Picric acid

• 2,4,6- trinitrophenol
• Trinitrophenol
• TNP
• Welter cal Bitter

Colorless to slightly yellow leaf or prismatic crystals that taste extremely bitter and detonate when heated

Fixed

1.76 g · cm -3

0.29

Soluble in ethanol (80 g · l-1 at 20 ° C ), benzene, little in water ( 14 g · l-1 at 20 ° C, 60 g · L-1 at 100 ° C)

Risk

Template: Infobox chemical / molecular formula search available

Picric acid (gr. πικρος, pikros = bitter) is the trivial name for 2,4,6- trinitrophenol (TNP ). The structure consists of a benzene ring with a hydroxyl group (-OH) and three nitro groups (- NO2) as a substituent. It belongs to the group of trinitrophenols, a group of six constitutional isomers. Its salts are called picrates.

History

By treating indigo with nitric Peter Woulfe could represent the first 1771 picric acid. However, addition of the yellow coloration of silk she had at first no more importance.

The substance was the first exploding, explosive floor - filler and has been used as Lyddit, ekrasit, Schimose or melinite from 1886, after the Frenchman Eugène Turpin had discovered the explosive properties of the previously known long acid.

1864 written by the German physician Wilhelm Erb a paper on Physiological and therapeutic effects of picric acid. In 1865 he habilitated with a work on this subject.

For the catastrophic Halifax Explosion in 1917 2.300 tons of picric acid were responsible.

The use of picric acid for staining of baked goods in the late 19th century was widespread and was known as Welter cal bitter, but this was prevented by an accumulation of poisoning cases.

Representation and extraction

Picric acid is produced by the sulfonation of phenol followed by treatment with nitric acid. There is, of the representation of chlorobenzene to 2,4- dinitrochlorobenzene, 2,4- dinitrophenol and its re- nitriding. A direct production of the substance is achieved by the oxynitridation of benzene by means of concentrated nitric acid in the presence of mercury ( II) nitrate. Previously picric acid was prepared from Akaroidharz.

Properties

Picric acid forms colorless to slightly yellow, strongly bitter-tasting crystals that are difficult in cold water, however, more soluble in boiling water and slightly soluble in ethanol and benzene. Due to the accumulation of electron-withdrawing nitro group ( - NO2) picric acid is through its phenolic hydroxyl group, a strong acid ( pKa = 0.29). Therefore, it forms salts with a number of inorganic and organic bases. The salts are called picrates. The result with ammonia, the ammonium picrate.

As a strong acid attacks aqueous picric acid base metals to under Pikratbildung.

Picric acid burns in the air with thick smoke; at a very rapid heating or by a detonation occurs detonation. Picric acid is sensitive to thermal (heat, fire) and mechanical ( impact, friction ) and load, as defined in the Explosives Act as a potentially dangerous substance. For shipping to be used as a laboratory chemical (see below) the crystallized acid is stabilized ( " desensitized " ) by adding a little water.

Some of the salts, for example, lead picrate are extremely sensitive to shock, friction and sparks. They thus behave as primary explosives. Ammonium was used as an explosive.

Also as picrates refers to the charge-transfer complexes formed with picric acid aromatics. These solids are often poorly soluble and colored. Because of the characteristic and sharp melting points (eg benzene picrate 84 ° C, toluene - picrate 88 ° C, anthracene picrate 138 ° C ) was used especially in the past as a detection reagent for aromatic compounds picric acid.

Picric acid is toxic. On the skin it can cause severe allergic reactions. The contamination with dusts or vapors should be avoided.

Use

Primarily serves the picric acid the dye industry for the preparation of 2 -amino-4 ,6-dinitrophenol ( picramic acid ). It was formerly used together with gum arabic and distilled water to make yellow ink. Another field of application is the analysis for the detection of organic amines, alkaloids and creatinine. These basic substances form yellow salts which were characterized by their melting point ( derivative formation).

In histology picric acid is used in the fixatives after Bouin ( Bouinsche solution)

The use of picric acid as a filler material for grenades ( as in the First World War) was discontinued because of the uncontrolled formation of very shock-sensitive Schwermetallpikraten. The picric acid was replaced here by TNT. In microscopy using picric acid as a constituent of fixatives ( for preservation of cellular structures ) and for staining preparations. Another application of picric acid is metallography. Here, the substance is used for etching metal surfaces, for example in the preparation of magnesium alloys, or steels at Seigerungsuntersuchungen. The etching of steels with Igeweskys reagent, a 5 - % solution of picric acid in anhydrous alcohol, carried out. Picric acid is also used to creatinine concentration measurement: Creatinine forms in alkaline solution with picric acid a Meisenheimer complex ( Jaffé reaction), whose red color is measured photometrically.

Terms of Use

Picric acid is within the meaning of the Explosives Act as explosive substance of the substance group A ( dry) or C ( with 25% water moistened ) in accordance with § 1 para 3 Explosives Act classified. For individuals dry picric acid is thus in accordance with § 27 Explosives permit. Dry picric acid in 1.1 or camp group I or as dangerous goods in Class 1.1 (Substances which have a mass explosion hazard ) is classified, moistened with 30% water in storage group 1.4.

As a commercial product Picric acid moistened with > 30 % water and desensitized it. Moistened (> 30 % water) to picric acid behaves as a flammable solid and is used for transport as flammable solid dangerous goods in Class 4.1 according to ADR.