CARBAZOTIC or PICRIC ACID. (Fs., Acide picrique ; GER., Pikrinsaure.) Formula This interesting and important substance is formed by the action of nitric acid upon carbolic acid, indigo, aloes, and a large number of other organic substances. It has become a very useful industrial product since the discovery and utilization of its remarkable colouring powers, and is now manufactured in considerable quantities from certain products of the distillation of coal, which contain carbolic acid.
Much care is required in the selection of the coal-tar oils employed, as those which are distilled from the coal at a, low temperature contain substances which, on treatment with nitric acid, give rise to products not only useless for dyeing purposes, but detrimental to the crystallization of the picric acid. On this account, only those which distil over at about 180° or 200° (356 to 392 F.), and whose density is comparatively high, should be chosen, the best being those whose boiling point is between 190° and 200° (374 and 392 F.). The manufacture is at present carried on as follows: Tho oils containing carbolic acid are mixed with LI strong solution of caustic potash or soda ; the alkali immediately unites with the carbolic acid, forming a dense liquid, which separates itself from the remaining portion of the oil, and forms a layer at the bottom of the vessel. If the oils are rich in carbolic acid, one-third of their weight of alkali is sufficient, but if they are poor the amount of alkali must be decreased in proportion. The alkaline phenate is withdrawn from the bottom of the vessel, and decomposed by means of hydrochloric or sulphuric acid, when the carbolic acid floats upon the surface of the solution of alkaline chloride or sulphate formed, and is drawn off for treatment with nitric acid. As the reaction between the carbolic and nitric acids is very violent, care is required when large quantities are employed, not only on account of the inflammable nature of the oils used, but the disengagement of gas is so copious and rapid that there is danger of the liquor being thrown over the sides of the vessel in which the reaction is carried on. Another important point is the amount of nitric acid employed; it depends entirely upon the richness of the oils, rectified as above, in carbolic acid, from six to eight times their weight being the average quantity required. The operation is usually carried on in an earthenware vessel, capable of holding from 12 to 15 gallons ; this vessel is fitted with two tubulures, and is placed in a metal dish filled with flue sand, and heated by a fire from below. In one of the tubulures is placed a glass funnel, the outlet of which is very narrow, in order to prevent the too rapid entrance of the acid ; the other is fitted with a curved glass tube, to carry away the nitrous fumes given off, and is attached to a suitable condensing apparatus. About 40 lb. of nitric sp. gr. 1.31, are run into the vessel,
which is very gradually and gently heated to about 50° (122 F.), at which point the heat is removed ; about 12 or 13 lb. of the rectified oil are now introduced in successive portions by means of the funnel, and on contact with the nitric acid a brisk effervescence ensues, large quantities of gas being given off and the temperature rising considerably. The picric acid dissolves as quickly as it is formed in the excess of nitric acid, small quantities of the latter being added from time to time in order to cool the mixture ; the reaction is allowed to continue for twelve hours, and, after having become perfectly cool, the mixture is again heated to 50°, and about 28 lb. of nitric acid are added. Another twelve hours is sufficient to complete the reaction, which is hastened, towards the end, by gentle heating from below. When all effervescence has ceased, the excess of nitric acid is distilled off, and condensed for fubire use, and the solution of picric acid is evaporated iu large reirthenware vessels, heated on a sand-bath to the consistence of honey, when it is left to cool. The thick yellow page thus obtained, after being washed free from nitric acid, is dissolved in boiling water, a little weak eulphuric acid being added in order to dissolve the resinous matter present. The solution is filtered, and set out in pans to crystallize; the re-allting crystals ere generally very impure, and are dissolved end re-crystallized until they are of a delicate lemon-yellow colour. These are di led and packed in casks for the use of the dycr.
Picrio acid is bitter to the taste, and very poisonous; it is soluble in wnt,er, alcohol, and ether, forming yellow solutions. Its colouring properties were first discovered by 31. Guinon, of Lyons ; it is used for dyeing silk and woollen goods a brigbt yellow, alum being employed as a mordant. Cotton and flaxen goods are not dyed by picric acid. Some alkaline pierates have been einployed instead of the acid hy some dyers, though their highly explosive nature renders them quite unfit for these purposes. A mixture of picrate of potash, chlorato (or nitrate) of potash, and charcoal iS used in France as an explosive for torpedoes, under the name of Design°lle's powder ; in this country, pierate of ammonia, nitrate of potash, and charcoal have been used as a bursting powder for shells, under the name of Abel's "picric powder" (see" Explosive Agents"). It has been said that picric acid was formerly largely used to adulterate beer, for the purpose of heightening the bitterness of its flavour.