Читать книгу Legal Chemistry - Alfred Naquet - Страница 16

Fig. 3.

The materials supposed to contain arsenic are boiled in water which has been rendered strongly alkaline by the addition of pure potassa. The fluid is then filtered, an excess of hydrochloric acid added, and a current of sulphuretted hydrogen conducted through it. If arsenic be present in the suspected fluid, it is soon precipitated as a yellow sulphide. In dilute solutions the formation of the precipitate fails to take place immediately, and only a yellow coloration of the fluid is perceptible; upon slightly boiling the solution, however, the precipitation of the sulphide is soon induced. The precipitate is collected on a filter, well washed with boiling water, and then removed, if present in a quantity sufficient to admit of this operation. It is next dissolved in ammonia,[B] and the solution so obtained subsequently evaporated to dryness on a watch-glass. The residue of sulphide of arsenic is placed in a tube closed at one end containing nitrate of potassa in a state of fusion: it is decomposed by this treatment into a mixture of sulphate and arsenate of potassa, the reaction being completed in about fifteen minutes. The mixture is now dissolved in water, and lime water added to the solution: a precipitate of arsenate of lime is formed, which is separated from the fluid by filtration, dried, mixed with charcoal, and introduced into a second tube. A few pieces of charcoal are then placed in the tube adjoining the mixture and exposed to a red heat, the part of the tube containing the arsenical compound being also heated. By this operation the arsenic acid is reduced to arsenic, which is deposited upon the cold portion of the tube in the form of a metallic mirror. This mirror is then identified by subsequent reactions. The method just described is no longer in use, although the precipitation of the arsenic by sulphuretted hydrogen is still often resorted to in its separation from the other metals with which it may be mixed. The destruction of the organic substances is, however, accomplished by means of chlorate of potassa and hydrochloric acid. To insure the complete precipitation of the arsenic, it is advisable to conduct sulphuretted hydrogen through the solution, at a temperature of 70° for twelve hours, and then allow the fluid to remain in a moderately warm place, until the odor of the gas is no longer perceptible, the vessel being simply covered with a piece of paper. The precipitate is next freed from the other metals possibly present, as directed in the general method of analysis, collected on a filter, and dissolved in ammonia. The ammoniacal solution is evaporated on a watch crystal, as previously described, and the residuary sulphide reduced to metallic arsenic. This reduction is effected by a process somewhat different from the one previously mentioned: the residue is fused, in a current of carbonic acid gas, with a mixture of carbonate of soda and cyanide of potassium. The apparatus employed is represented in Fig. 3: a, is an apparatus producing a constant supply of carbonic acid. Upon opening Mohr's clamp, g, the gas passes into the flask h, which contains sulphuric acid; it is then conducted, by means of the tube i, into the reduction tube k, which has an interior diameter of 8 mm. This tube is represented, in half size, in Fig 4.

Fig. 4.

The reduction is performed as follows: The sulphide of arsenic is ground in a small mortar, previously warmed, together with 12 parts of a mixture consisting of 3 parts of carbonate of soda and 1 part of cyanide of potassium, both salts being perfectly dry. The powder thus obtained is placed upon a piece of paper rolled in the form of a gutter, and introduced into the reduction tube. The latter is then turned half round its axis, so as to cause the mixture to fall in de without soiling the other parts of the tube. The paper is now withdrawn and the apparatus mounted. Upon opening the clamp g, and strongly heating the mixture by either the flame of a gas or an alcohol lamp, a mirror-like ring of metallic arsenic is deposited at h, if this poison be present in the substances under examination. When the coating is too minute to permit of perfect identification, it should be driven by heat to a thinner part of the tube; in this way it is rendered easily visible, being condensed upon a smaller space.

The above process possesses the advantage of not allowing arsenic to be confounded with any other body; it also permits of a quantitative estimation of the poison present. For this purpose, it is only necessary to previously weigh the watch-crystal, upon which the ammoniacal solution of sulphide of arsenic was evaporated, and to determine its increased weight after the evaporation; the difference of the two weighings multiplied by 0.8049, gives the corresponding weight of arsenious acid, and by 0.6098, the weight of the corresponding amount of metallic arsenic.