576 PICRIC ACID 



PHTHaiiXC ACID. A crystallised substance produced by the action of nitric 

 acid on rubian. See MADDER. 



PHYTOGRAPHY. See NATURE-PRINTING. 



PI ASS ABA FIBRE, obtained from the Attalea funifera, and used in Brazil for 



rope-making. It is this palm which also yields Coquilla nuts. 



PICAIKEARS. A colourless oil in wood-tar, discovered by Keichenbach. See DIS- 

 TILLATION, DESTRUCTIVE ; NAPHTHA ; PYROXILIC SPIRIT. 



PICKXiES are various kinds of vegetables and fruits preserved in vinegar. The 

 preparation of pickles belongs rather to a book on cookery. The peculiar and 

 beautiful green colour which has been frequently imparted to pickles is due in 

 nearly all rases to the use of a salt of copper. This is in the highest degree injurious, 

 and cannot be too strongly deprecated. The presence of copper may be detected by 

 putting the blade of a perfectly clean knife, or still better, a polished piece of soft 

 iron, into the suspected pickle ; it will, if copper be present, become coated in a short 

 time with a cupreous film. It is satisfactory to find that most of our large pickle- 

 manufacturers are content to sacrifice the colour, at one time so much looked 

 to ; and they now furnish the public with pickles which are free from any metallic 

 contamination. 



PICOIiINE, C 12 H 7 N (C 6i SL'V). A nitryle base, isomeric with aniline, discovered 

 by Anderson in coal-naphtha and bone-oil. It is also contained in the shale-naphtha 

 and crude chinoline. 



PICRIC ACID. One of the products of the action of nitric acid upon phenol : 

 it may also be obtained from a considerable number of other organic compounds, 

 amongst which may be mentioned an Australian resin, from Xanthorrhcea hustUis (Sten- 

 house), salicin, indigo, &c ; but the best source is undoubtedly impure phenol, or even 

 the coal-tar oils that distil at 180 and 200 Cent. (Laurent). See CARBOLIC ACID. 



The reaction between nitric acid and phenol is very violent. It is therefore 

 necessary to observe many precautions when operating upon considerable quantities 

 of material. When the first violent action has ceased, fresh quantities of nitric acid 

 are added, and the mixture is heated in order to facilitate the reaction. On allowing 

 the mixture to cool, after having added water, a yellowish, very bitter mass is 

 obtained, which is washed with water in order to remove the excess of nitric acid. 

 This mass consists of impure picric acid, and treated with cold or hot water, it 

 furnishes solutions which, when filtered, may be employed for the ordinary processes 

 of dyeing. 



It is, however, preferable to purify the acid and to prepare it in the crystallised 

 condition. 



For this purpose two processes may be employed. The yellow mass may be 

 extracted by boiling water sufficiently acidulated with sulphuric acid to render com 

 paratively insoluble the yellow resinous matters. (These yellow matters are produced 

 by an incomplete transformation, partly of the phenol, but principally of the neutral 

 oils, and other foreign matters accompanying it, and which have also been attacked by 

 the nitric acid.) The picric acid crystallises from the solution (the more easily for 

 being acidulated with sulphuric acid), and is deposited in the form of crystalline 

 plates of a light yellow colour. But these crystallisations cause the loss of a con- 

 siderable quantity of substance, and by no means eliminate completely the yellow 

 tarry matter. It is therefore better to convert the impure acid into a salt which may 

 be easily purified, and afterwards to precipitate the acid from it. The picrate of 

 potassium is very applicable for this purification, as it is only slightly soluble in cold 

 water, whilst it is readily soluble in boiling. 



But in operating upon a large scale, the filtration of large quantities of the salt 

 becomes extremely difficult, as the liquids, even when boiling and contained in heated 

 funnels, have a great tendency to crystallise on the filters, which then become 

 choked. 



Several manufacturers therefore adopt another process, which consists in saturating 

 a boiling solution of picric acid with carbonate of soda, excess being avoided, for 

 fear of dissolving the yellow resinous matter. The boiling matters are filtered to 

 separate this resin, and to the filtrate a further quantity of carbonate of soda is 

 added. This causes the bulk of the picrate of soda to crystallise out, as this salt 

 is nearly insoluble in solutions containing an excess of alkaline carbonate. The small 

 quantity of picrate still remaining in the mqther-}iquor.s may be precipitated by the 

 addition of a salt of potassium. 



The crystallised picrate of soda thus obtained is then dissolved, and its boiling 

 solution is decomposed by an excess of sulphuric acid. The picric acid thus separated 

 being very insoluble in the mother-liquors containing the acid sulphate of soda, 

 crystallises almost entirely on cooling ; when drained, washed with a little cold water, 

 and pressed, it is almost chemically pure. 



