l)$

Symbol Equation (Example: Sulfuric acid and Potassium hydroxide): $H_2SO_4(aq) + 2KOH(aq) \rightarrow K_2SO_4(aq) + 2H_2O(l)$

Extended Content (Extended Only)

There is no additional supplement-specific content for the procedural description of titrations in this section. However, the calculation of concentrations using titration data is covered in Topic 3: Stoichiometry.

Key Equations

• Titre Calculation: $\text{Titre} = \text{Final Burette Reading} - \text{Initial Burette Reading}$
• Average (Mean) Titre: $\text{Mean Titre} = \frac{\text{Sum of concordant titres}}{\text{Number of concordant titres}}$

Common Mistakes to Avoid

• ❌ Wrong: Including the "rough" (first) titration when calculating the mean titre.
  • ✓ Right: Only use concordant results (those within 0.10 cm³ of each other) for the average.
• ❌ Wrong: Reading the burette from the top of the liquid curve.
  • ✓ Right: Always read the volume from the bottom of the meniscus at eye level.
• ❌ Wrong: Forgetting to remove the funnel from the top of the burette after filling.
  • ✓ Right: Always remove the funnel, as remaining drops could fall into the burette during the titration, changing the volume reading.
• ❌ Wrong: Using a beaker instead of a conical flask.
  • ✓ Right: Use a conical flask to allow for vigorous swirling without losing any of the reaction mixture.

Exam Tips

• Burette Readings: Always record burette readings to two decimal places (e.g., 22.40 cm³, not 22.4 cm³). The second decimal digit should be either a '0' or a '5'.
• Command Words: If asked to "Describe how to perform a titration," ensure you mention rinsing the equipment (burette with acid, pipette with alkali) to prevent contamination and dilution.
• Real-world contexts: Titrations are often used to find the concentration of vinegar (ethanoic acid), the purity of a sample, or the concentration of a commercial cleaning product.
• Table Formatting: In practical exams, always present your results in a clear table with "Initial", "Final", and "Titre" rows/columns, all including units (cm³).