Acid–Base Titrations

An acid–base titration is a method to find the exact amount (concentration) of an acid or an alkali by reacting them together until neutralisation. Neutralisation is when hydrogen ions and hydroxide ions form water: $$\text{H}^+ (aq) + \text{OH}^- (aq) \rightarrow \text{H}_2\text{O} (l)$$

Key apparatus and their roles

• Burette: a long tube with a tap that delivers accurate, variable volumes (read to 2 d.p., e.g. 23.45 cm³).
• Volumetric pipette: measures one fixed, accurate volume (e.g. 25.0 cm³) into a conical flask.
• Conical flask: holds the measured solution; easy to swirl.
• Indicator: shows the end-point with a clear colour change.
• White tile: helps see the colour change clearly.

Choosing a suitable indicator

• Phenolphthalein: colourless in acid, pink in alkali; good for strong acid vs strong base, and weak acid vs strong base.
• Methyl orange: red in acid, yellow in alkali (orange at end-point); good for strong acid vs weak base or strong acid vs strong base.
• Do not use universal indicator (the change is too gradual).

Method (technique you must know)

1. Rinse the burette with the solution that will go in it; fill it and remove air bubbles. Record the initial reading at eye level (bottom of the meniscus).
2. Use a volumetric pipette to place a fixed volume (e.g. 25.0 cm³) of the other solution into a conical flask. Add 2–3 drops of indicator.
3. Do a rough titration to find the end-point. Then repeat carefully, adding from the burette drop by drop near the end-point while swirling.
4. Get two (or more) concordant titres (within 0.10 cm³) and average them.

Identifying the end-point

The end-point is when the indicator changes to its final colour and stays changed after gentle swirling (for about 30 seconds). This is where the reacted amounts match the reaction ratio.

Calculations you need

Use the relationship $n = c \times V$, where $n$ is moles, $c$ is concentration in mol dm⁻³, and $V$ is volume in dm³ (so 25.0 cm³ = 0.0250 dm³).

Common mistakes to avoid

• Using the wrong indicator (end-point is hard to see).
• Forgetting to remove air bubbles from the burette tip.
• Not averaging concordant titres.

Real-world connection: titrations are used to check the exact acidity of foods and to control the strength of cleaning products.