To 3600 g. of concentrated sulfuric acid, in a 5-l. flask placed
in an empty water bath, are added 360 g. of technical trinitrotoluene,
while the mixture is stirred mechanically. Sodium dichromate
(Na2Cr2O7 2H2O) is now added in small quantities (PRECAUTION: see Notes),
with constant stirring, until the temperature of the mixture reaches 40'0;
the empty water bath is now filled with cold water and the addition
of sodium dichromate continued at such a rate that the temperature
remains at 45-55'0. In all 540 g. of sodium dichromate are added,
the addition taking one to two hours. When all has been added,
the mixture, which has now become very thick, is stirred for two
hours at 45-55'0, and poured into a crock containing 4 kg.
of crushed ice. The insoluble trinitrobenzoic acid is filtered off,
and carefully washed with cold water until free from chromium salts.
On drying it weighs 320-340 g.
The product is now mixed with 2 l. of distilled water at 35'0
in a 5-l. flask provided with a stirrer, and 15 per cent sodium
hydroxide solution is dropped in with continuous stirring until
a FAINT red color is just produced. Should this disappear,
it is restored by the addition of a few drops more. When it has
persisted for five minutes, the color is discharged by the addition
of a few drops of acetic acid, and the insoluble unattacked
trinitrotoluene filtered off and washed with a little water.
The trinitrobenzoic acid is precipitated from the filtrate
by the addition of a slight excess of 50 per cent sulfuric acid.
The solution is chilled, and the acid filtered and washed free
from salts with ice water. When dried in air it weighs 230-280 g.
(57 to 69 per cent of the theoretical amount).