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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.
An acid-base indicator is a weak acid or a weak base. The undissociated form of the indicator is a different color than the iogenic form of the indicator. An Indicator does not change color from pure acid to pure alkaline at specific hydrogen ion concentration, but rather, color change occurs over a range of hydrogen ion concentrations. This range is termed the color change interval. It is expressed as a pH range.