What Is a Chemical Indicator?

A chemical indicator is a substance that undergoes a distinct observable change when conditions in its solution change. This could be a color change, precipitate formation, bubble formation, temperature change, or other measurable quality.

Another type of indicator that may be encountered in chemistry and other sciences is a pointer or light on a device or instrument, which may show pressure, volume, temperature, etc. or the condition of a piece of equipment (e.g., power on/off, available memory space).

The term "indicator" comes from the Medieval Latin words indicare (to indicate) with the suffix -tor.

Examples of Indicators

• A pH indicator changes color over a narrow range of pH values in solution. There are many different pH indicators, which display different colors and act between certain pH limits. A classic example is litmus paper. Blue litmus paper turns red when it's exposed to acidic conditions, while red litmus paper turns blue under basic conditions.
• Fluorescein is a type of adsorption indicator. The dye is used to detect the completed reaction of the silver ion with chloride. Once sufficient silver is added to precipitate chloride as silver chloride, excess silver is adsorbed onto the surface. Fluorescein combines with adsorbed silver to produce a color change from greenish-yellow to red.
• Other types of fluorescent indicators are designed to bond to selected molecules. The fluorescence signals the presence of the target species. A similar technique is used to label molecules with radioisotopes.
• An indicator may be used to identify the endpoint of a titration. This may involve the appearance or disappearance of a color.
• Indicators may indicate the presence or absence of a molecule of interest. For example, lead tests, pregnancy tests, and nitrate tests all employ indicators.

Desirable Qualities of a Chemical Indicator

To be useful, chemical indicators must be both sensitive and easily detectable. It need not, however, show a visible change. The type of indicator depends on how it's being used. For example, a sample analyzed with spectroscopy may employ an indicator that wouldn't be visible to the naked eye, while a test for calcium in an aquarium would need to produce an obvious color change.

Another important quality is that the indicator doesn't change the conditions of the sample. For example, methyl yellow adds a yellow color to an alkaline solution, but if acid is added to the solution, the color remains yellow until the pH is neutral. At this point, the color changes from yellow to red. At low levels, methyl yellow does not, itself, change the acidity of a sample.

Typically, methyl yellow is used at extremely low concentrations, in the parts per million range. This small amount is sufficient to see a visible change in color, but not enough to change the sample itself. But what If an enormous amount of methyl yellow were added to a specimen? Not only might any color change be invisible, but the addition of so much methyl yellow would change the chemical composition of the sample itself.

In some cases, small samples are separated from larger volumes so that they can be tested using indicators that produce significant chemical changes.