Indicators are large organic molecules used in chemistry to determine a substance’s pH. They change to different colors depending on whether they are added to an acid, a base (also known as an alkali) or a neutral substance. Most indicators are themselves weak acids and respond to changes in the hydrogen ion concentration.
The most common of all indicators is litmus paper. Litmus paper works by absorbing solutions and changing color according to their relative pH. Below pH 4.5, the paper turns red. Above pH 8.2, the paper turns blue. Deep reds and deep blues therefore indicate solutions that are, respectively, strongly acid and strongly alkali. Litmus paper turns purple when exposed to a neutral solution. Litmus itself is a weak acid.
Phenolphthalein is a colorless, weak acid that is commonly used as an indicator in titration experiments to signal the completion of reactions between acids and bases. It dissociates in water to form pink anions. When phenolphthalein is mixed with an acid, the concentration of the anions isn’t high enough for the pink color to be observable, so the solution remains clear. When mixed with an alkali, the concentration of the anions becomes enough for their pink color to be observable.
Bromothymol blue is most commonly used as an indicator for weak acids and bases as it is most effective for substances between pH 6 and pH 7.6, when the color change is most distinct. Bromothymol blue is a yellow color when mixed with an acid and a blue color when mixed with a base or a neutral substance. It is often used to help maintain the pH of fish tanks and swimming pools.
A universal Indicator is a solution containing a mixture of indicators. It provides a gradual change in color over a wider pH range than the individual indicators would. The approximate pH of a solution can be identified by adding a few drops of universal indicator to it. Red indicates an acidic solution; purple suggests it is alkali; while a yellow/green color means it has a natural pH.