Universal Indicator and Litmus

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A range of solutions with Universal Indicator
A test for acidity or alkalinity with Litmus paper

Universal Indicator and Litmus are the two most common pH indicators in laboratories.

  • Universal Indicator is a pH indicator composed of a solution of several compounds that exhibits several smooth color changes over a pH value range from 0 to 14 to indicate the acidity or alkalinity of solutions.
  • Litmus is a dye made from lichens. The main use of litmus is to test whether a solution is acidic or basic. Wet litmus paper can also be used to test for water-soluble gases that affect acidity or alkalinity; the gas dissolves in the water and the resulting solution colors the litmus paper. For instance ammonia gas, an alkaline, colors the red litmus paper blue.

Usage[edit]

Both Litmus and Universal Indicator are used as a liquid form or in the form of a paper strip:

  • The paper form of Litmus is a strip of colored paper that changes color to red if the solution is acidic and to blue if the solution is basic. The strip can be placed directly onto a surface of a wet substance or few drops of the solution can be dropped onto the strip using dropping equipment.
  • The main components of liquid form for Universal Indicator are thymol blue, methyl red, bromothymol blue, and phenolphthalein. Litmus is rarely used in liquid form. This mixture is important because each component loses or gains protons depending upon the acidity or alkalinity of the solution being tested. It is best to use this type of a universal indicator in a colorless solution. This will increase the accuracy of the indicator.