A universal indicator is a substance that changes color based on the pH level of a solution. It is used to determine the acidity or base level of a substance. Universal Indicators are made up of a mixture of substances, but common indicators are litmus, phenolphthalein and methyl orange.
Litmus is obtained from certain lichens and is a commonly used acid-base indicator due to the wide range over which the color change occurs. The color change begins at around pH7, which is exactly half way along the pH scale, making it useful for detecting acids and bases. The color ranges from red in acid, changing to blue in alkaline.
Phenolphthalein changes color from colorless to pink at around pH 8.3 to 10, which makes it a useful test for substances that are highly alkaline. Methyl orange is the opposite of phenolphthalein in that it changes color between pH 3.1 and 4.4, making it useful for detecting substances that have a high acidity. The colors are yellow in the base and red in acid.
Liquid pH Universal Indicators work by putting a few drops of the solution into the substance that the user is testing. The color that appears as a result of the reaction gives the level of acidity within the substance.