24 6.5 Activity: measuring pH of different water samples (aim, method, expected result)
Aim. To compare the pH of sea water, river water and well water.
Materials. A pH meter or pH test strips; samples of the three waters (sea water, river water, well water); sample cups; distilled water; a stirring rod; paper towels.
Method with a pH meter.
- Calibrate the meter following the maker's instructions (in practice using standard buffer solutions, and rinsing the electrode with distilled water), then dry the electrode gently with paper towel (never rub it).
- Prepare samples: number the cups, pour a small amount of each water sample into its own cup.
- Test: immerse the electrode in the sample, stir gently, and record the reading once it has stabilised.
- Repeat for each sample, rinsing and drying the electrode between samples so as not to carry one sample into the next.
Method with test strips. Dip a strip into each sample for a few seconds, remove it and compare the colour with the standard colour chart on the container.
- Expected results. Sea water is the most alkaline of the three (about 8.1–8.3 because of dissolved carbonates).
- River water is near neutral (about 7–8).
- Well water depends on the rock layers: limestone areas give slightly alkaline water (7.5–8.5), others slightly acidic.
Safety and accuracy. Wear gloves and eye protection when sampling polluted water; take several readings and average; do not touch the electrode bulb.