Class practical

Indigestion is caused by excess acid in the stomach. Indigestion tablets neutralize some of this acid. This experiment shows how you can measure the amount of hydrochloric acid neutralised by one tablet. This is one measure of the effectiveness of the tablet.

Lesson organisation

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 work-place 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.

Apparatus Chemicals

Eye protection

Each working group requires:

Burette (30 cm3 or 50 cm3 capacity) (Note 1)

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

Each working group requires:

Dilute hydrochloric acid of appropriate concentration, 100 cm3(Note 1)

Two indigestion tablets, one of the same brand to be tested by all groups, and another tablet from a range of brands available to the class (Note 2)

Original packets from which the tablets are taken, together with price information for each packet

Methyl orange indicator solution (or alternative)

Deionised or distilled water, about 100 cm3

Refer to Health & Safety and Technical notes section below for additional information.

Health & Safety and Technical notes

Read our standard health & safety guidance

Wear eye protection. 

1 Dilute hydrochloric acid, HCl(aq) - see CLEAPSS Hazcard and CLEAPSS Recipe Book. 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.

2 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 and CLEAPSS Recipe Book. The methyl orange indicator should be available in a dropper bottle on the teacher’s bench.

Procedure

a 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.

b Add about 25 cm3 of deionised water to the flask, followed by three drops of methyl orange indicator.

c 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 re-use the acid in the beaker – this should be rinsed down the sink.)

d 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.

e 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.

f Record the volume of acid used.

g 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 pre-tested 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 they ought, if possible without prompting, 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.

Health & Safety checked, August 2016

Credits

This Practical Chemistry resource was developed by the Nuffield Foundation and the Royal Society of Chemistry.

© Nuffield Foundation and the Royal Society of Chemistry

Website

BUPA - Information on indigestion and the variety of indigestion tablets. 

However, there is a lack of easily found information on the quantitative composition of simple indigestion tablets on the web.

Page last updated October 2015