Title: Methyl Red
CAS Registry Number: 493-52-7
CAS Name: 2-[[4-(Dimethylamino)phenyl]azo]benzoic acid
Additional Names: C.I. Acid Red 2; C.I. 13020
Molecular Formula: C15H15N3O2
Molecular Weight: 269.30
Percent Composition: C 66.90%, H 5.61%, N 15.60%, O 11.88%
Literature References: Prepn by diazotization of anthranilic acid and coupling with dimethylaniline: Clarke, Kirner, Org. Synth. 2, 47 (1922). See also Colour Index vol. 4 (3rd ed., 1971) p 4043.
Properties: Glistening, violet crystals from toluene. mp 181-182°. pKa1 2.5; pKa2 9.5; pKb 4.8. Almost insol in water; sol in alcohol and in acetic acid.
Melting point: mp 181-182°
pKa: pKa1 2.5; pKa2 9.5; pKb 4.8
Use: As indicator in 0.1% alcoholic soln; pH: 4.4 red, 6.2 yellow. Used for titrating NH3, weak organic bases, e.g., alkaloids; not suitable for organic acids, except oxalic and picric acid. Methyl red is easily reduced, thereby losing its color, and readings should be made promptly. It is gradually being replaced by sulfonphthalein indicators, such as bromcresol green, which are more stable and exhibit a sharper change in color.