Required Practical: Neutralisation Titration

This lesson covers AQA Required Practical 2 (Chemistry) — carrying out a titration to determine the reacting volumes of an acid and an alkali, as specified in the AQA GCSE Combined Science Trilogy specification (8464).

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Aim

To find the exact volume of acid needed to neutralise a known volume of alkali using a titration technique with an indicator.

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Apparatus

Item	Purpose
Burette (50 cm³)	To add the acid gradually and measure the volume used precisely
Pipette (25 cm³) and pipette filler	To measure an exact volume of alkali
Conical flask (250 cm³)	To hold the alkali and indicator
White tile	To see the colour change clearly
Indicator (phenolphthalein or methyl orange)	To show the end point of the reaction
Dilute hydrochloric acid	The acid titrant
Sodium hydroxide solution	The alkali (analyte)
Clamp stand and boss	To support the burette
Funnel	To fill the burette

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Method

flowchart TD
    A["1. Rinse and fill the<br/>burette with dilute acid<br/>Record the start reading"] --> B["2. Use a pipette to<br/>transfer 25.0 cm³ of<br/>NaOH into a conical flask"]
    B --> C["3. Add 2–3 drops of<br/>indicator to the flask"]
    C --> D["4. Place the flask on<br/>a white tile under<br/>the burette"]
    D --> E["5. Add acid from the<br/>burette, swirling<br/>the flask constantly"]
    E --> F["6. Near the end point,<br/>add acid DROP BY DROP<br/>until the indicator<br/>just changes colour"]
    F --> G["7. Record the final<br/>burette reading and<br/>calculate the titre"]
    G --> H["8. Repeat until you<br/>obtain concordant<br/>results (within 0.10 cm³)"]

    style A fill:#1565c0,color:#fff
    style C fill:#7b1fa2,color:#fff
    style F fill:#d32f2f,color:#fff
    style H fill:#2e7d32,color:#fff

Step-by-Step Detail

1. Fill the burette: Rinse the burette with the acid, then fill it using a funnel. Remove the funnel after filling. Record the start reading to the nearest 0.05 cm³.

2. Measure the alkali: Use a pipette (not a measuring cylinder) to transfer exactly 25.0 cm³ of sodium hydroxide solution into a conical flask.

3. Add indicator: Add 2–3 drops of indicator (e.g. phenolphthalein — starts pink in alkali, turns colourless at end point; or methyl orange — starts yellow in alkali, turns orange/red at end point).

4. Position the flask: Place on a white tile so you can clearly observe the colour change.

5. Add acid: Open the burette tap and add acid to the alkali. Swirl the flask continuously to ensure thorough mixing.

6. End point: As you approach the end point, add acid one drop at a time. The end point is the point at which the indicator changes colour permanently (i.e. does not revert when swirled).

7. Record: Note the final burette reading. The titre is the final reading minus the start reading.

8. Repeat: Carry out the titration at least three times. Use concordant results (titres that agree within 0.10 cm³) to calculate a mean titre. Ignore any anomalous results.

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Choosing an Indicator

Indicator	Colour in Alkali	Colour at End Point	When to Use
Phenolphthalein	Pink	Colourless	Strong acid + strong alkali
Methyl orange	Yellow	Orange/Red	Strong acid + strong alkali

Exam Tip: You must not use universal indicator for a titration because it changes colour gradually over a range of pH values. Titration indicators must give a sharp, single colour change at the end point.

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Recording Results

Titration	1 (rough)	2	3	4
Final reading / cm³	26.50	25.30	25.25	25.35
Start reading / cm³	0.00	0.00	0.00	0.00
Titre / cm³	26.50	25.30	25.25	25.35

• Rough titre: The first attempt — used to find the approximate volume, but not included in the mean.
• Concordant titres: Titres 2, 3, and 4 are concordant (within 0.10 cm³).
• Mean titre: (25.30 + 25.25 + 25.35) ÷ 3 = 25.30 cm³

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Key Skills

Reading a Burette

• The burette reads from top to bottom (0.00 at the top, 50.00 at the bottom).
• Read the bottom of the meniscus at eye level.
• Record to the nearest 0.05 cm³.

Why Use a Pipette?

A pipette delivers an accurate and precise fixed volume (e.g. 25.0 cm³). A measuring cylinder is less precise and less accurate.

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Sources of Error

Error	How to Minimise
Not reading the burette at eye level	Always read at the bottom of the meniscus at eye level
Overshooting the end point	Add dropwise near the end point
Air bubbles in the burette	Flush the tap before starting to remove air bubbles
Not rinsing the burette with acid first	Rinsing prevents dilution of the acid
Using too much indicator	Use only 2–3 drops — excess indicator can affect the end point

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Common Mistakes

Mistake	Correction
Using universal indicator	Universal indicator gives a gradual colour change — use phenolphthalein or methyl orange for a sharp end point
Including the rough titre in the mean	The rough titre is for practice — only average concordant results
Reading the burette at the top of the meniscus	Always read at the bottom of the meniscus
Not repeating the experiment	Repeat until at least two concordant results are obtained

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Worked Example

Question: A student carried out a titration and obtained titres of 24.80, 24.90, 24.85 and 27.10 cm³ (the first was a rough run). Calculate the mean titre.