9.2a - Determination of pH (Leaving Cert Chemistry): Revision Notes
9.2a - Determination of pH
Experiment Summary
In this experiment, the pH of various water samples is determined using either a pH metre, pH paper or a universal indicator solution.
The pH scale ranges from 0 (highly acidic) to 14 (highly basic), with a pH of 7 indicating neutral water. Determining the pH of water is essential for assessing water quality and its potential effects on plant and aquatic life.
Materials and Apparatus Required
Chemicals
- Water samples (e.g., river, lake, or tap water)
- Universal indicator solution (or pH paper)
- Distilled water (for cleaning the pH metre probe)
Apparatus
- pH metre (or pH paper with colour chart)
- Beakers (100 cm³)
- Dropping pipette
- Safety glasses
Safety Precautions
- Wear safety glasses throughout the experiment.
- Avoid contact with potentially contaminated water samples.
- Ensure the pH metre is rinsed thoroughly between different samples to prevent cross-contamination.
Method
Using pH Paper:
- Pour a small volume of the water sample into a clean beaker.
- Dip a strip of pH paper into the water sample and compare the resulting colour to the pH chart provided with the paper.
- Record the pH value.
Using Universal Indicator Solution:
- Add 2-3 drops of universal indicator solution to a small beaker of water sample and stir.
- Compare the resulting colour to the pH colour chart and record the pH value.
Using a pH Meter:
- Calibrate the pH metre according to the manufacturer's instructions using buffer solutions.
- Rinse the pH probe with distilled water and place it in the water sample.
- Allow the reading to stabilise, then record the pH value.
- Rinse the probe with distilled water before measuring the next sample.
Repeat the procedure for each water sample being tested.
Results
| Sample | pH (pH Paper) | pH (Universal Indicator) | pH (pH Metre) |
|---|---|---|---|
| River water | 7.2 | 7.0 | 7.1 |
| Lake water | 6.5 | 6.3 | 6.4 |
| Tap water | 7.8 | 7.6 | 7.7 |
Example Questions with Answers
Q1: What could cause a water sample to have a relatively high pH value?
A high pH may result from water running over limestone, which dissolves calcium carbonate, raising the pH.
Alternatively, intense photosynthetic activity from algae can increase pH due to the consumption of carbon dioxide.
Q2: What could cause a water sample to have a relatively low pH value?
Low pH values can be due to the presence of organic material that decomposes to form acidic substances or from industrial pollutants that lower the pH.
Q3: Why is it important to rinse the pH metre probe between samples?
Rinsing the pH probe prevents contamination between different samples, ensuring accurate pH readings.
Q4: What are the environmental impacts of extreme pH values in water?
Extremely high or low pH values can harm aquatic life by affecting biological processes, making the water either too acidic or too basic for organisms to survive.