Methylorange

• Sodium -4-( dimethylamino) - azobenzene -4' -sulfonic acid
• Helianthin
• Orange III
• Gold Orange
• Tropaeolin D
• C. I. 13025
• C. I. Acid Orange 52

Orange solid

Fixed

1.28 g · cm -3

> 300 ° C

Poorly in water (approximately 5 g · l -1)

Risk

60 mg · kg -1 ( LD50, rat, oral)

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Methyl orange belongs to the group of azo dyes. It is the sodium salt of 4'-( dimethylamino) -azobenzene -4 -sulfonic acid and is most commonly used as a pH indicator.

Properties

Methyl orange is orange crystals. In a strongly acidic pH range, the azo group is protonated. In addition, the base group is deprotonated in the pH range from 3.1 to 4.4 and there is a color change from red to yellow orange.

Use

Methyl orange is used as the acid-base indicator.

Synthesis

Sulfanilic acid is diazotized with nitrous acid and then converted to 4- Benzoldiazoniumsulfonsäure:

Then the 4- Benzoldiazoniumsulfonsäure with N, N -dimethylaniline (green) is coupled and transferred to the sodium salt with sodium hydroxide solution and precipitated:

The resulting sodium salt - at the same time, an azo dye - is methyl orange.