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Methyl red (2-(N,N-dimethyl-4-aminophenyl) azobenzenecarboxylic acid) , also called C.I. Acid Red 2, is an indicator dye that turns red in acidic solutions.It is an azo dye, and is a dark red crystalline powder. Methyl red is a pH indicator; it is red in pH under 4.4, yellow in pH over 6.2, and orange in between, with a pKa of 5.1.ethyl red (2-(N,N-dimethyl-4-aminophenyl) azobenzenecarboxylic acid) , also called C.I. Acid Red 2, is an indicator dye that turns red in acidic solutions.
Acid - Base Indicators: The most common method to get an idea about the pH of solution is to use an acid base indicator. An indicator is a large organic molecule that works somewhat like a  color dye. Whereas most dyes do not change color with the amount of acid or base present, there are many molecules, known as acid - base indicators , which do respond to a change in the hydrogen ion concentration. Most of the indicators are themselves weak acids. The most common indicator is found on litmus paper.