Experiment: Effect of dilution and concentration on pH

The pH scale measures how acidic or alkaline a solution is, and this experiment shows how pH depends on the concentration of ions in solution. It is a favourite context for Paper 3 because it involves a clear manipulated variable, the degree of dilution or the concentration, and a clear responding variable, the pH. This guide sets out the method, the qualitative observations to expect, and the science process skills the practical papers reward. Our teachers use it to make the link between concentration and pH concrete rather than memorised.

Aim

To investigate how diluting an acid with water, and changing its concentration, affects the pH of the solution.

Apparatus and materials

• Measuring cylinders and beakers
• Glass stirring rod
• Universal indicator solution and a colour chart, or a pH meter
• Wash bottle of distilled water
• Dropper (teat pipette)
• A dilute acid, for example hydrochloric acid, of a known starting concentration
• For comparison, a weak acid such as ethanoic acid of the same concentration

Procedure

1. Measure a fixed volume of the acid into a beaker, and record the concentration you start with.
2. Test the pH of this undiluted acid by adding a few drops of universal indicator and matching the colour to the chart, or by using a pH meter.
3. Prepare a diluted sample by measuring a smaller volume of the acid and adding distilled water to make it up to the original total volume, stirring to mix.
4. Test the pH of the diluted sample in the same way, using the same volume of solution and the same number of drops of indicator.
5. Repeat step 3 and step 4 to make a series of increasingly dilute samples, each diluted further than the last.
6. To compare concentration and strength, test an acid of the same concentration but different strength (for example ethanoic acid against hydrochloric acid) using the same method.
7. Record every pH reading in a table beside its dilution or concentration.

Expected observations

For the strong acid, the undiluted solution turns universal indicator red, showing a low pH. As the acid is diluted step by step, the colour changes through orange towards yellow, showing the pH rising towards 7, but it does not pass 7, because water is neutral and only dilutes the acid. A more concentrated acid gives a lower pH (a redder colour) than a more dilute one. At the same concentration, the strong acid shows a lower pH than the weak acid, because the strong acid ionises more completely.

Inference and conclusion

The pH of an acid depends on the concentration of hydrogen ions in the solution. Diluting the acid with water lowers the hydrogen ion concentration, so the pH rises towards 7. Increasing the concentration of the acid raises the hydrogen ion concentration, so the pH falls. Comparing acids of equal concentration shows that a strong acid, which ionises completely, has a lower pH than a weak acid, which ionises only partially. The conclusion is that pH is a measure of hydrogen ion concentration, changed both by dilution and by the strength of the acid.

Science process skills (Paper 3 style)

Stating a hypothesis. A suitable hypothesis is: as an acid is diluted with water, its pH increases, because dilution lowers the concentration of hydrogen ions.

Identifying variables. The manipulated variable is the concentration of the acid (or the degree of dilution); the responding variable is the pH of the solution; the controlled variables are the type of acid, the temperature, the volume of solution tested and the amount of indicator used.

Tabulating and plotting. Tabulate the concentration (or dilution factor) against the pH, then plot pH on the vertical axis against concentration on the horizontal axis, and describe the trend shown by the graph.

Making an inference. Infer from the graph that pH increases as concentration decreases, and connect this to the falling hydrogen ion concentration.

Safety precautions

• Wear safety goggles, because the acid is corrosive and can damage the eyes.
• Add acid to water when diluting, not water to acid, so that any heat released is spread out safely.
• Handle the concentrated acid with care and wipe up spills at once, because it can burn skin and clothing.
• Keep the same small volumes for testing, so that splashes are limited and comparisons stay fair.

Common errors

• Using different volumes or amounts of indicator for each test. This makes the colour comparison unfair; keep the volume and the drops of indicator the same every time.
• Judging the colour under different lighting. Match the universal indicator colour to the chart under the same light each time, or the pH read from the colour will be inconsistent.
• Forgetting to stir after diluting. An unmixed solution gives an uneven pH reading; stir every diluted sample.
• Confusing concentration with strength. A dilute strong acid can have a similar pH to a concentrated weak acid; keep the two ideas separate when explaining results.
• Expecting the pH to pass 7 on dilution. Diluting an acid moves the pH towards 7 but not beyond it.

How our teachers use this experiment

In one-to-one SPM Chemistry lessons, taught in English from RM50 per hour, we use this practical to fix the difference between concentration and strength, the single most common confusion in the pH topic. Because the same reasoning about hydrogen ion concentration runs through the whole Acids, Bases and Salts chapter of SPM Chemistry, and the dilution investigation is a natural Paper 3 practical (Paper 3 is a practical test assessing science process skills), understanding it well here supports many later questions on pH and neutralisation.