Acid-Base Titration: Finding the Concentration of Hydrochloric Acid

This experiment uses a titration to find the concentration of hydrochloric acid by reacting it with sodium hydroxide of known concentration.

Titration is a precise method used in laboratories, industry and quality control. It develops careful measuring, observation and calculation skills.

An acid reacts with a base (or alkali) in a neutralisation reaction to form a salt and water. Hydrochloric acid and sodium hydroxide give sodium chloride and water.

The equation shows that 1 mole of acid reacts with 1 mole of alkali. At the end-point the moles of alkali added equal the moles of acid in the flask.

1. Rinse and fill the burette with sodium hydroxide solution, then remove the funnel and fill the tip.
2. Pipette 25.0 cm3 of hydrochloric acid into a clean conical flask.
3. Add a few drops of phenolphthalein. The solution stays colourless.
4. Record the starting reading at eye level from the bottom of the meniscus.
5. Rough titration: add alkali quickly while swirling until the colour changes. This shows the approximate titre.
6. Accurate titrations: add alkali quickly until about 2 cm3 before the rough titre, then drop by drop, swirling, until one drop gives a permanent faint pink.
7. Record the final reading and calculate the titre.
8. Repeat until two titres agree within 0.10 cm3 (concordant).

• The solution in the flask is colourless at the start.
• Near the end-point, pink appears where the alkali enters but disappears on swirling.
• At the end-point one drop gives a permanent faint pink.
• The rough titre is usually a little higher than the accurate titres because alkali is added quickly.

The pH changes very slowly at first, then rises sharply near the end-point. This sharp change is why one drop can change the indicator colour.

Worked example. Concordant titres of 24.90 and 25.00 cm3 give a mean of 24.95 cm3.

1. Moles NaOH = 0.100 x 24.95 / 1000 = 0.002495 mol
2. Ratio 1 : 1, so moles HCl = 0.002495 mol
3. Concentration HCl = 0.002495 / (25.0 / 1000) = 0.0998 mol/dm3

State the result with units and link it to your evidence, for example: the concentration of the hydrochloric acid is 0.0998 mol/dm3, calculated from two concordant titres that agree within 0.10 cm3.

Your confidence depends on how close the concordant titres are. If your titres agree closely, your results are precise. Whether they are accurate depends on avoiding systematic errors.

Repeating the titration and using concordant results reduces the effect of random error.

• Wear eye protection and tie back long hair.
• Use a pipette filler; never pipette by mouth.
• Rinse skin splashes with plenty of water and tell your teacher.
• Clean up spills at once.
• Fill burettes below eye level to avoid splashing the face.

These are important practical skills linked to this experiment. This is not a claim about how often any question has appeared in past papers; check your own syllabus and past papers.

• Describing the method and the reason for each step.
• Reading a burette and recording titres to two decimal places.
• Explaining the purpose of the rough titration.
• Identifying concordant results and calculating a mean.
• Calculating moles and concentration.
• Choosing a suitable indicator and describing the colour change.
• Explaining sources of error and improvements.

• Record every reading to two decimal places, usually ending in 0 or 5.
• Draw a results table before you begin with headings and units.
• Use a rough titration, then accurate ones, until two are concordant.
• Show your working: formula, substitution, answer with units.
• Give the answer to a sensible number of significant figures, usually three.
• For error questions name the error, its effect and a specific improvement.

• Quality control: manufacturers use titrations to check acidity or concentration of food, drinks and medicines.
• Environmental testing: titrations help measure acidity of water and soil samples.
• Agriculture: checking soil acidity guides decisions about fertilisers and lime.
• Industry: titration helps ensure products have the correct concentration.

These key points summarise the experiment.

1. Titration finds the volume of one solution that exactly reacts with a known volume of another.
2. The equation is HCl + NaOH -> NaCl + H2O, a 1 : 1 reaction.
3. Phenolphthalein is colourless in acid and pink in alkali.
4. The end-point is the first permanent faint pink.
5. A rough titration shows roughly where the end-point is.
6. Add alkali drop by drop near the end-point and swirl the flask.
7. Read the burette at eye level from the bottom of the meniscus, to two decimal places.
8. Concordant titres agree within 0.10 cm3.
9. Calculate the mean using concordant titres only.
10. Moles = concentration x volume (in dm3).
11. Concentration = moles / volume in dm3.
12. Rinse the burette with the solution it will hold and the pipette with its own solution; rinse the flask only with water.
13. Remove the funnel before titrating, because drips from it would add extra volume.
14. Convert cm3 to dm3 by dividing by 1000.

Use these links to revise the ideas in this experiment and practise related skills.

• Glossary: Volume
• Glossary: Decimal
• Glossary: Rounding
• Lesson: Measuring accurately
• Lesson: Mean
• Experiment: Observing Onion Epidermal Cells Under a Light Microscope
• Experiment: Investigating Ohm's Law

Practise again with new questions