~Picric Acid, Tri-nitro-Phenol, or Carbazotic Acid.~--Picric acid, or a tri-nitro-phenol (C_{6}H_{2}(NO_{2})_{3}OH)[2:4:6], is produced by the action of nitric acid on many organic substances, such as phenol, indigo, wool, aniline, resins, &c. At one time a yellow gum from Botany Bay (_Xanthorrhoea hastilis_) was chiefly used. One part of phenol (carbolic acid), C_{6}H_{5}OH, is added to 3 parts of strong fuming nitric acid, slightly warmed, and when the violence of the reaction has subsided, boiled till nitrous fumes are no longer evolved. The resinous mass thus produced is boiled with water, the resulting picric acid is converted into a sodium salt by a solution of sodium carbonate, which throws down sodium picrate in crystals. Phenol-sulphuric acid is now, however, more generally used, and the apparatus employed for producing it closely resembles that used in making nitro-benzol. It is also made commercially by melting carbolic acid, and mixing it with strong sulphuric acid, then diluting the "sulpho- carbolic"[A] acid with water, and afterwards running it slowly into a stone tank containing nitric acid. This is allowed to cool, where the crude picric acid crystallises out, and the acid liquid (which contains practically no picric acid, but only sulphuric acid, with some nitric acid) being poured down the drains. The crude picric acid is then dissolved in water by the aid of steam, and allowed to cool when most of the picric acid recrystallises. The mother liquor is transferred to a tank and treated with sulphuric acid, when a further crop of picric acid crystals is obtained. The crystals of picric acid are further purified by recrystallisation, drained, and dried at 100 deg. F. on glazed earthenware trays by the aid of steam. It can also be obtained by the action of nitric acid on ortho-nitro-phenol, para-nitro-phenol, and di-nitro-phenol (2:4 and 2:6), but not from meta-nitro-phenol, a fact which indicates its constitution.[B] [Footnote A: O. and p. phenolsulphonic acids. C_{3}H_{4}(OH).SO_{3}H + 3HNO_{3} = C_{6}H_{2}(NO_{2})_{3}OH + H_{2}SO_{4} + 2H_{2}O. (Picric acid).] [Footnote B: Carey Lea, _Amer. Jour. Sci._, (ii.), xxxii. 180.]