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The pH of a solution can be measured by several methods. The most common methods in schools use Universal Indicator solution or paper, or a pH meter. The pH meter is usually more accurate than the other methods. It can measure to 0.01 of a pH unit.

Use the following pH meter readings to work out the concentration of H⁺ ions in solution (and before you reach for your calculator, have a go at estimating the values as they are convenient numbers):

pH 4.30

The concentration is 5.0 x 10^–5 mol dm^–3.

pH 3.70

The concentration is 2.0 x 10^–4 mol dm^–3.

Was it tricky to estimate?

If you found it tricky to have a go without a calculator, a handy hint is to remember that log 2 = 0.3 and log 5 = 0.7. The numbers in the pH examples given above are reasonably convenient. Estimating the value of log or pH calculations is a useful tool for teachers to get a 'feel' for the likely answer. This handy technique, requiring only simple addition or subtraction of logs, is explored in the next topic.