What is a universal indicator?

	Vocabulary
	anthocyanin acid alkaline base indicator pH solution universal indicator

A universal indicator is a solution which undergoes several color changes over a wide range of pH's. The color is used to "indicate" pH directly. Universal indicators are usually mixtures of several indicators.

The easiest universal indicator to prepare is red cabbage juice. Stew red cabbage in water and pour off the water for use as an indicator solution. The pigment in red cabbage juice is anthocyanin, which changes color from red in acid solution to purplish to green in mildly alkaline solution to yellow in very alkaline solution. The color changes for red cabbage juice are shown to the right.

Another universal indicator is used in the classic 'chemical stoplight' demonstration, which changes from green to yellow to red as the pH goes from alkaline to acidic. The indicator is a solution that is 0.05% in each of the following: methyl red, methyl yellow, thymol blue, and bromthymol blue in ethanol[1].

References

This recipe is taken from J. T. Riley, J. Chem. Ed., (54), 29 (1977).

Author: Fred Senese senese@antoine.frostburg.edu

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