What Is an Indicator? A Dye That Changes Colour With Acidity

An indicator is a weak acid or base whose two forms, the one holding a proton and the one having lost it, absorb light differently and therefore appear different colours. In acidic conditions the proton-holding form dominates, in alkaline conditions the other does, and in between both are present in proportions set by the acidity. The transition occurs over roughly two units of pH centred on the compound's own dissociation constant, which means each indicator changes colour over its own particular range and is useless outside it. The visible change happens because the structure of the molecule shifts when the proton leaves, altering the arrangement of alternating bonds that determines which wavelengths are absorbed. That connection between molecular structure and colour is the same chemistry that underlies dyes generally, which is why so many indicators are dyes and why the two fields developed together.

Different indicators are chosen for different purposes because their ranges differ:

• •Litmus, extracted from lichens, red in acid and blue in alkali, changing near neutral and famous for being the first anyone meets
• •Phenolphthalein, colourless in acid and pink in alkali, changing in the alkaline range and used for titrating a strong base
• •Methyl orange, red in acid and yellow in alkali, changing in the acidic range and used for titrating a strong acid
• •Bromothymol blue, yellow to blue across the near-neutral range, widely used in biology for detecting carbon dioxide
• •Universal indicator, a mixture giving a continuous range of colours across the whole scale rather than a single change
• •Red cabbage extract, which contains pigments spanning a wide range and is the standard demonstration anyone can do at home

The practical use is in titration, where a measured solution is added to another until the reaction is complete, and the indicator signals the endpoint. Choosing correctly matters because the acidity at the completion point depends on what is reacting, and using an indicator that changes at the wrong acidity gives a systematically wrong answer. A strong acid neutralised by a strong base reaches completion at neutral, so almost any indicator works. A weak acid neutralised by a strong base reaches completion in the alkaline region, which requires an indicator changing there. A weak base neutralised by a strong acid reaches completion in the acidic region and requires one changing there. The steepness of the change matters too, since a sharp transition in acidity near completion means any indicator in that region changes abruptly, giving a clear endpoint.

The observation that certain plant extracts change colour with acidity is old, and turning it into a measurement was a gradual achievement. Seventeenth-century work recorded that syrup of violets and other plant infusions changed colour with what were then called acids and alkalis, and used the effect to classify substances long before anyone knew what an acid was. Litmus from lichens was in use by then. The modern understanding had to wait for the theory of acids as proton donors and for the definition of the pH scale in the early twentieth century, which gave the colour changes a quantitative meaning and allowed indicators to be characterised by the range they change over. Synthetic dye chemistry in the nineteenth century supplied a large supply of new candidates, since the same industry producing dyes for textiles was producing compounds whose colour depended on structure in exactly the way an indicator requires.

The principle is applied well outside laboratories. Soil testing kits use indicators to determine whether ground is suitable for particular plants, which matters because nutrient availability depends strongly on acidity. Swimming pool and aquarium test kits work the same way. Some plant pigments change colour with soil acidity while still in the plant, which is why hydrangea flowers differ in colour depending on where they grow and can be altered deliberately by treating the soil. Indicator strips and papers impregnated with mixtures give a rough reading anywhere. Medical test strips use the same chemistry for urine testing. Electronic measurement has replaced indicators where precision is needed, since a glass electrode gives a continuous numerical reading rather than a change at one point, and indicators remain standard where cheapness, simplicity and not needing power matter more.

An indicator is a weak acid whose proton-holding and proton-free forms absorb light differently, changing colour over about two units of acidity centred on its own dissociation constant. Each one therefore works only in its own range. In a titration the indicator must change where the reaction actually completes, which depends on whether the acid and base are strong or weak.