« PreviousContinue »
tals which might be mixed with the sulphide of arsenic, the precipitate is most advantageously treated with dilute ammonia. For this purpose, the filter containing the moist precipitate is spread out in a porcelain dish, and a little water •poured on it; the precipitate is then divided into a thin, homogeneous paste, by means of a feather, and ammonia added, drop by drop, as long as solution takes place, or until an excess of the solvent is clearly indicated by the odor. Sulphide of arsenic and the organic substances dissolve in dilute ammonia, and the other metallic sulphides, which might have been precipitated by the sulphureted hydrogen, remain undissolved; only a small quantity of sulphide of antimony might, also, enter into solution. The ammoniacal liquid, which is of a dark-brown color, is strained through a small filter, and the residue carefully washed with ammoniacal water. This residue has to be examined for antimony, lead, copper, mercury, etc.
The treatment of the precipitate with dilute ammonia may, also, be thus executed : dilute ammonia is poured, drop by drop, on the filter containing the moist precipitate, while still in the funnel, care being taken, if the precipitate is of a slimy and viscous consistency—as is, almost invariably the case when organic matter is present to bring it into contact with the liquid by stirring it with a feather; or the dilute ammonia is applied by means of a washing-bottle.
Instead of the dilute liquid ammonia, a saturated solution of carbonate of ammonia may be employed for the purpose of extracting the sulphide of arsenic from the precipitate; the latter reagent acts less solving on sulphide of antimony, which might be present, but acts, also, less energetic on the sulphide of arsenic.
- been shown by Becker, that the precipitated sulphide of arsenic be
comes sensibly oxydized, after the whole of the sulphureted hydrogen has passed off.---(Archiv der Pharm. Vol. 56, p. 287.)
The ammoniacal solution, together with the washings, is then placed in a porcelain capsule, and evaporated to dryness at a gentle heat; the sulphide of arsenic, if present in the precipitate, remains, mixed with the organic matter, as a dark-brown residne.
$ 12. The only conclusive proof of the presence of arsenic in this residue, and, consequently, in the substances under examination, is its separation in the metallic state, its reduction to metallic arsenic. To effect this reduction, the sulphide or the acids of arsenic, prepared from the sulphide by oxidation, may be employed. But, even if the reduction is to be made directly from the sulphide, after the manner hereafter to be described, it is not admissible to use the residue itself without any further preparation, since the products of the destruction of the organic matter may cause the reaction to become quite indistinct. The organic substances, contained in the residue, must, therefore, first be destroyed, either by nitric acid and fusion with nitrate of soda, or by nitric acid and sulphuric acid; we then obtain a mass, containing arsenic acid, which may be used directly for the process of reduction, or from which pure sulphide of arsenic may be prepared.
$ 13. The safest method to effect the destruction of the organic matter, and the oxidation of the sulphide of arsenic, is fusion with nitrate of soda. Some highly
concentrated or, better still, fuming nitric acid is poured on the residue, in the porcelain capsule, and the acid evaporated at a gentle heat. If the residue which is now left is still of a dark color, this treatment with nitric acid has to be repeated, until the residue, while moist, appears of a yellow color. To the residue some caustic soda is now added, which serves to neutralize the excess of acid without effervescence and to soften the mass, and, afterwards, some finely powdered carbonate of soda, and some nitrate of soda, also finely powdered, and the whole placed in a porcelain crucible. The capsule is cleaned out with a little carbonate of soda, which is then added to the mass in the crucible. The contents of the crucible are now thoroughly dried, and then heated by means of a spirit-lamp, care being taken that the temperature rises very gradually. At first the mass turns brown, or black, becomes afterwards decolorized, without deflagration, and, finally, fuses to a colorless liquid. The whole of the organic matter is then completely destroyed.
The fused mass contains, if arsenic was present, arsenate of soda, besides nitrate, nitrite, sulphate, and carbonate of soda ; and, in case the precipitate contained antimony, antimonate of soda. The mass is treated with water; if antimonate of soda was present, this remains undissolved, and is separated by filtration; the liquid is strongly acidulated with dilute sulphuric acid, precaution being taken that no loss is incurred by spirting, and evaporated in a porcelain capsule. Towards the end of the operation some more sulphuric acid is added, to make sure of its being present in sufficient quantity to expel the whole of the nitric and nitrous acids; the only safe indication for this is the appearance of the thick sulphuric acid vapors, and the evaporation must be continued until this takes place.-(Meyer, Annalen der Chemie und Pharmacie, Bd. 66. p. 237.) The elimination of the antimony, and the complete destruction of the organic substances, greatly recommends this treatment of the crude sulphide of arsenic. To prevent volatilization of the arsenic as chloride of arsenic, it is requisite that all materials, viz.: the nitric acid, the caustic soda, the carbonate and nitrate of soda, be perfectly free from chlorine...
The mass obtained by evaporation, usually a colorless and very acid liquid, is very well adapted for the preparation of metallic arsenic by the process of Marsh (see below).
If the preparation of sulphide of arsenic is intended, for the purpose of subsequent reduction by another process, the mass is most conveniently treated · with a concentrated solution of sulphurous acid; the excess of this acid removed by heat, and a current of sulphureted hydrogen passed through the moderately hot liquid, until the whole of the arsenic is precipitated. The sulphide is collected on a small filter, well washed, and, finally, dissolved in dilute ammonia by pouring the solvent, drop by drop, on the filter, as above described. The solution, on evaporation in a porcelain dish, yields the pure sulphide.
$ 14. Fresenius recommends to effect the destruction of the organic matter of the residue in question—the residue from the evaporation of the ammoniacal solution of the crude sulphide of arsenic ($ 11)-by means of nitric acid and concentrated sulphuric acid.
The residue is mixed with some fuming nitric acid, and heated, over a water-bath, to dryness; this residue is moistened with some pure concentrated and heated sulphuric acid, placed upon the water-bath for about two or three hours, and then exposed to a higher temperature (above 150° C.), until it is converted into a charred mass; this mass is treated with from ten to twenty parts of distilled water, filtered and washed. The liquid, thus obtained, is well adapted for Marsh's test; but when used for the purpose of precipitating the arsenic as sulphide, the latter is not, or at least not always, obtained perfectly free from organic matter.
§ 15. Instead of treating the precipitate (s 10), obtained from the solution of the substance under examination by means of sulphureted hydrogen, with dilute ammonia, to evaporate the solution and heat the residue with nitric acid, etc., as mentioned in the preceding paragraph, it has been recommended to treat the precipitate, together with the filter, directly with nitric acid, etc. But since it is so easy a matter to exclude the paper of the filter, why not do it? And since it is always advisable to test for the presence of other sulphides, insoluble in ammonia, why then omit the treatment with ammonia ?
$ 16. We will now take into consideration the re- • duction of the arsenic, which, as above (S 12) stated, is the only conclusive proof for the presence of the poison, partly because the arsenic, in its metallic state, is easily recognized as such, and partly because the process of reduction can be executed in such a manner as to exclude the possibility of its being