The pH of a substance can be found by dissolving a small amount of the substance in deionised water and adding a few drops of Universal Indicator solution. The colour produced is compared with a pH chart.

Class practical or demonstration

In this experiment students prepare a series of solutions by dilution. Each solution approximates to a pH number. The students then confirm what they have done by using Universal Indicator. The experiment shows that a solution with a given pH number differs in concentration from the one with the next pH number by a factor of 10.

Lesson organisation

This could be done as a demonstration or as a class experiment. To save time, students can work in groups of four. One pair of students makes the acidic solutions; the other pair makes the alkaline solutions. They then put the two sets of solutions together to make one set covering the pH range from 1 to 13.

Apparatus Chemicals

Eye protection

Each working group (4 students) will require:

Test-tubes, 13 (Note 1)

Test-tube rack(s) with sufficient space for 13 test-tubes

Beakers (100 cm3), 2

Measuring cylinders (10 cm3), 2

Dropping pipettes (optional)

Deionised or distilled water

Dilute hydrochloric acid, 0.1 M

Dilute sodium hydroxide solution, 0.1 M

Universal indicator solution (HIGHLY FLAMMABLE), full range, ideally in small dropping bottles

pH indicator chart

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. 

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

Dilute sodium hydroxide solution, NaOH(aq), (IRRITANT at concentration used) - see CLEAPSS Hazcard and CLEAPSS Recipe Book. 

Universal indicator solution (HIGHLY FLAMMABLE) - see CLEAPSS Hazcard and CLEAPSS Recipe Book. 

1 Test-tubes with a capacity of around 10 cm3 are ideal. The test-tubes should be as clean as possible. Test-tubes, dropping pipettes and measuring cylinders should be washed in tap water and then rinsed with deionised or distilled water.

Procedure

Students 1 and 2 

a Number the test-tubes 1–7.

b Half-fill test-tube 1 with the hydrochloric acid solution.

c Transfer 1 cm3 of the hydrochloric acid into the measuring cylinder. Add distilled or deionised water to the measuring cylinder, up to the 10 cm3 mark.

d Pour some of the resulting diluted solution from the measuring cylinder into test-tube 2, enough to come to a similar height as the solution in test-tube 1.

e Carefully, pour away all but 1 cm3 of the solution remaining in the measuring cylinder. Now add distilled or deionised water to the measuring cylinder up to the 10 cm3 mark. Pour the resulting solution into test-tube 3. Continue in this way until you have solutions in test-tubes 1 to 6. Put only distilled or deionised water into test-tube 7.

Students 3 and 4
f Repeat instructions a – e using the sodium hydroxide solution instead of hydrochloric acid. Number the test-tubes 8–13.

Both groups
g Put the two racks of test-tubes together so that the solutions are in order 1 to 13. The test-tubes now have solutions in them with pH 1 (test-tube 1) to pH 13 (test-tube 13).

h Add a drop of Universal indicator to each test-tube. Rock each test-tube from side to side to mix the contents. Add more Universal indicator solution to each test-tube if needed to allow the colours to be seen more clearly. Be sure to add the same number of drops of indicator to each test-tube.

i Compare the colours of the solutions with the pH indicator chart.

Teaching notes

The depth of discussion will depend on the level of the students involved.

The pH of the solutions in test-tubes 5, 6, 8 and 9 will not be very accurate. It is not possible to get pH 7 by diluting the pH 6 solution. Successive dilutions past 6 give solutions with pH progressively closer to, but never quite reaching, pH 7. The same applies to further dilutions on the alkaline side of neutral.

The colour you get in test-tube 7 might tell you something about the quality of your deionised water.

Students sometimes worry about overfilling the measuring cylinder to a level above the 10 cm3 mark. However, this will not make much difference to the overall outcome.

Draw out that the students have had to dilute solutions by 10 to change from one pH number to the next. A solution of pH 1 is ten times more acidic (has a greater concentration of hydrogen ions) than a solution of pH 2. A solution with pH 4 is not ‘very acidic’ as is often stated.

The letter p in pH stands for ‘power’ (or ‘potenz’ in German) and refers to the concentration of hydrogen (H+) ions in the solution when expressed in the form 10-n mol dm-3, where n is the pH. The relationship between pH number and hydrogen ion concentration can be expressed as
pH = – log10 [H+(aq)].

Universal indicator is a mixture of indicators made in such a way as to give, as far as possible, a different colour for each pH number. Students should notice that it is not very effective at the extremes of the range.

Health & Safety checked, 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

Page last updated October 2015