Arsenic Acid is manufactured on a large scale for this industry. The arrangement for the preduction of aqueous arsenic acid for magenta manufacture is thus described by H. Cant, of Evans, Cant, and Co. A series of stoneware pans A (Fig. 490) fit hermetically into an iron water bath B, heated by means of an iron coil. The pans are fittLd with cemented covers, having two openings, in one of wbich a glass funnel F is placed, and to the other is securely attached au earthenware tube for leading away the nitrous oxide. Stoneware jars J, fitted with tubulures and stop cocks, aro filled with nitric acid (sp. gr. 1.400) ; the stop-cocks are opened so as to allow the acid to dribble slowly, as required, into the jars A, which contain arsenious acid mixed with a certain quantity of water. Thus the arsenious acid is expeditiously converted into arsenic acid of the degree of hydration required for magenta manufacture, whilst the nitrous fumes are collected in a series of large Woulfe's bottles. By means of an aspirator, these vapours are drawn out of the pans A accom panied by air, and are thus oxidized into nitrous acid, and ultimately into nitric acid for re-use. An outlet near the bottom of the pans allows the arsenic acid solution to be drawn off as required. Thie method of producing arsenic acid, and converting the nitrous oxide into a profitable product, enables arsenic acid t,o be produced for a trifle more than the cost of the arsenious and nitric acids, and removes the objectionable features of the manufacture ; several attempts to utilize the refuse arseniates and arsenites, reeulting from the manufacture of rosaniline salts, have been made, but with little success.
Nicholson (No. 519, 1878) proposes nitric and hydrochloric acids, instead of arsenic acids, for the production of colours having rosaniline for their base. Three parts by weight of commercial aniline (such as is generally used for the production of red aniline dyes), about one part nitric acid (sp. gr. 1.420), and one part hydrochloric acid (sp. gr. 1.160), are heated at 177°-204° (350° 400° F.) -until the desired colouring matters are produced; this is ascertained by withdrawing samples as the operation proceeds. The contents are removed, and the colouring matter is extracted by boiling water ; it may be used direct, or purified; or the ba,se, rosaniline, may be separated by means of an alkali or alkaline earth. Though the extra trouble, and modification of plant, required by this process may operate against its introduction, there can be but one opinion on the merits of au invention which dispenses with a seriously poisonous product.
An improvement in the same direction, by the recovery of the arsenic from the waste products of this manufacture, converting it at the same time into a source of profit, is to separate the rosaniline baee by means of ammonia, by which the arsenious and arsenic acids contained in the " melt " are converted into arsenite and arseniate of ammonia, both of which are soluble; from these salts, it is proposed to expel the ammonia by heat, and collect it for use over again, obtaining the arsenic in a form ready for reconversion into arsenic aei 1. The objectionable feature in this is the
handling of a readily soluble poisonous material.
Toluidine, or Coupkr's Reds.—Coupier prepares hie reds from (1) pure aniline and pure nitro totuol ; (2) ordinary commercial aniline and ordinary nitro-benzol ; (3) nitro-tuluol and toluidine, or nitro-xylol and xylidine ; using iron and hydrochloric acid iu each case. The products from the first two yield reds identical with ordinary rosaniline ; to the last, he givea the name " rosa toluidine," " toluidine red," or " xylidine red." The apparatus and procedure may be as for rosaniline.
Erythrobenzine.—Laurent and Casthelaz gave this name to a red colouring matter obtained from nitro-benzol, iron filings, and hydrochloric acid, no aniliue being added, although in the reaction it may be produced ; it agrees with rosaniline, with which it is probably identical. Equal parts by weight nitro-benzol and aniline are gradually heated together in an enamelled iron vessel to about 200° (392° F.) ; the heat is continued until the masa becomes pasty, and will solidify, on cooling, to a brittle body reaembling crude rosaniline. It ia powdered, boiled in water with carbonate of soda to precipitate the colouring matter, and further treated as for purifying rosaniline. This yields a fine red colouring matter, and, aa regards the aniline used, is equal in yield to the arsenic acid method. Nitro-tolnol and aniline give a colour approaching that of roaaniline ; toluidine and nitro-toluol give a red with a decided violet shade. The reds obtained in thia way are better adapted for confectionery, colouring liquids, &c., than " rubine," which ia made with nitrate of mercury instead of arsenic, and has been largely used for the above purposes.
Xylidine .Red.—Hofmann's xylidine red is obtained by heating pure xylidine and pure aniline with an oxydant capable of formiog rosaniline ; it gives a splendid crimson. It is manufactured and purified as rosaniline (q. v.).
Roaaniline and the bases formed in its manufacture are capable of combining with the ethyl and methyl radicals, and from some of them important colouring matters are obtained. See Hofmann'a Violet, 8co.
Tannate of rosaniline is formed as a bright red precipitate, by adding a cold and dilute solu tion of a salt of rosaniline to a, aolution of tannic acid ; from warm and concentrated solutions, a brownish-red brittle masa ia produced. By dissolving in alcohol or wood-spirit, it dyes yellowish or orange-red tints. Treated with nitric or hydrochloric acid, it yields a colouring matter passing gradually from violet to blue ; by regulating the supply of acid, any desired tint may be obtained. It is purified by washing with water, aud dissolved in wood-spirit ; its solution diluted with water can be used for dyeing.