pH Indicator Definition and Examples

A pH indicator or acid-base indicator is a compound that changes color in solution over a narrow range of pH values. Only a small amount of indicator compound is needed to produce a visible color change.

When used as a dilute solution, a pH indicator does not have a significant impact on the acidity or alkalinity of a chemical solution.

The principle behind the function of an indicator is that it reacts with water to form the hydrogen cation H⁺ or hydronium ion H₃O⁺. The reaction changes the color of the indicator molecule.

Some indicators change from one color to another, while others change between colored and colorless states. pH indicators are usually weak acids or weak bases. Many of these molecules occur naturally.

For example, the anthocyanins found in flowers, fruits, and vegetables are pH indicators. Plants containing these molecules include red cabbage leaves, rose petal flowers, blueberries, rhubarb stems, hydrangea flowers, and poppy flowers. Litmus is a natural pH indicator derived from a mixture of lichens.

For a weak acid with formula HIn, the equilibrium chemical equation would be:

HIn (aq) + H₂O (l) ⇆ H₃O⁺ (aq) + In^- (aq)

At a low pH, the concentration of the hydronium ion is high and the equilibrium position lies to the left. The solution has the color of the indicator HIn. At a high pH, the concentration of hydronium is low, the equilibrium is to the right, and the solution has the color of the conjugate base In^-.

In addition to pH indicators, there are two other types of indicators used in chemistry. Redox indicators are used in titrations involving oxidation and reduction reactions. Complexometric indicators are used to quantify metal cations.

Examples of pH Indicators

• Methyl red is a pH indicator used to identify pH values between 4.4 and 6.2. At low pH (4.4 and lower) the indicator solution is red. At high pH (6.2 and above) the color is yellow. Between pH 4.4 and 6.2, the indicator solution is orange.
• Bromocresol green is a pH indicator used to identify pH values between 3.8 and 5.4. Below pH 3.8 the indicator solution is yellow. Above pH 5.4 the solution is blue. Between pH values of 3.8 and 5.4, the indicator solution is green.

Universal Indicator

Because indicators change colors over different pH ranges, they may sometimes be combined to offer color changes over a wider pH range.

For example, "universal indicator" contains thymol blue, methyl red, bromothymol blue, thymol blue, and phenolphthalein. It covers a pH range from less than 3 (red) to greater than 11 (violet). Intermediate colors include orange/yellow (pH 3 to 6), green (pH 7 or neutral), and blue (pH 8 to 11).

Uses of pH Indicators

pH indicators are used to give a rough value of pH of a chemical solution. For precise measurements, a pH meter is used.

Alternatively, absorbance spectroscopy may be used with a pH indicator to calculate the pH using Beer's law. Spectroscopic pH measurements using a single acid-base indicator are accurate to within one pKa value. Combining two or more indicators increases the accuracy of the measurement.

Indicators are used in a titration to show the completion of an acid-base reaction.