From the yellow filtrate, colouring matter can be obtained by neutralizing with. soda, and leaving it to settle, when it will form a dark-brown powder ; its solution may be precipitated with alumina to form pigments or lakes.
Dinitro-antEraquinone is heated in a concentrated (sp. gr. 1.3 to 1-4) solution caustic soda or potash at 170°-220° (388°-428° F.) until the blueish-violet colour ceases to become more intense. The cooled mass is dissolved in boiling water, and filtered. The hot filtrate is treated with hydrochlorio acid, which gives rise to a brownish-yellow preciPitate ; this, after it has been well washed, is ready for direct d)eing and printing.
The residue on the filter consists principally of " regenerated" anthraquinone, which, by transformation into nitro-anthraquinone, becomes a further source of colouring matter, from which .pure alizarine can be obtained, by extraction with ether or other suitable solvent.
Alizarine is sold in the form of a yellowish-brown thick solution, or pasty fluid, containing 10, 15, or 20 per cent. of colour, and is sent out in wooden casks. Quite recently, methods have been discovered by which a product containing 80 per cent. of pure, perfectly soluble colouring matter can be obtained. Dry alizmine could be easily prepared ; but, owing to the difficulty of reducing it to a powder or paste again with water, the goods become spotted, and a larger quantity is consumed ; it is on this account also, that it contains rarely so much as 20 per cent. dry alizarine. It is used for dyeing violets, lilacs, and Turkey reds. The paste dissolves readily in caustic soda, yielding a splendid violet-blue solution.
Pure alizarine can be obtained from this paste, by dissolving in weak caustic soda solution ; on adding chloride barium, and boiling, a dense precipitate is formed, which is filtered off and well washed with water ; it is then diffused in water, and decomposed with an acid, when the orange precipitate, after filtering, washing, and drying, consists of nearly pure alizarin°.
Kopp separates yellow alizariue from alizurine verte by dissolving in hydrocarbon and treating with alkali.
Isopurpurine, Anthrapurpurine, or Yellow Allzarine.—This is obtained from the alizarine prepared by Gessert Bros., by dissolving in ammonia, and adding hydrate baryta ; the precipitate is boiled in water, and the red liquid is filtered ; from this the isopurpurine is precipitated by an acid, filtered off, and washed, the process being repeated. If sulphuric acid is used, the isopurpurine is taken up with alcohol, from which it can be obtained in crystals. It is an orange-red substance
with all the properties of alizariue, but it dissolves in soda with more of violet-red coloration ; in ammonia, it givos a reddish-brown colour. Iu dyeing, its shades of red are similar to alizarine, but purer ; the purples are more blue ; and the blacks, more intense. When used to dye Turkey-red, it produces a brilliant scarlet of remarkable permanence. lts alkaline solution gives a spectrum resembling tbat of alizarine.
Perkin obtains it from crude alizarine, by the following process :—The latter is first dissolved in water containing a little carbonate soda ; the solution is shaken with recently precipitated alumina, which combines with the alizaiine, settles as a lake, and is filtered off; the filtered liquid is heated with hydmchloric acid, and the colouring matter thus precipitated is filtered off, washed, and dried. It is further freed from anthrafiavic acid, and other impurities, by repeated boiling with alcohol, digesting with a boiling solution of soda, and washing with the same. It is then dissolved in boiling water, and precipitated with chloride barium ; it is collected on a filter, washed with warm water, and decomposed by boiling with carbonate soda ; from this solution, when filtered, hydrochloric acid precipitates the anthrapurpurine.
Caro (1876, No. 1229) obtains alizarine orange, by acting on commercial alizarine with nitrous acid. From this is obtained a material having the properties of purpurine, by dissolving it in 10 parts by weight sulphuric acid (op. gr. 1.848), and heating to 150° (302° F.), until gases cease to he evolved, when the colouring matter is found in solution. On addition of water, it is precipitated.
Bibliography—G. W. Gesner, Coal, Petroleum, &c.' (New York : 1865); Reimann, 4Technologie des Anilins' (Berlin : 1866); M. C. Knab, Etudes Bur les Goudrons ' (Paris : 1867); W. H. Perkin, Aniline or Coal-tar Colours ' (Cantor Lectures, Society of Arts : 1869) ; Girard and De Laire Derives de la Ilouille ' (Paris : 1872) ; Bolley and Kopp, Matieres Colorantes' (Ziirich: 1873); W. Crookes, 'Wagner's Chemical Technology' (London : 1874); C. Schorlemmer, 'Carbon Compounds' (London : 1874); W. Crookes, Auerbach's Anthracene ' (London : 1877); E. J. Mills, 4Destructive Distillation ' (London : 1877); Chemical News ; " Journal of Gas-lighting ; ' Specifications of Patents.