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debted to Mr. Bischoff. The solution of hypochlorite of soda must not contain free chlorine; it is prepared by passing chlorine gas into a solution of carbonate of soda, or by adding a solution of carbonate of soda to strong chlorine-water. A drop of this solution is placed, by means of a glass-rod, near the spot, and then made to flow over it.
If.a drop of sulphide of ammonium is placed, by means of a glass-rod, on an arsenic-spot, and gently heated over a spirit-lamp, the spot is completely dissolved. On allowing the liquid to evaporate, which may be accelerated by blowing, a pure yellow residue of sulphide of arsenic remains behind. An antimony-spot, when treated in the same manner, yields an orange-red residue of sulphide of antimony. This is another very characteristic distinguishing test, discovered by H. Rose. If the sulphide of arsenic is moistened with a drop of hydrochloric acid, it remains undissolved, while the sulphide of antimony dissolves very readily; on the other hand, a solution of carbonate of ammonia dissolves the sulphide of arsenic, and acts not on the sulphide of antimony.
Both the spots of arsenic as well as those of antimony disappear when moistened with nitric acid of from 1.26 to 1.3 spec. gr. To the solution thus obtained a solution of nitrate of silver, or of nitrate of silver and ammonia (prepared by adding exactly so much ammonia to a solution of nitrate of silver that the precipitate is re-dissolved), is added, by means of a glass-rod. The liquid is then carefully neutralized with ammonia, which is done by holding a glassrod, moistened with ammonia, over the spot. If the spot consisted of arsenic, a yellow precipitate of
arsenite of silver will be produced; if it was an antimony-spot, no reaction takes place. Another very important distinguishing test! A drop of strong sulphureted hydrogen water, when added to the solution of an arsenic-spot, produces a lemonyellow precipitate of sulphide of arsenic; when added to the solution of an antimony-spot, an orangered precipitate of sulphide of antimony.
The metallic mirror of the reduction-tube shows, of course, the same behavior to reagents as the spots on porcelain.
By these reactions the nature of the arsenic is established beyond any doubt, and all the other tests which have been proposed, and which are much less conclusive, and more exposed to fallacy, are thus made superfluous. For the sake of completeness, however, we will mention some of them.
$ 28. If a piece of phosphorus, moistened with water, is introduced into a porcelain capsule, and the dish containing the spots placed over it, the spots, if consisting of arsenic, disappear very soon; but very slowly, if consisting of antimony.-(Cottereau.) This is owing to the formation of ozone, which rapidly oxidizes the arsenic, and affects but slowly the antimony.-Schönbein.) The places from which the arsenic-spots have been removed, powerfully redden a moistened litmus-paper; the places whence the antimony-spots have been removed, have no effect on litmus-paper.--(Schönbein.)
If some iodine is introduced into a porcelain capsule, and the dish containing the spots placed over it, the arsenic-spots assume a pale, yellowish-brown color, which changes, on exposure to air, within a few min
utes, to yellowish-brown. The reaction disappearg on continued exposure to air, or on gently heating. Antimony-spots, under the same circumstances, assume a carmelite-brown color, which, on exposure to air, passes into orange, and which is permanent. If the place from which the yellow arsenic-spots have disappeared is moistened with a drop of sulphureted hydrogen water, or if exposed to the influence of sulphureted hydrogen gas, a new yellow spot immediately appears, which is quickly removed on addition of ammonia. The spots of iodide of antimony which, as before mentioned, do not disappear on exposure to air, are converted, by sulphureted hydrogen, into sulphide of antimony, which resists the action of dilute ammonia for a considerable time.(Lassaigne.)
If a drop of bromine is introduced into a porcelain capsule, and the dish, containing the spots, placed over it, the arsenic-spots very soon assume a fine, lemon-yellow color; the antimony spots, within a still shorter period, become orange-red. Both kinds of spots become colorless on exposure to air, and sulphureted hydrogen produces the same reactions as in the preceding case.—(Slater.)
A concentrated solution of iodate of potassa imparts to the arsenic-spots a cinnamon color, and soon after dissolves them, while the antimony-spots resist its action for two or three hours. Chlorate of potassa gradually dissolves the spots of arsenic, but not those of antimony. Nitroprussiate of potassa, on the other hand, dissolves the antimony-spots, but not the arsenic-spots.—(Slater.)
$ 29. If the tube containing the metallic mirror, the reduction-tube, is connected with an apparatus which affords dry sulphureted hydrogen gas, the current being so regulated that the quantity of the escaping gas is just sufficient to burn with flame, and heat is applied by means of a common spirit-lamp from without to within, i. e., against the current of the gas, yellow sulphide of arsenic will be formed if the metallic mirror consisted of arsenic, but orangecolored or black sulphide of antimony, if it consisted of antimony.-Pettenkofer, Fresenius.)
If, then, the tube is fastened to an apparatus which affords dry hydrochloric acid gas (for which purpose it is only necessary to put some chloride of sodium into a large excess of sulphuric acid, and to apply a gentle heat), and the gas passed through the tube, without heating, the sulphide of antimony entirely disappears; chloride of antimony is formed, which is very volatile in a current of hydrochloric acid gas. If the escaping gas is transmitted through water, the antimony is retained in the latter, and may readily be detected by sulphureted hydrogen and other reagents. Sulphide of arsenic remains under these circumstances perfectly unchanged, not being affected by the gas at common temperature. By sucking a little ammonia into the tube it is again obtained in solution, which may serve for other experiments.(Fresenius. Annalen der Chem. und Pharm., vol. xliii., p. 361.)
§ 30. Antimoneted hydrogen causes, like arseneted hydrogen, a dark precipitate in a solution of nitrate of silver. Similar as the action of the silver salt on the two gases may appear at the first glance, a closer examination will show a decided difference. The oxide of silver oxidizes both the arsenic and the hydrogen of the arseneted hydrogen; metallic silver is, consequently, deposited, and the liquid contains arsenious acid besides the excess of the silver-salt and free nitric acid.* On carefully adding some dilute ammonia to the filtered solution, & yellow precipitate of arsenite of silver' will, therefore, bé thrown down, either immediately or after addition of some nitrate of silver, in case the latter should be wanting. Of the antimoneted hydrogen only the hydrogen is oxidized by the oxide of silver; the antimony is precipitated with the reduced silver, either alone or in combination with the latter as antimoniet of silver, the filtered liquid does not contain the slightest trace of antimony, and ammonia does, of course, not produce any precipitate.
This difference in the behavior of a solution of nitrate of silver to arseneted and antimoneted hydrogen furnishes a very reliable method to distinguish arsenic from antimony; hence, the test with the silver-solution ought never to be neglected. The facility to completely remove the silver from the solution by hydrochloric acid, admits of the application of various other reagents to the liquid even after the characteristic yellow precipitate has been produced. Whether a solution of terchloride of gold possesses any advantages over
[* The mutual decomposition may be thus expressed: in the case of arseneted hydrogen,
6 (Ago. N05) + AsH = 6Ag + AsO3 + 3H0 + 6N0S; in the case of antimoneted hydrogen,
3 (Ago : N05) + SbH AgoSb + 3H0 + 3N06.]