Experiment: Acid–base titration

Titration is the standard way to find out how much acid exactly neutralises a fixed amount of alkali, and it is one of the most heavily examined practicals in the Form 4 Acids, Bases and Salts chapter. It builds directly on preparing a standard solution, because a reliable titration needs a solution of accurately known concentration. This guide sets out the method, the observations, and the Paper 3 skills that the practical assessment rewards.

Aim

To determine the volume of dilute acid, for example hydrochloric acid, that exactly neutralises a fixed volume of alkali, for example sodium hydroxide solution, using an indicator to show the end point.

Apparatus and materials

• Burette (for example, a 50 cm³ burette) with a retort stand and clamp
• Pipette (for example, a 25 cm³ pipette) and a pipette filler
• Conical flask
• White tile
• Filter funnel and small beakers
• Wash bottle of distilled water
• Dilute hydrochloric acid and sodium hydroxide solution
• A suitable indicator, for example phenolphthalein

Procedure

1. Rinse the pipette with a little of the alkali, then use the pipette filler to measure a fixed volume of the alkali into a clean conical flask.
2. Add two or three drops of phenolphthalein to the flask; the solution turns pink.
3. Rinse the burette with a little of the acid, then fill it with the acid, making sure the space below the tap is filled and there is no air bubble.
4. Record the initial burette reading, taking the bottom of the meniscus at eye level.
5. Stand the conical flask on the white tile below the burette so the colour change is easy to see.
6. Run the acid into the flask, swirling the flask continuously to mix the contents.
7. As the end point approaches, shown by the pink colour fading more slowly where the acid lands, add the acid drop by drop.
8. Stop the instant the pink colour just disappears and the solution becomes colourless. This is the end point.
9. Record the final burette reading and find the volume of acid used (the titre).
10. Repeat the titration until two titres agree closely (concordant results), and use the average of the concordant titres.

Expected observations

The alkali with phenolphthalein is pink. As acid is added and swirled, the pink colour flashes and fades. At the end point, a single drop of acid changes the whole solution from pink to colourless, and the colour does not return on swirling. Concordant titres are close to one another, which shows the readings are reliable.

Inference and conclusion

At the end point the acid has exactly neutralised the alkali, so the number of moles of acid that reacted equals the number of moles of alkali multiplied by the mole ratio in the balanced equation. For a reaction in which one mole of acid reacts with one mole of alkali, the moles of acid equal the moles of alkali at the end point. If the concentration and volume of one solution are known, the concentration of the other can be calculated from the average titre. The conclusion is that neutralisation is complete at the end point, and the fixed volume relationship lets us find an unknown concentration.

Science process skills (Paper 3 style)

Operational definition of the end point. Define the end point operationally as the moment when one drop of acid changes the indicator colour permanently, here, from pink to colourless, with continuous swirling.

Identifying variables. If the investigation compared the titre needed for alkalis of different concentrations, the manipulated variable is the concentration of the alkali, the responding variable is the volume of acid needed to reach the end point, and the controlled variables are the volume of alkali pipetted, the concentration of the acid, and the indicator used.

Tabulating data. Draw a titration table with rows for final reading, initial reading and volume of acid used, and columns for the rough titration and the accurate titrations, all volumes recorded to the same precision as the burette allows.

Making inferences. Explain why only concordant titres are averaged: a rough first titration overshoots, so it is used only to find the approximate end point, not in the average.

Safety precautions

• Wear safety goggles, because both the acid and the alkali are corrosive and can damage the eyes.
• Use a pipette filler and never mouth-pipette, so corrosive solution is not drawn into the mouth.
• Clamp the burette securely and fill it below eye level, so acid is not spilled onto the face when filling.
• Wash any acid or alkali splashes off the skin at once with plenty of water, because both can burn.

Common errors

• An air bubble in the burette tip. A bubble that escapes during the titration adds to the apparent titre; run out a little acid first to fill the tip.
• Not rinsing the burette and pipette. Water left inside dilutes the solutions and changes the titre; rinse each with the solution it will hold.
• Reading the burette with parallax. Always read the bottom of the meniscus at eye level, or the volume is wrong.
• Overshooting the end point. Adding acid too fast near the end point takes the colour past the change; add dropwise and swirl.
• Averaging a rough titre. Include only concordant accurate titres in the average.

How our teachers use this experiment

In one-to-one SPM Chemistry lessons, taught in English from RM50 per hour, we drill the titration routine, rinse, pipette, indicator, fill without a bubble, titrate to a dropwise end point, repeat for concordant results, until it is second nature. Because titration questions recur across SPM Chemistry and are a classic Paper 3 practical (Paper 3 is a practical test assessing science process skills), a clean, well-explained method here protects easy marks that many students lose on technique alone.