Methyl orange
The molecule methyl orange is commonly used as an indicator in acid-base equilibrium reactions. In base form, on the left in the figure, the color is yellow. Adding a proton yields the structure on the right, colored red. Note that this color change occurs over the pH range from approximately 3-4.
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"Chemical Principles/Solution Equilibria: Acids and Bases."
http://en.wikibooks.org/wiki/Chemical_Principles/Solution_Equilibria:_Acids_and_Bases%23Indicators
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