Investigating the effect of acid on indigestion tablets

IntroductionEducational data logger

DrDAQ is used here with the pH probe to find the pH of various indigestion remedies and the effect of adding dilute Hydrochloric acid.

The principle is that the stomach produces hydrochloric acid. This acid provides a medium for the digestion of protein by the enzyme protease. It also kills microbes that may be in the food. An excess of acid can lead to indigestion and indigestion tablets (may) work by neutralising the excess acid. The practical is also an interesting way of discussing what is meant by the best result and there are lots of opportunities to discuss fair testing, evaluating and alternative methods.

The practical is most suitable for Key Stage 3, when discussing acids, alkalines and pH. It can also be used to explore the importance of preliminary work (pH changes are only obvious when the acid is added to the indigestion tablets, not the other way around). The understanding can then help when doing science course work.

Equipment required

  • DrDAQ data logger connected to a PC.
  • One pH probe (DrDAQ part number DD011)
  • 1 M hydrochloric acid.
  • 4 types of indigestion tablets (e.g. Andrews Antacid, Rennie, Milk of Magnesia, Alka Seltzer).
  • Four 100 cm3 beakers.
  • 3 ml pipette.
  • Pestle and mortar.
  • Distilled water.

Experiment setup

  • Crush one indigestion tablet using the pestle and mortar.
  • Add 50 cm3 of distilled water and mix.
  • Measure the pH using the pH probe, start recording the pH after about 1 minute (when the pH is relatively stable).
  • After about 50 seconds add 1 ml of dilute hydrochloric acid.
  • Monitor the change in pH.

Carrying out the experiment

The beakers must be stirred regularly for the experiment to be fair. The pH will fluctuate slightly, especially when stirred. Focus should be made on pH changes greater than one.


  • How acid or alkaline are the indigestion tablets?
  • Why does the pH change when the acid is added?
  • Which is the best indigestion remedy? The one which is most alkaline to begin with, the one which shows the least change in pH or the one which returns to its (close to its) starting pH in the shortest amount of time?
  • Was the experiment a fair test (the tablets are different sizes and some dissolve)? How could the problem(s) be addressed?

Further study

  • Try repeating the experiment by adding the indigestion tablet to the acid.
  • How else could the pH of the indigestion tablet could be measured? (i.e. using universal iIndicator).
  • What advantages and limitation are there of measuring the pH with the probe?
  • What does pH measure?
  • What chemicals are in indigestion tablets and how do they react with the acid?