Universal Indicator Definition

Universal indicator papers over different pH values.
Universal indicator papers over different pH values. GUSTOIMAGES / Getty Images

A universal indicator is a blend of pH indicator solutions designed to identify pH of a solution over a wide range of values. There are several different formulas for universal indicators, but most are based on a patented formula developed in 1933 by Yamada. A common mixture includes thymol blue, methyl red, bromothymol blue, and phenolphthalein.

Color change is used to identify pH values. The most common universal indicator colors are:

Red 0 ≥ pH ≥ 3
Yellow 3 ≥ pH ≥ 6
Green pH = 7
Blue 8 ≥ pH ≥ 11
Purple 11 ≥ pH ≥ 14

However, the colors are specific to the formulation. A commercial preparation comes with a color chart that explains the expected colors and pH ranges.

While a universal indicator solution may be used to test any sample, it works best on a clear solution because it's easier to see and interpret the color change.

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Helmenstine, Anne Marie, Ph.D. "Universal Indicator Definition." ThoughtCo, Dec. 9, 2017, thoughtco.com/definition-of-universal-indicator-605761. Helmenstine, Anne Marie, Ph.D. (2017, December 9). Universal Indicator Definition. Retrieved from https://www.thoughtco.com/definition-of-universal-indicator-605761 Helmenstine, Anne Marie, Ph.D. "Universal Indicator Definition." ThoughtCo. https://www.thoughtco.com/definition-of-universal-indicator-605761 (accessed May 21, 2018).