The pH scale and neutralisation: describe the use of universal indicator or a wide range indicator to measure the approximate pH of a solution

The idea

Universal indicator is a mixture of dyes that changes colour gradually across the pH scale, so it shows whether a solution is strongly acidic, weakly acidic, neutral, weakly alkaline or strongly alkaline. To use it, add a few drops to the solution in a clean test tube or spotting tile, or dip a clean glass rod into the solution and touch it onto universal indicator paper. Compare the resulting colour with the colour chart and read off the nearest pH value. The reading is approximate because colour matching is subjective and the indicator gives a broad colour change rather than a sharp end point. A wide range indicator works in the same way, covering a broad pH interval.

What earns the marks

• Universal indicator is a mixture of indicators that produces a different colour at different pH values across the pH scale.
• The indicator is added to the solution, or the solution is applied to indicator paper, and the colour produced is compared with a standard colour chart.
• The colour chart links each colour to an approximate pH value, allowing the solution to be classified as acidic, neutral or alkaline.
• The result is approximate because colour matching is subjective and the indicator changes colour gradually over a range of pH values.
• A wide range indicator covers a broad pH interval and is used in the same way as universal indicator to estimate pH.
• The solution should be tested in a clean container with a clean dropping pipette or glass rod to avoid contamination by residues from previous tests.

Common mistakes and how to put them right

• Treating the universal indicator reading as exact.

  Put it right: The colour chart gives an approximate pH, so a reading such as pH 3 should be described as approximate rather than precise.

• Confusing universal indicator with a single indicator such as litmus.

  Put it right: Litmus only shows acidic or alkaline, whereas universal indicator gives a range of colours linked to pH values.

• Using a dirty test tube or pipette.

  Put it right: Residues from a previous solution can change the colour and give a false pH reading, so apparatus must be clean.

Exam tips

• State clearly that the colour is matched to a chart to obtain an approximate pH value, not just that the indicator changes colour.
• Use the colour chart language of the specification, such as red for strong acid and purple for strong alkali, and link each colour to an approximate pH.
• When asked to describe a method, include the practical steps: place the solution in a clean container, add a few drops of universal indicator, and compare the colour with the chart.