Strong and Weak Acids with pH Sensors – Beyond Universal Indicator

Many students begin their study of acids and alkalis using universal indicator. It’s colourful, quick, and perfectly fine for basic classification. But when we introduce GCSE and A-Level Chemistry, universal indicator simply isn’t precise enough. To understand strong and weak acids, students need to measure pH accurately — and the best way to do that is with a digital pH sensor.

Using PASCO or similar pH probes, students can see how two acids of the same concentration can behave very differently, revealing the hidden chemistry behind dissociation, equilibrium, and hydrogen ion concentration.

---

The Core Idea – Not All Acids Are Equal

A strong acid dissociates completely in water:

$$\text{HCl} \rightarrow \text{H}^+ + \text{Cl}^-$$

A weak acid only partially dissociates:

$$\text{CH}_3\text{COOH} \rightleftharpoons \text{H}^+ + \text{CH}_3\text{COO}^-$$

Even if both solutions are 0.1 mol dm⁻³, their pH values will be very different. A universal indicator colour chart cannot show this clearly — but a pH sensor can.

---

The Experiment

Equipment:

• PASCO (or equivalent) pH sensor

• 0.1 M hydrochloric acid (strong)

• 0.1 M ethanoic acid (weak)

• Distilled water

• Data-logging software

Method:

1. Calibrate the pH sensor using standard buffer solutions.

2. Measure and record the pH of 0.1 M HCl.

3. Measure and record the pH of 0.1 M ethanoic acid.

4. Compare values and plot them on a pH–concentration graph.

5. For A-Level students, repeat using 0.01 M and 0.001 M solutions to show how pH changes with concentration.

Typical results:

• 0.1 M HCl → pH ≈ 1

• 0.1 M CH₃COOH → pH ≈ 2.8

Students immediately see that weak acids release far fewer hydrogen ions.

---

The Science

Strong acids dissociate fully, so hydrogen ion concentration equals the concentration of the acid.
Weak acids are governed by an equilibrium expressed by their Ka value.
The pH of a weak acid depends on:

• its initial concentration

• its acid dissociation constant

• the position of equilibrium

Using a pH sensor allows students to measure these differences instead of memorising them.

---

Skills Highlight

• Accurate pH measurement with digital probes

• Using data acquisition software to log and graph pH

• Interpreting dissociation and Ka values

• Comparing theoretical vs experimental pH

---

Why It Works in Teaching

This experiment shows students that acid strength is not about “how dangerous it is”, but about how much it dissociates.
With precise digital data, students finally understand why strong acids have low pH even at low concentration — and why weak acids require equilibrium calculations at A-Level.