pH scale

Read our standard health & safety guidance

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 (see 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)

Access to:

Deionised or distilled water
Dilute hydrochloric acid, 0.1 mol dm^-3 (Low hazard at this concentration)
Dilute sodium hydroxide solution, 0.1 mol dm^-3 (Irritant at this concentration)

Universal indicator solution (Highly flammable), full range, ideally in small dropping bottles
pH indicator chart

Technical notes

Dilute hydrochloric acid (Low hazard at concentration used) Refer to CLEAPSS Hazcard 47A and Recipe card 31
Dilute sodium hydroxide solution (Irritant at concentration used) Refer to CLEAPSS Hazcard 91 and Recipe card 65
Universal indicator solution (Highly flammable) Refer to CLEAPSS Hazcard 32 and Recipe card 36

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

HEALTH & SAFETY: Wear eye protection

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 – j Repeat instructions a – e using the sodium hydroxide solution instead of hydrochloric acid. Number the test-tubes 8–13.

Both groups

k 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).

l 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.

m 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.

See also Making a pH indicator

Health & Safety checked, June 2007