The sulfonation of phenol at low temperatures produces the o-sulfonic acid, and at high temperatures the p-sulfonic acid along with more or less of the di- and even of the trisulfonic acids according to the conditions of the reaction. All these substances yield picric acid as the final product of the nitration.

Unless carefully regulated the production of picric acid from phenol is accompanied by losses, either from oxidation of the material with the production of red fumes which represent a loss of fixed nitrogen or from over sulfonation and the loss of unconverted water-soluble nitrated sulfonic acids in the mother liquors. Olsen and Goldstein have described a process which yields 220 parts of picric acid from 100 parts of phenol. In France, where dinitrophenol was used during the first World War in mixtures with picric acid which were loaded by pouring, Marqueyrol and his associates have worked out the details of a four-stage process from the third stage of which dinitrophenol may be removed if it is desired. The steps are: (1) sulfonation; (2) nitration to the water-soluble mononitrosulfonic acid; (3) nitration to dinitrophenol, which is insoluble in the mixture and separates out, and to the dinitrosulfonic acid which remains in solution; and (4) further nitration to convert either the soluble material or both of the substances to picric acid. The process is economical of acid and gives practically no red fumes, but the reported yields are inferior to those reported by Olsen and Goldstein. The

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