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.