Acid-Base Indicator Definition and Examples

pH Indicators in Chemistry

Beakers of acidic and basic solutions after litmus indicator has been added. Litmus changes its color depending on the pH of the solution.
Beakers of acidic and basic solutions after litmus indicator has been added. Litmus changes its color depending on the pH of the solution. GIPhotoStock / Getty Images

Acid-Base Indicator Definition

An acid-base indicator is either a weak acid or weak base that exhibits a color change as the concentration of hydrogen (H+) or hydroxide (OH-) ions changes in an aqueous solution. Acid-base indicators are most often used in a titration to identify the endpoint of an acid-base reaction. They are also used to gauge pH values and for interesting color-change science demonstrations.

Also Known As: pH indicator

Acid-Base Indicator Examples

Perhaps the best known pH indicator is litmus. Thymol Blue, Phenol Red and Methyl Orange are all common acid-base indicators. Red cabbage can also be used as an acid-base indicator.

How an Acid-Base Indicator Works

If the indicator is a weak acid, the acid and its conjugate base are different colors. If the indicator is a weak base, the base and its conjugate acid display different colors.

For a weak acid indicator with the genera formula HIn, equilibrium is reached in the solution according to the chemical equation:

HIn(aq) + H2O(l) ↔ In-(aq) + H3O+(aq)

HIn(aq) is the acid, which is a different color from the base In-(aq). When the pH is low, the concentration of the hydronium ion H3O+ is high and equilibrium is toward the left, producing the color A. At high pH, the concentration of H3O+ is low, so equilibrium tends toward the right side of the equation and color B is displayed.

An example of a weak acid indicator is phenolphthalein, which is colorless as a weak acid, but dissociates in water to form a magenta or red-purple anion. In an acidic solution, equilibrium is to the left, so the solution is colorless (too little magenta anion to be visible), but as pH increases, the equilibrium shifts to the right and the magenta color is visible.

The equilibrium constant for the reaction may be determined using the equation:

KIn = [H3O+][In-] / [HIn]

where KIn is the indicator dissociation constant. The color change occurs at the point where the concentration of the acid and anion base are equal:

[HIn] = [In-]

which is the point where half of the indicator is in acid form and the other half is its conjugate base.

Universal Indicator Definition

A particular type of acid-base indicator is a universal indicator, which is a mixture of multiple indicators that gradually changes color over a wide pH range. The indicators are chosen so mixing a few drops with a solution will produce a color that can be associated with an approximate pH value.

Table of Common pH Indicators

Several plants and household chemicals can be used as pH indicators, but in a lab setting, these are the most common chemicals used as indicators:

IndicatorAcid ColorBase ColorpH RangepKIn
thymol blue (first change)redyellow 1.5
methyl orangeredyellow 3.7
bromocresol greenyellowblue 4.7
methyl redyellowred 5.1
bromothymol blueyellowblue 7.0
phenol redyellowred 7.9
thymol blue (second change)yellowblue 8.9
phenophthaleincolorlessmagenta 9.4

The "acid" and "base" colors are relative.

Also note some popular indicators display more than one color change as the weak acid or weak base dissociates more than once.