During the solution of the trinitrobenzoic acid, the temperature should not be below 35'0, owing to the slight solubility of trinitrobenzoic acid in cold water. The heat of neutralization raises the temperature to 45-55'0, but the latter temperature should not be exceeded, since any trinitrobenzene formed at this point would later be removed with the unreacted trinitrotoluene. Care must be taken that no more alkali is added than is just sufficient to produce the faint red color. If an excess of alkali is added it produces a permanent color, which is not removed by acid and colors the final product. When once the evolution of carbon dioxide sets in, the flame must be cut down so as to avoid the formation of a thick layer of froth which might foam over. 3. Other Methods of Preparation 1,3,5-Trinitrobenzene can be prepared by heating _m_-dinitrobenzene with nitric acid and sulfuric acid to 120'0;[1] by heating 2,4,6-trinitrotoluene with fuming nitric acid in a sealed tube at 180'0 for three hours;[2] by heating 2,4,6-trinitrobenzoic acid or its sodium salt with water, alcohol, dilute sodium carbonate or other suitable solvent.[3] [1] Ber. 9, 402 (1876); Ann. 215, 344 (1882). [2] Ber. 16, 1596 (1883). [3] D. R. P. 77,353; Frdl. 4, 34 (1894). XXV 2, 4, 6-TRINITROBENZOIC ACID C6H2(NO2)3CH3 + 3O(Na2Cr2O7 + H2SO4)--> C6H2(NO2)3CO2H + H2O Prepared by H. T. CLARKE and W. W. HARTMAN. Checked by J. B. CONANT and J. J. TOOHY. 1. Procedure 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.