6119. Methyl Red

Nomenclature

CAS number: 493-52-7
2-[2-[4-(Dimethylamino)phenyl]diazenyl]benzoic acid; 2-[[4-(dimethylamino)phenyl]azo]benzoic acid; C.I. Acid Red 2; C.I. 13020.
C15H15N3O2; mol wt 269.30.
C 66.90%, H 5.61%, N 15.60%, O 11.88%.

Description and 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.

Chemical structure

Properties

Glistening, violet crystals from toluene. mp 181-182°. d 1.31. pKa1 2.5; pKa2 9.5; pKb 4.8. Almost insol in water; sol in alcohol and in acetic acid.

Derivative

Sodium salt.

Nomenclature

CAS number: 845-10-3
C15H14N3NaO2; mol wt 291.28.
C 61.85%, H 4.84%, N 14.43%, Na 7.89%, O 10.99%.

Derivative

Hydrochloride.

Nomenclature

CAS number: 63451-28-5
C15H15N3O2.HCl; mol wt 305.76.
C 58.92%, H 5.27%, N 13.74%, O 10.47%, Cl 11.60%.

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. Prepn of agar plates in microbiology. Food science applications include assessment of food penetration into packaging and pork quality during cold storage.