Chemical Properties
dark red crystalline powder
Uses
Used as pH indicator.
Definition
ChEBI: An azo dye consisting of benzoic acid substituted at position 2 by a 4-[(dimethylamino)phenyl]diazenyl group.
Uses
As indicator in 0.1% alcoholic solution; 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.
Hazard
Questionable carcinogen.
Preparation
2-Aminobenzoic acid?diazo, and N,N-dimethylaniline coupling.
General Description
Methyl red solution is an azo dye which turns to red when pH is below 4.4 (yellow pH < 6.2, orange pH 4.4-6.2). Some bacteria utilize glucose to form large amounts of acid with the result that the pH value of the medium falls distinct. Other species produce no or less free acid. This difference can be visualized by using methyl red. This test is used to differentiate enteric bacteria.
Biochem/physiol Actions
Methyl Red is a maroon red crystal azo dye. Methyl Red is a pH indicator and changes color at a pH of 5.5. Methyl Red is widely used in saliva sampling method. In addition, it is also employed in carbohydrate and lactic acid detection. Methyl Red has been effectively used for intrageneric differentiation of Enterobacteriaceae.
Properties and Applications
moderate soluble in ethanol, insoluble in water. Alcohol solution to join hydrochloric acid for purple; Add sodium hydroxide to dim yellow.
Purification Methods
The acid is extracted with boiling toluene using a Soxhlet apparatus. The crystals which separate on slow cooling to room temperature are filtered off, washed with a little toluene and recrystallised from glacial acetic acid, *benzene or toluene followed by pyridine/water. Alternatively, dissolve it in aqueous 5% NaHCO3 solution, and precipitate it from a hot solution by dropwise addition of aqueous HCl. Repeat this until the extinction coefficients do not increase. [Beilstein 16 IV 504.]
