Using indigestion tablets to neutralise an acid

Burettes are expensive and require a certain amount of skill to use. What follows here assumes that the class has been judged capable of doing this experiment using a burette.

The practical work should if possible start with the apparatus ready at each workplace in the laboratory to avoid vulnerable and expensive glassware (the burette) being collected from an overcrowded central location.

The experiment to test a single tablet should take no more than 25 minutes. If it is extended to compare the effectiveness of two or more brands of tablet, allow another 15–20 minutes per tablet. Different groups can be allocated one ‘standardisation’ brand of tablet to test, and one other brand, so that the class can compare a number of different brands in a one-hour lesson.

Equipment

Apparatus

• Eye protection
• Burette, 30 cm³ or 50 cm³ capacity (note 3)
• Conical flask, 100 cm³
• Beaker, 100 cm³
• Pestle and mortar
• Stirring rod
• Spatula
• Filter funnel, small (about 35 mm diameter)
• White tile (optional)
• Burette stand and clamp

Chemicals

• Dilute hydrochloric acid of appropriate concentration, 100 cm³ (note 4)
• Two different indigestion tablets – one from the standardisation brand to be tested by all groups, and one from a range of brands available to the class (note 5)
• Original packets from which the tablets are taken, together with price information for each packet
• Methyl orange indicator solution (or alternative) (note 6)
• Dionised or distilled water, about 100 cm³

Health, safety and technical notes

• Read our standard health and safety guidance.
• Wear eye protection.
• If your school still uses burettes with glass stopcocks, consult the CLEAPSS Laboratory Handbook, section 10.10.1, for their care and maintenance. This experiment will not be successful if the burettes used have stiff, blocked or leaky stopcocks. Modern burettes with PTFE stopcocks are much easier to use, require no greasing, and do not get blocked. Burettes with pinchcocks of any type are not recommended; while cheap, they also are prone to leakage, especially in the hands of student beginners.
• Dilute hydrochloric acid, HCl(aq) – see CLEAPSS Hazcard HC047a and CLEAPSS Recipe Book RB043. The concentration of hydrochloric acid should not need to be greater than 0.4 M. The concentration needed depends on the formulation of the tablets being tested. The aim is for each tablet tested to require around 20–30 cm³ of hydrochloric acid to neutralise. This can either be calculated from the tablet formulation if this is straightforward (some contain ingredients such as sodium alginate which make the calculation unreliable), or by running a test titration using an acid concentration of 0.1 M. The latter result can then be used to calculate a suitable concentration.
• Indigestion tablets – it is sensible to select brands of tablets for which a comparison is straightforward, with active ingredients restricted to carbonates, bicarbonates and/or hydroxides, avoiding those containing other active ingredients. A total of, say, four or five brands should be sufficient for an interesting exercise in comparing brands.
• Methyl orange indicator – see CLEAPSS Hazcard HC032 and CLEAPSS Recipe Book RB000. The methyl orange indicator should be available in a dropper bottle on the teacher’s bench.

Procedure

1. Crush a tablet using a pestle and mortar and carefully transfer it to a conical flask, using a spatula to ensure complete transfer as far as possible. Rinse any remaining fragments into the flask with a few cm³ of deionised water.
2. Add about 25 cm³ of deionised water to the flask, followed by three drops of methyl orange indicator.
3. Using a small funnel, pour a few cm³ of the dilute hydrochloric acid provided into the burette, with the tap open and a beaker under the open tap. Once the tip of the burette is full of solution, close the tap and add more of the solution up to the zero mark. (Do not reuse the acid in the beaker – this should be rinsed down the sink.)
4. Add acid from the burette into the flask, 1–2 cm³ at a time, while slowly swirling the flask. Continue to add the acid until a red colour begins to be seen in the flask that quickly returns to yellow-orange.
5. When it begins to take longer for this to happen, add a smaller quantity of acid at a time – eg 0.5 cm³ – until you reach a point where the red colour remains after one minute.
6. Record the volume of acid used.
7. Rinse the flask with water, and repeat the experiment with a different indigestion tablet. Refill the burette, if necessary.

Teaching notes

Titrating a powdered tablet containing insoluble ingredients such as calcium carbonate, magnesium carbonate and magnesium hydroxide is slow, as you need to allow for the solid to react with the acid. If the tablets have been pretested for their expected titre values, students can be instructed to add acid from the burette rapidly to a point 5 cm³ below the lowest expected value for the brands being tested – this should save time.

The experiment is designed to raise ‘fair test’ principles for discussion, and students are expected to comment with rational arguments on the validity of the comparisons they make. In particular, and if possible without prompting, they ought to read the instructions on each packet concerning the recommended dosage. Many comparisons are likely to be ‘grey’ rather than ‘black and white’. This could lead to suggestions for further investigations for improving comparisons, but it is unlikely that these will be feasible at school.

This experiment is likely to be more useful in investigative-style work for 14–16 year olds, rather than illustrating the development of understanding of the concept of acidity. However, this experiment enhances such understanding for many students.

Student questions

1. Explain why each group is asked to test one tablet of the same brand, and one of another brand.
2. Use your results to draw what conclusions you can about the value represented by each brand.
3. Is this a fair way of comparing brands of indigestion tablet? Explain your answer.
4. From the list of active ingredients on the packets, write word equations for the reactions that take place in your flask during the titrations.
5. Write a symbol equation for at least one of the reactions that takes place in the flask.