What is pH and how is it measured?
Short answer
pH is a scale from 0 to 14 that shows how acidic or alkaline a solution is. Below 7 is acidic, 7 is neutral and above 7 is alkaline. It is measured with universal indicator, which changes colour, or more accurately with a digital pH meter.
Step by step
- Add a few drops of universal indicator, or dip in indicator paper.
- Compare the colour with the chart: red for strong acid, green for neutral, purple for strong alkali.
- For an exact value, rinse a pH probe, place it in the solution and read the meter.
- Remember that each step of 1 on the scale means a tenfold change in hydrogen ion concentration.
Worked example
Common pH values
- Stomach acid: about 1 to 2
- Lemon juice: about 2
- Pure water: 7
- Soap: about 9 to 10
- Oven cleaner: about 13
Common mistakes
- Thinking a higher pH means more acidic. It is the opposite.
- Calling every alkali a base without noting that alkalis are bases that dissolve in water.
- Forgetting that neutralisation of an acid and an alkali produces a salt and water.
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