Methyl Red

Updated: 2026-08-30

Overview

Methyl Red is an azo dye that serves as a pH indicator, changing color from red to yellow as pH increases. First synthesized in the early 20th century, it has become a standard laboratory reagent with particular importance in microbiology and analytical chemistry. The compound's full chemical name is 2-(N,N-dimethyl-4-aminophenyl) azobenzenecarboxylic acid. Its reliable color transition between pH 4.4 (red) and 6.2 (yellow) makes it valuable for visual endpoint determination in titrations and microbial culture media.

Physical and Chemical Properties

As an azo compound, Methyl Red exhibits characteristic stability and vivid coloration. The crystalline powder has limited water solubility but dissolves well in organic solvents like ethanol and acetic acid, which are commonly used to prepare indicator solutions. The dye's color change mechanism involves protonation/deprotonation of the carboxylic acid group and associated changes in electron delocalization. In strongly acidic conditions, the protonated form appears red, while the deprotonated yellow form dominates in neutral to slightly acidic solutions.

Main Applications

In microbiology laboratories, Methyl Red is essential for the Methyl Red-Voges Proskauer (MR-VP) test, which differentiates enteric bacteria based on their glucose fermentation pathways. The test helps identify Escherichia coli and other members of the Enterobacteriaceae family. Industrial applications include use as a pH indicator in various chemical processes and quality control procedures. Some specialized formulations incorporate Methyl Red in pH test papers and diagnostic kits where visual pH determination is required.

Safety and Storage

While not extremely hazardous, Methyl Red requires proper handling precautions. Laboratory personnel should wear gloves and eye protection to prevent skin contact and eye irritation. The powder can generate dust, so work should be conducted in well-ventilated areas or under fume hoods. For storage, keep containers tightly sealed in cool (below 25°C), dry conditions away from strong light. Shelf life typically exceeds three years when stored properly. Contamination with strong acids or bases should be avoided as it may affect the indicator's performance.

B2B Procurement Guide

When sourcing Methyl Red commercially, buyers should specify required purity levels (typically 95-99% for laboratory use) and confirm the CAS number (493-52-7) to ensure product authenticity. Packaging options range from small 25g bottles to kilogram quantities for industrial users. Quality certifications like ACS grade or ISO compliance may be required for certain applications. Lead times vary by supplier, with specialty chemical distributors often maintaining stock while manufacturers may require production scheduling. Bulk purchases generally offer better unit pricing but require verification of storage capabilities.