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of magnesia, and ammonia. The liquid is mixed with ammonia in excess, then chloride of ammonium and, subsequently, sulphate of magnesia added. A crystalline precipitate of arsenate of magnesia-ammonia is thrown down, which is washed on a filter or in a glass, with ammoniacal water. It is dried and used for reduction. This precipitate, as well as the arsenate of lime, may, also, very advantageously, be used for Marsh's test; it is, for this purpose, dissolved in dilute sulphuric acid.
$ 49. The use of arsenate of lime for the preparation of an arsenic mirror, has also been recommended by Berzelius. This chemist prepared the solution, requisite for the precipitation, in the following manner: the purified sulphide of arsenic, as resulting from the evaporation of the ammoniacal solution (v. $ 11), is removed from the small porcelain dish, or watch-crystal, by mixing and rubbing it with a little pulverized nitre. Another portion of nitre is then fused in a porcelain crucible, or in a glass-tube, and the mixture of sulphide of arsenic and nitre added in small portions to the fused mass; the oxidation is thus rapidly effected. The fused mass which contains arsenate, carbonate, sulphate, nitrate, and nitrite of potassa, is dissolved in a small quantity of hot water, and the solution mixed with an excess of freshly prepared lime-water; the mixture is heated to ebullition, and then treated as above described. It is this method, as will be seen on comparison, which Meyer employed to effect the destruction of the organic matters in the crude sulphide of arsenic, substituting only nitrate of soda for common nitre, and adding carbonate of soda to remove any anti
The fused mass,
mony which might be present. thus obtained, contains a considerable quantity of alkaline carbonate; it is dissolved in water, the solution acidulated with nitric acid (not sulphuric acid, to prevent the formation of sulphate of lime), heated to ebullition (to drive out the carbonic acid), and precipitated with lime-water. If it is intended to precipitate the arsenic acid with a salt of magnesia, sulphuric acid may be taken instead of nitric. I must again repeat that there is no better method for the destruction of the organic matters.
$ 50. At a later period, Berzelius proposed the use of the sulphide of arsenic for the production of arsenic-mirrors. The method of reduction which he recommended, gave results, surpassing in delicacy and accuracy, all others hitherto known for the detection of arsenic in legal investigations. The sulphide of arsenic resulting from the evaporation of the ammoniacal solution (v. $ 11), is mixed, while still moist, with dry carbonate of soda, and the mixture formed into the shape of a small cylinder, a little water being added, if necessary. The cylinder is dried at a very gentle heat, and placed, by means of a glass-rod, at the point (d) of the reduction-tube (c) of the apparatus. (Fig. 10.)
The apparatus consists of a gas-bottle, for the evolution of hydrogen gas from zinc and dilute sulphuric acid, a tube (a) filled with loose cotton, and a chloride of calcium tube (b). The reduction-tube, made of glass of tolerable thickness, is connected with the latter by means of a cork, as shown in the figure. The different parts of the apparatus having been joined together, some moderately diluted sul
phuric acid is poured through the funnel-tube on the zinc and water, previously introduced into the bottle, and thus a current of hydrogen gas of mode. rate velocity produced. As soon as the air is completely driven out, the reduction-tube is very gently heated at (d) and before this point, by means of a common spirit-lamp, and thus the complete dessication of the mass very readily effected. The moisture collects beyond the point (d), and is easily removed from the tube by application of the flame.
When the mass has become perfectly dry, and the reduction-tube, beyond the point (d), sufficiently cool, the place (d) is heated with a powerful flame, when a beautiful mirror of metallic arsenic will be deposited beyond this spot, as illustrated by Fig. 7, (p. 57).
H. Rose, in his investigations on the action of alkaline carbonates on sulphide of arsenic, gives the following explanation of the formation of the metallic mirror. The humid mixture of tersulphide of arsenic and carbonate of soda contains arsenite of soda and sulpharsenite of soda, resulting from the
mutual decomposition of soda and sulphide of arsenic to sulphide of sodium and arsenious acid (3 Na O + As S = 3 Na S + As O), the former entering into combination with the sulphide of arsenic, the latter with the soda. On applying heat, both compounds give off arsenic, the arsenite being thereby converted into arsenate, and the sulpharsenite into sulpharsenate (5 As S = 3 As S' + As“).
Besides, the arsenic acid of the arsenate becomes reduced by the hydrogen, and thus increases the amount of free arsenic. It was formerly supposed, and I shared this opinion, that the sulphosalt was also decomposed by the hydrogen, that a sulphohydrate was formed, and arsenic liberated. But this is, according to H. Rose, not the case; the sulphosalt remains unchanged, and hence only part of the arsenic is obtained.—(Pogg. Annalen. Bd. 90, p. 565; Pharm. Centralblatt. 1854. p. 46.*)
However this may be, there is this objection to be raised against the method of Berzelius, that the reduced and vaporized arsenic is never completely deposited in the tube, but that a portion of it is always carried off with the current of the gas. This becomes very evident if the gas is kindled before the process of reduction has commenced; the flame is the ordinary hydrogen-flame; but, as soon as reduction takes place, the characteristic white coloration of the flame becomes apparent, and, if a porcelain dish is held into it, very beautiful spots are deposited. This circumstance is by some considered as an advantage of the method, since it affords, like the method of Marsh, not only a metallic mirror, but at the same time the characteristic spots. As early as 1830, the white color of the flame in this process was noticed by Wackenroder, and considered very characteristic; he collected the arsenious acid in a closed glass-tube, held over the flame, to be subsequently tested with sulphureted hydrogen.
* If a mixture of dry tersulphide of arsenic with carbonate of soda is heated, arsenate and sulpharsenate of soda are formed, and arsenic set free (5 As S = 3 As Sø + As“); but, if the pentasulphide is heated with carbonate of soda, the arsenate and sulpharsenate are formed without liberation of arsenic. If the operation is performed in a current of hydrogen gas, the last-named mixture also affords an arsenicmirror, owing to the reduction of the arsenic acid.—(H. Rose.)
Since the amount of escaping arsenic increases with the velocity of the current, the evolution of gas should be very moderate. I was formerly of opinion that the arsenic passed off as arseneted hydrogen, but this Fresenius has shown not to be the case, because the whole of the arsenic is deposited on passing the gas through a series of tubes, filled with loose cotton.
As in the process of Marsh, so it is in the method of Berzelius, absolutely necessary that the materials for the evolution of the hydrogen gas be free from arsenic. To exclude any possible intermixture of arseneted hydrogen with the hydrogen which serves for reduction, it has been recommended by Duflos and Hirsch to fill one half of the tube (a) Fig. 10 (the portion nearest to the gas-bottle), with loose cotton, soaked in a solution of corrosive sublimate, a salt which decomposes the arseneted hydrogen.
$ 51. Duflos and Hirsch have proposed another change in the method of Berzelius, namely, the substitution of the sulphide of arsenic by an arsenate.