Titrations

A titration is a practical method to find the concentration of a solution by reacting it exactly with another solution of known concentration. Think of adding drops of lemon juice to tea until it tastes “just right” – the “just right” point in chemistry is called the end-point.

Key apparatus

• Burette: a tall tube with a tap that delivers accurate, variable volumes (used for the solution of known concentration).
• Volumetric pipette: measures a fixed, accurate volume (often 25.0 cm³) of the unknown solution into a conical flask.
• Indicator: changes colour at the end-point.
• Conical flask and white tile to see colour changes clearly.

Typical procedure (acid–base)

1. Rinse and fill the burette with the standard solution (e.g. NaOH). Record the initial reading.
2. Use a volumetric pipette to place a known volume (e.g. 25.0 cm³) of the unknown solution (e.g. HCl) into a conical flask. Add a few drops of indicator.
3. Run the solution from the burette into the flask while swirling until the first permanent colour change appears – this is the end-point.
4. Record the final burette reading. The volume delivered is the titre. Repeat to get two concordant titres (within about 0.10 cm³).

Indicators and end-point

• Phenolphthalein: colourless in acid, pink in alkali (good for strong base titrations).
• Methyl orange: red in acid, yellow in alkali (good for strong acid titrations).
• End-point = first permanent colour change that lasts while swirling.

Calculations you need

Use moles and concentration: $n = c \times V$ where volume $V$ is in dm³. Convert using $V(\text{dm}^3) = \dfrac{V(\text{cm}^3)}{1000}$.

Use the balanced equation to find reacting mole ratios.

Common mistakes

• Forgetting to convert cm³ to dm³ before using $n = cV$.
• Overshooting the end-point by adding too much from the burette at once.
• Thinking the burette must start at 0.00 cm³ – it does not. Use final − initial readings.