A pH indicator is a chemical compound that is added in small amounts to a solution so that the pH (acidity or alkalinity) of the solution can be determined easily. Hence a pH indicator is a chemical detector for protons (H⁺). Normally, the indicator causes the color of the solution to change depending on the pH.

pH indicators themselves are frequently weak acids or bases. When introduced into a solution, they may bind H⁺ (Hydrogen ion) or OH^- (hydroxide) ions. The different electronic configuration of the bound indicator causes the indicator's color to change.

Because of the subjective determination of color, pH indicators are susceptible to imprecise readings. For applications requiring precise measurement of pH, a pH meter is frequently used.

pH indicators are frequently employed in titrations in analytic chemistry and biology experiments to determine the extent of a chemical reaction.

Tabulated below are several common laboratory pH indicators. Indicators usually exhibit intermediate colors at pH values inside the listed transition range. For example, phenol red exhibits an orange color between pH 6.6 and pH 8.0. The transition range may shift slightly depending on the concentration of indicator in solution and on the temperature at which it is used.

Indicator	Color at low pH	Transition pH range (approximate)	Color at high pH
Methyl violet	blue-violet	0.0-1.6	yellow
Thymol blue (first transition)	red	1.2-2.8	yellow
Methyl yellow	red	2.9-4.0	yellow
Bromophenol blue	yellow	3.0-4.6	violet
Congo red	blue	3.0-5.2	red
Methyl orange	red	3.1-4.4	yellow
Litmus	red	4.5-8.3	blue
Bromocresol purple	yellow	5.2-6.8	violet
Bromothymol blue	yellow	6.0-7.6	blue
Phenol red	yellow	6.6-8.0	red
Thymol blue (second transition)	yellow	8.0-9.6	blue
Phenolphthalein	colorless	8.2-10.0	pink
Thymolphthalein	colorless	9.4-10.6	blue
Alizarin Yellow R	yellow	10.1-12.0	red
Indigo carmine	blue	11.4-13.0	yellow

• Universal Indicator is a blend of different indicators that exhibits several smooth color changes over wide range of pH values.

• Anthocyanins are a class of compounds that occur in many different plants; they appear red in acidic solutions and blue in bases. Extracting anthocyanins from red cabbage leaves to form a crude acid-base indicator is a popular introductory chemistry demonstration.