Methyl Red appears as lustrous purple crystals or reddish-brown powder. Though barely soluble in water, it dissolves freely in ethanol, chloroform and glacial acetic acid. Widely used as a pH indicator for acid-base titration, it delivers sharp color shifts to pinpoint titration endpoints accurately.
Chemical Properties
Melting point | 179-182 °C (lit.) |
Boiling point | 412.44°C (rough estimate) |
Density | 0.839 g/mL at 25 °C |
Bulk density | 300-500kg/m3 |
Vapor density | 9.3 (vs air) |
Refractive index | 1.5930 (estimate) |
Fp | 11 °C |
Storage temp. | Store at +5°C to +30°C. |
Solubility | ethanol: soluble1mg/mL |
Form | Solid |
Pka | 4.95(at 25℃) |
Colour Index | 13020 |
Color | Reddish-violet |
PH Range | 4.4(red)-6.3(yellow) |
Odor | Odorless |
Water Solubility | Practically insoluble in water. Soluble in alcohol, acetic acidSoluble in ethanol. Insoluble in water. |
λmax | 410nm, 530nm, 427nm, 519nm |
ε(extinction coefficient) | ≥10000 at 282-288nm in methanol |
Merck | 14,6119 |
BRN | 750102 |
Stability | Stable. Incompatible with strong oxidizing agents. |
Major Application | Waveguides, LED sensors, photoresists, liquid crystals, sol-gel matrix, optical sensors, paints, toys, detection of thiols, food freshness sensors, dental product, saliva sampling method, detecting lactic acid, carbohydrates |
InChIKey | CEQFOVLGLXCDCX-MSUUIHNZSA-N |
CAS DataBase Reference | 493-52-7 |
NIST Chemistry Reference | Benzoic acid, 2-[[4-(dimethylamino)phenyl]azo]-(493-52-7) |
IARC | 3 (Vol. 8, Sup 7) 1987 |
EPA Substance Registry System | Methyl red (493-52-7) |
A 0.1% ethanolic solution of methyl red turns red at pH 4.4 and shifts to yellow at pH 6.2.
It works well for titrations of ammonia and weak organic bases including alkaloids. For organic acid assays, it is only suitable for oxalic acid and picric acid.
The indicator tends to fade under reducing conditions, so test readings must be taken without delay.
Nowadays, methyl red is being progressively substituted by sulfonphthalein indicators such as bromocresol green, which boast better stability and much sharper color changes.
