Methyl red

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Methyl red
Methyl red sample solution.jpg
Methyl red in solid form and as solution at pH < 4.4 (red), pH 4.4-6.2 (orange) and pH > 6.2 (yellow)
Names
IUPAC name
2-(N,N-dimethyl-4-aminophenyl) azobenzenecarboxylic acid
Preferred IUPAC name
2-{[4-(Dimethylamino)phenyl]diazenyl}benzoic acid
Other names
C.I. 13020
C.I. Acid Red 2
Properties
C15H15N3O2
Molar mass 269.30 g/·mol
Appearance Deep red solid
Odor Odorless (fresh)
Faint fish-like (old)
Density 1.31 g/cm3
Melting point 179–182 °C (354–360 °F; 452–455 K) (decomposes)
Boiling point Decomposes
Almost insolule
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
Hazards
Safety data sheet FisherSci
Related compounds
Related compounds
Methyl orange
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
Infobox references

Methyl red is an azo dye indicator used for determining the pH of a solution. It has the chemical formula C15H15N3O2.

Properties

Chemical

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.

Physical

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.[1]

Availability

Methyl red can be purchased from chemical suppliers, in both powdered and solution form. Can also be found online.

Preparation

Methyl red can be prepared by diazotization of anthranilic acid with hydrochloric acid and sodium nitrite, followed by reaction with dimethylaniline.

Projects

Handling

Safety

Like most dyes, methyl red will stain most objects and due to its insolubility in water it's very difficult to wash.

Methyl red is classed by the IARC in group 3, and not considered to cause cancer in humans.

Storage

Methyl red should be stored in closed amber bottles, away from light and acids.

Disposal

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.

References

  1. Neuhaus; Zeitschrift fuer Kristallographie, Kristallgeometrie, Kristallphysik, Kristallchemie; vol. 103; (1941); p. 297,303

Relevant Sciencemadness threads