Methyl red in solid form and as solution at pH < 4.4 (red), pH 4.4-6.2 (orange) and pH > 6.2 (yellow)
| IUPAC name
2-(N,N-dimethyl-4-aminophenyl) azobenzenecarboxylic acid
| Preferred IUPAC name
| Other names
C.I. Acid Red 2
|Molar mass||269.30 g/·mol|
|Appearance||Deep red solid|
|Odor|| Odorless (fresh)|
Faint fish-like (old)
|Melting point||179–182 °C (354–360 °F; 452–455 K) (decomposes)|
|Solubility|| Soluble in glacial acetic acid, benzene, chloroform, ethanol, formic acid, lipids|
Slight soluble in acetone
Almost insoluble in petroleum ether, toluene
|Vapor pressure||~0 mmHg|
|Safety data sheet||FisherSci|
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
Methyl red is an azo dye indicator used for determining the pH of a solution. It has the chemical formula C15H15N3O2.
Methyl red turns red in pH below 4.4, orange between 4.4 and 6.2 (more clear at 5.1) and yellow in pH over 6.2.
Methyl red is a deep red solid, almost insoluble in water, but more soluble in organic solvents, such as glacial acetic acid, hot acetone, benzene, chloroform, ethanol, formic acid, as well as lipids. While odorless, old samples have a faint fish-like smell. Although in some literature its density is given the value of 0.791 g/cm3, in reality the value 1.31 g/cm3 is the correct one, as methyl red sinks when added to water.
Methyl red can be purchased from chemical suppliers, in both powdered and solution form. Can also be found online.
Like most dyes, methyl red will stain most objects and due to its insolubility in water, it's very difficult to wash off.
Methyl red is classed by the IARC in group 3, and not considered to cause cancer in humans.
Methyl red should be stored in closed amber bottles, away from light and acids.
Methyl red can be neutralized with a strong oxidizing solution, such as piranha solution, chromic acid or Fenton's reagent, followed by neutralization and then poured down the drain after all the methyl red has been neutralized. Since most of methyl red neutralized this way is either as solution or traces, there is little risk or splashing.
- Neuhaus; Zeitschrift fuer Kristallographie, Kristallgeometrie, Kristallphysik, Kristallchemie; vol. 103; (1941); p. 297,303