Methyl red (methyl red) is a commonly used acid-base indicator.

Appearance and properties: dark red crystalline powder

Density: 0.839 g/mL at 25oC

Boiling point: 479.5oC at 760mmHg

Melting point: 178-182oC

Flash point: 11oC

Refractive index: 1.592

Stability: Stable. Incompatible with strong oxidizing agents.

Storage condition: -20oC

Vapor pressure: 2.89E-10mmHg at 25°C

Production method

1. Obtained by diazotization of anthranilic acid and reaction with N, n-dimethylaniline.

2. Add 12kg water and 9.1L industrial hydrochloric acid to 5kg anthranilic acid, stir to mix thoroughly, and then directly add cold to 0℃: The pre-prepared 34% of industrial sodium nitrite solution (8kg) is slowly added to the bottom of the container containing anthranilic acid hydrochloride under stirring, and the diazotization reaction is carried out, and the reaction temperature is controlled at 0 ~ 5℃: When the sodium nitrite solution is used to remove about 5kg, the starch potassium iodide test paper and Congo red test paper are continuously tested until it turns blue, stop adding sodium nitrite solution, continue to stir for 10min, and then the test is still blue, and the reaction reaches the end point. After that, a solution of 40g urea and 60ml water is added. Half an hour later, 6.1L dimethylaniline was added under rapid agitation, at the same time, ice was added to keep the solution temperature at 0 ~ 5℃, stirring for 2 ~ 3H, adding 5kg of crystalline sodium acetate, continuing to stir for 3 ~ 4h: let it sit overnight, suction filtration. The resulting crystals were divided into 150kg water and 2.5kg sodium carbonate, heated by steam and filtered, the filtrate was added with 1:1 hydrochloric acid solution to the Congo red test paper to turn blue, a large amount of crystallization was precipitates, after drying, 6 times pure water was added, heated by steam, and then 1:1 hydrochloric acid solution was added, dried, after passing the test, 3 times ethanol was added, and the reflux was 3 ~ 3 4h, after cooling crystallization is the finished product.

Usage:

Methyl red is one of the commonly used acid-base indicators, the usual concentration is 0.1% ethanol solution,pH4.4(red)-6.2(yellow). It is also used to stain live protozoa.