An ingenious tap arrangement has been designed by M. Chev(.: to obviate the necessity for discharging the colour from nitric acid in the manner described. It is shown in Figs. 155 and 156, and will be best understood from the section given in the hitter drawing. The products of distillation enter at A. Tho first portions contain the colouring gases, and the tap being turned as shown iu Fig. 1511, are conducted to a range of Woulfe's bottles for condensation in the usual way. After a time, when the gas is beginning to lose its colour, the tap is turned so as to close up B, and the acid fumes are concluded along C to a separate range of bottles, which thus yield an almost colour less product. Towards the close Of the distillation, if the red fumes reappear, the current can be readily passed through B again into the bottles containing the first-collected and impure acid.
The range of bottles is occasionally warmed by the waste heat from the retorts in order to prevent their fracture by the first, hot, acid which comes over, the fire being diverted into a second flue as soon as the operation has well begun. It is very questionable, however, if this refinement is either ncoessary or advisable.
The condensing apparatus of M. D. Plisson, shown in Figs. 157 to 163, possesses some ad vantages over the usual Woulfe's bottle arrangement. Fig. 157 gives a vertical section passing through the siphon nozzle of a bottle. a is a small stoneware pipe fixed to the bottom of the vessel, with a small opening at b to admit a certain portion of the contents of the bottle. This pipe rises abovo the level of the liquid and issues through the side of the condenser at c. Into the neck of this bottle the tubular end A of an upper ves-el is inserted, while another communication pipe B forms a connection with the next bottle placed alongside. A shoulder d fits into the dished ek at e, and is carefully luted. Figs. 158 to 163 give various views of the condensing apparatus fitted together. Fig. 158 a side view, Fig. 159 an end view of a portion to the right of the line X, Figs. 160 to 163 different plans of Fig. 158. A, Fig. 158, represents the retort into which the nitre and sulphuric acid are introduced. The evolved gases pass off through B into the bottle marked 1: here a portion condenses, the remainder passing on through C into bottle No. 2, and so on in the direction of the arrows. The gas condensed in the bottles 4 and 5 falls down into the lowest range 1, 2, and 3, fitted with siphon nozzles, through which the condensed liquid flows into the main D, and finally into the receiver marked 6. F F, Figs. 160 and 161, are the handles.
The pipe G takes the vapours that may be evolved in bottle 6 and conveys them back into the con densers to be liquefied. H and I are auxiliary bottles to catch whatever gas may be uncondensed
in the main ranee.
The cost of a ten of good commercial acid is about 23/. 10s., allowing 25s. for the nitre cake which is removed from the retort, broken up, and sold to the manufacturers of sulphate of soda for mixture with the charge of common salt. The operation consumes about 1 ton of coal to a ton of nitrio acid, labour figures for 404. per ton, packages for 25s., and wear and tear for 10s. The selling price varies from '2 1. to 31d. per lb., accenting to quality. This crude sulphate—" or nitre cake " —is usually of about the following composition : Sulphate of soda .. ••• •. .. Free sulphuric acid .. .. • • .. .. Insoluble .. • .. • • .. 1.29 100-00 This sample would be esteemed by the alkali manufacturer on account of the largo amount of free acid contained.
Anhydrous nitric acid—nitric anhydride—is obtained by decomposing nitrate of silver with chlorine gas. Both should be perfectly dry, and the operation should bo slowly performed. The following equation represents the decomposition :— 2 = 2 AgC1 0 + Nitrate of silver. Chlorine. Chloride of silver. Oxygen. Nitric anhydride.
The silver salt, in well dried crystals, is placed in a U-tube, which is immersed in a bath of water with a supernatant layer of oil, and heated by means of a spirit lamp placed below. The chlorine gas is admitted from any suitable gas-holder, and dried by being passed through tubes containing chloride of calcium, and pumice-stone moistened with sulphuric acid. With the tube containing the silver salt is connected another tube, immersed in a freezing mixture, at the bottom of which is a small receiver to separate a small quantity of volatile nitrous anhydride which is produced during the operation. When the apparatus is fitted together the nitrate of silver is heated to about 175°, and a stream of chlorine gas passed over it at the rate of about GO cubic in. in twenty-four hours. After the crystals of nitrate have been thoroughly dried the temperature should be gradually lowered to about 70°. The chlorine then decomposes the salt, freeing nitric acid and oxygen, and forming chloride of silver. The oxygen finally passes off, and the nitric anhydride condenses in tho cooled receiver. Some little difficulty in this process arises from the fact that the acid acts upon the caout ohouc joinings. The U-tubes, however, cannot well be in one piece, or absolutely joined together, as is sometimes recommended, by melting the ends, as it is necessary to separate the parts iu order to remove the condensed nitrous compound.