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MANUFACTURE OF PICRIC ACID 511

FIG. 117. General view of nitrators for the production of picric acid [8].

The mixture of phenolsulphonic acids is diluted with 737.5 1. of water. Usually

wash-waters are used for this purpose. The concentration of phenolsulphonic acid

in the resulting solution is 18.4% calculated on phenol.

Nitration is carried out in two stages. In the first stage the nitrator is filled with

a nitrating mixture composed of 70% HNO 3 , 10% H 2 SO 4 and 20% H 2 O. The

quantity of the mixture corresponds to that calculated for the introduction of one

nitro group. Phenolsulphonic acid is added to the nitrating mixture at 30°C at the

beginning and at 52°C at the end. During the first stage mononitro derivatives are

formed.

After the first stage the temperature in the nitrator is raised to 60°C and the

second stage begins. It consists in adding the same nitrating mixture. During the

first hour of mixing the temperature increases to 110-115°C. Altogether 1250 kg

of nitrating mixture are added (i.e. 2.75 kg of HNO 3 for 1 kg of phenol).

This method is not accompanied by the evolution of large quantity of nitrogen

oxides. The nitration is ended when the content of the nitrator changes colour

from red (the colour of phenolsulphonic acids) to yellow (the colour of picric acid).

The yield is 667.6 kg of picric acid, i. e. 220% by weight (theoretical yield 243%).

The spent acid contains 1.0% and 1.5% of picric and oxalic acid respectively.

It is diluted with double its weight of water; water from washing picric acid is being