In chemistry, an indicator is defined as a substance that undergoes distinct observable change when the conditions of its solution change. Litmus is the most commonly used indicator in the laboratory.
Indicators are substances that change color when they are added to acidic or alkaline solutions. The litmus indicator turns red in acidic solutions and blue in alkaline solutions. Universal indicator is a mixture of several different indicators. Universal indicator can exactly determine how strongly acidic or alkaline a particular solution is. In strongly acidic solutions, the universal indicator turns red whereas in strongly alkaline solutions, the universal indicator turns dark purple. In a neutral solution, the universal indicator turns green.
An indicator is used in titration to determine the endpoint of a reaction. Titration is a laboratory technique that is used to determine the unknown concentration of a reactant. As titration is a delicate process, indicators must be chosen carefully by taking into consideration the pH at the equivalence point of a titration. Methyl orange is an indicator whose color changes from red to orange in the acidic pH range of 3.1 to 4.4. Phenolphthalein is an indicator whose color changes from colorless to pink in the alkaline pH range of 8.0 to 10.0.