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$ 33. We will now consider the method of reduction of Fresenius and Babo. It is based on the observation, that the sulphides of arsenic, the arsenites and arsenates, when fused with a mixture of cyanide of potassium and carbonate of soda, yield metallic arsenic; and that this metal is obtained in the shape of a lustrous mirror in all cases where the base of the salt is not at the same time reduced, or, if it suffer reduction, where an arseniuret is formed, which, on application of heat, parts with all or part of its arsenic. In reducing a sulphide of arsenic in this manner sulphocyanide of potassium is formed, and in reducing an arsenite or arsenate the cyanide of potassium is changed to cyanate of potassa.
Fresenius recommends the use of the sulphide of arsenic as obtained by the treatment of the crude sulphide with nitric and sulphuric acid, extraction of the mass with water, and precipitation of the solution with sulphureted hydrogen (S 14). The precipitate is collected on a filter, washed, dissolved in ammonia, and the solution evaporated in a porcelain dish. The dish with the sulphide of arsenic may be dried, over a water-bath at 40°C, and the weight of the sulphide before reduction be ascertained, whence the weight of the arsenic or arsenious acid may be deduced.
It has already been mentioned (S 14), that by proceeding in this manner the sulphide of arsenic is not always with certainty obtained perfectly free from organic matter, even by a skillful operator. For this reason I consider it much more appropriate to employ the pure sulphide of arsenic as resulting from the treatment of the crude sulphide with nitric acid, subsequent fusion of the residue with nitrate and carbonate of soda, etc. (8 12), and which is perfectly free from organic substances. The sulphide of arsenic obtained after this method is collected on a filter, washed, dissolved on the filter in ammonia, and the solution evaporated in a porcelain dish. Its weight may then, if necessary, be determined; it is sure to be the tersulphide AsSo; 100 parts correspond to 80.4 of arsenious acid, and to 60.9 of metallic arsenic.
H. Rose has observed, that in fusing sulphide of arsenic with cyanide of potassium, not the whole of the arsenic becomes reduced, but that part of it forms a sulpho-salt, which is not acted upon by cyanide of potassium. If the sulphide of arsenic contains a certain amount of sulphur, no reduction whatever is effected; hence the tersulphide affords a greater quantity of reduced arsenic than the pentasulphide. The reduction of the arsenic acid to arsenious acid by means of sulphurous acid (S 13), ought, therefore, not to be neglected.
It has been stated before, that cyanide of potassium reduces the whole of the arsenic from arsenic and arsenious acids; hence, the most rational way of procedure appears to be the conversion of the pure sulphide of arsenic, obtained in the manner just described, into arsenic acid, and the application of this compound for the process of reduction. Concentrated nitric acid is added to the sulphide of arsenic, contained in the porcelain dish, and the acid evaporated by heat; this is repeated several times, if necessary; to remove the last traces of nitric acid, the mass is repeatedly moistened with water, and
evaporated to dryness; the final residue is again softened with water, some dry carbonate of soda added, so that an alkaline mass is formed, and the whole again evaporated to a perfectly dry mass, which now is eminently adapted for the process of reduction.
§ 34. The apparatus of reduction, as constructed by Fresenius and Babo, is represented in Fig. 6. (A)
Fig. 6. is a large gas-bottle, for the preparation of carbonic acid. It is filled to about one-half with water and large pieces of marble or compact limestone; chalk should not be taken, since it does not afford a constant current. A funnel tube (a) passes through one of the apertures of the doubly perforated cork to near the bottom of the flask. Into the other aperture, a tube (b) is inserted, which conducts the gas into the small bottle (B), partially filled with concentrated sulphuric acid, by which the gas is at once washed and dried. Through the tube (c), the car bonic acid passes into the reduction-tube (C), represented in Fig. 7.
The apparatus having been arranged, the sulphide of arsenic (or, better still, the mass obtained by nentralization with carbonate of soda, and in which the arsenic is in the shape of an arsenite or arsenate), is mixed in a heated mortar—a mortar of agate is preferable to any other kind—with about ten parts of a pulverized and well-dried mixture of three parts of anhydrous carbonate of soda, and one part of cyanide of potassium; the powder is placed on a narrow strip of smooth paste-board, which is bent up at the sides, and pushed into the reduction-tube down to (e). By turning the tube, the mixture is placed at (d e), without soiling any other portion of the tube. The pasteboard is then carefully removed.
The tube, when filled in this manner, is fitted over the cork of the tube (c) of the apparatus.
Hydrochloric acid is now poured through the funnel-tube to produce a moderate current of carbonic acid gas, and the mixture carefully dried, which is done by very gently heating the tube throughout its whole length by the flame of a spirit-lamp. When every deposition of water has disappeared, and the current of gas become so slow that only one gasbubble passes through the sulphuric acid every second, the reduction-tube is heated to ignition at the place (c). When this point is reached, heat is also applied to the mixture, beginning at (d), and slowly progressing towards (e) until the whole of the arsenic is driven off.
The reduced arsenic is deposited at (h), while a very small portion makes its escape at (i), and imparts to the air its peculiar alliaceous odor. Towards the end of the operation the second lamp is gradually advanced to (c), and thus the arsenic, which is deposited in the wider part of the tube, driven to (h). This done, the tube is closed at the narrow end, and the mirror driven by heat from (i) to (h), when it presents a very fine and purely metallic appearance. The tube is then cut off at (e), the open end closed with a cork, and deposited ad acta.
In driving the arsenic from one place to another, the greatest circumspection must be observed, since a loss of arsenic is almost invariably experienced, even if the current of the gas is very feeble. It would appear that the gaseous arsenic is much less easily deposited from the heavy carbonic acid gas than from the light hydrogen gas. I have already ($ 16,) had occasion to observe that, in my opinion, the presence of a very small quantity of arsenic cannot so conclusively be demonstrated by this process as by the process of Marsh.
The presence of the compounds of antimony in the mass subjected to reduction, does, in the process of Fresenius and Babo, not influence the result, because not a trace of the reduced antimony is volatilized, and because the reduced antimony does not retain the arsenic. This is a great advantage of the process in question; it excludes the possibility of mistaking antimony for arsenic. The presence of tin also does