Indicators

Is the lemon juice really acidic? And the dish soap basic? Tasting is not an option, and with dish soap it would even be dangerous. Luckily there are substances that simply show you the answer: they change colour depending on whether they are dipped into something acidic or basic. Such substances are called indicators, and you probably even have one in your fridge.

What an indicator shows

An indicator is a dye that reacts to acids and bases, that is, to acidic and to basic solutions. How acidic or basic something is, is described by the pH value: the scale runs from 0 to 14. Values below 7 mean acidic, exactly 7 is neutral (like pure water), values above 7 mean basic. The indicator translates this invisible value into a colour you recognise instantly.

Red cabbage juice: the indicator from the kitchen

The juice of red cabbage contains a natural dye that works as an indicator. On its own, in neutral water, it is violet. Add it to something acidic like vinegar or lemon juice and it turns red. In a base like soapy water it turns blue-green to green, and the stronger the base, the greener it gets. That is why red cabbage is even called Blaukraut (blue cabbage) in some German regions: cooked in lime-rich water it turns blue, with a dash of vinegar it stays red. In the lab, people also use litmus (red with acid, blue with base) and universal indicator, which shows a different colour for each pH range, for example green in a neutral solution.

Why this matters

pH measurements show up more often than you might think: at the swimming pool, the water's pH is checked so it neither irritates your eyes nor stops the chlorine from working. Gardeners measure the soil's pH because many plants only grow well in a certain range. And in an aquarium, the pH helps decide whether the fish feel comfortable. Indicators are behind all of it, often as handy test strips.