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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.
1 An acid-base indicator (e.g., phenolphthalein) changes color depending on the pH. 2  Redox indicators are also used. 3  A drop of indicator solution is added to the titration at the beginning; the endpoint has been reached when the color changes. Indicator: A substance that changes color in response to a chemical change. 2  An acid-base indicator (e.g., phenolphthalein) changes color depending on the pH. 3  Redox indicators are also used. 4  A drop of indicator solution is added to the titration at the beginning; the endpoint has been reached when the color changes.