No stomach for it:modelling the effect of antacid medication |
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Introduction |
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Lesson organisation
Organisation may depend on the number of pH probes and meters you have, or the range of antacids you want to try. Students working in pairs would each be able to investigate one or two antacids.
Apparatus and Chemicals
For the class - set up by technician/teacher:
• Hydrochloric acid, dilute, 0.01 mol dm3, 100 cm3 for each antacid for each working group (refer to Hazcard 47A and note 1)
• Universal indicator solution, in dropping bottles (note 2)
• Antacids, with details of dosage from packaging
For each group of students:
• Beaker, 100 cm3, 2 per antacid to be tested
• Mortar and pestle
• Measuring cylinder, 100 cm3
Technical notes and safety
1 Hydrochloric acid is described on Hazcard 47A as irritant at concentrations above 2.0M, causes burns and is irritating to the respiratory system. The acid used here is much more dilute and presents a minimal hazard to students.
2 Universal indicator - see Hazcard 32 and Recipe Card 36. The bottled solution is highly flammable.
Procedure
SAFETY: Take care when making up the dilute acid.
Preparation by the teacher
a Make up the dilute hydrochloric acid by serial dilution (1 in 10, twice) from 1 mol dm-3 acid.
(See note 1.)
b Copy (and enlarge if necessary) the details of typical doses of antacids from the packaging.
c Set up a few beakers of 50 cm3 of water with indicator to show what a neutral pH would look like.
Click here to see a full list of Health and Safety procedures
Investigation
d Measure 50 cm3 of dilute acid into each of two beakers and add enough Universal indicator to get a clearly visible colour. (See note 2.)
e Sit both beakers on a sheet of white paper.
f Keep one beaker for comparison as small changes in the acid pH range can be hard to see.
g Add a normal dose of antacid to the other beaker and watch the colour change. If the antacid is in tablet form, crush the tablet in a mortar with a pestle before adding to the acid.
h Repeat with other antacids.
i Decide which antacid is making the greatest change, or the quickest change. Record any other
observations - such as effervescence.
j If using a pH probe, plot a graph of pH against time over 10-15 minutes.
Teaching notes
The approximate relaxed volume of our stomach is 50 cm3, but it is able to expand to nearly 4 dm3.
The lowest pH of secreted acid is about 0.8, but it is diluted in the stomach to an ideal pH of around 1.4. The stomach secretes acid to produce the optimum pH for the action of pepsin. An excess of acid is sometimes produced, which results in acid indigestion (in the short term) or could result in ulceration of the stomach lining (if high concentrations of acid persist). Antacids have been developed to treat short-term excesses. Other pharmaceuticals are used to treat long-term imbalance of acid production.
Students may be surprised how little the pH changes when the antacid is added.
It is interesting to compare liquids with powders, and to see just how slowly an uncrushed tablet reacts.
There are ingredients other than antacids in many over-the-counter preparations that have an effect on indigestion. Some include a mucilaginous component that coats the stomach lining and may protect the lining tissue from damage by acid. This could lead into a more detailed exploration of the structure of
the stomach and the different tissues that make up the organ.
Discuss the issues associated with long-term use of antacid preparations. Ideas are listed below.
• Pepsin operates best at acid pHs, so using antacids before meals, or immediately after, could reduce the rate of digestion.
• The body has many mechanisms that maintain balance. Is it possible that taking antacid medication regularly would, in fact, stimulate the gastric lining to make more acid to restore normal pH?
Further information
www.rsc.org/education/teachers/learnnet/pdf/LearnNet/rsc/Kev51-60.pdf
This is from the RSC's 'Classic Chemistry Experiments' - a formal titration of preparations of indigestion
tablets with hydrochloric acid. You could use this as a more quantitative extension activity linked to the above investigation.
Click here to download this activity as a Microsoft Word document
