2. Nitric acid throws down a white precipitate, and takes it up again when added in excess.
3. The Infusion of Galls causes a dirty, yellowish-white precipitate; but it will not act on a solution which contains much less than two grains per ounce.
4. The best liquid reagent is Hydrosulphuric acid. In a solution containing only an eighth part of a grain per ounce, it strikes an orange-red colour, which, when the excess of gas is expelled by heat, becomes an orange-red precipitate; and if the proportion of salt is greater, the precipitate is thrown down at once.—The colour of the precipitate is so peculiar as to distinguish it from every other sulphuret; but if any doubt regarding its nature should occur, it may be known by collecting it, dissolving it with the aid of gentle heat in hydrochloric acid, and adding water to the solution; which will then yield a white precipitate, the sesquioxide of antimony in union with a little chlorine.
5. When the solution is put into Marsh’s apparatus for detecting arsenic [p. [211]], the flame yields a dark brownish-black, obscurely shining crust on a surface of porcelain held across it, and a white crystalline powder if the porcelain be held just above the flame. The dark crust is antimony, the white one its oxide. The former has only a distant resemblance to the brilliant stain of arsenic, notwithstanding all that has been said of their similarity. It is well, however, to use some other test for distinguishing the two metals besides their appearance; and the most convenient is a solution of chloride of lime, which instantly makes an arsenical crust disappear, but does not affect an antimonial one.
Tartar-emetic, like the soluble salts of mercury and copper, is decomposed by various organic principles. All vegetable substances that contain a considerable quantity of tannin have this effect; of which an example has been already mentioned in the action of infusion of galls. Decoctions of cinchona-bark decompose it still more effectually. The animal principles do not act on tartar-emetic, with the exception of milk, which is slightly coagulated by a concentrated solution. Many vegetable and animal substances, though they do not decompose it, alter the operation of the fluid tests. Thus tea, though it does not effect any distinct decomposition of the salt, will prevent the action of gall-infusion; and French wine gives a violet tint to the precipitates with that test and with acids.[[1130]] Hydrosulphuric acid, however, acts under all circumstances, and always characteristically, whatever the colour of the fluid may be. Dr. Turner found that when transmitted through a diluted solution in tea, porter, broth, and milk, with certain precautions to be mentioned presently, he procured a precipitate which either showed its proper colour at once, or did so at the margin of the filter on which it was collected.[[1131]]
The circumstances now referred to render it necessary to resort to other means, besides the simple application of liquid reagents, for the purpose of detecting tartar-emetic in complex organic mixtures. This subject has been ably investigated, first by Dr. Turner,[[1132]] and afterwards by Professor Orfila.[[1133]] The result of the researches of both seems to me to be that the most convenient method yet proposed is the following.
Process for Tartar-emetic in Organic Mixtures.—If the subject of analysis be not already liquid enough, add distilled water. Then acidulate with a little hydrochloric and tartaric acids; the former of which throws down some animal principles, while the latter dissolves readily all precipitates formed with tartar-emetic by reagents or organic principles except the sulphuret. Filter the product.
1. Subject a small portion of the liquid to a stream of hydrosulphuric acid gas, and if it be perceptibly coloured orange-red, treat the whole liquid in the same way; boil to expel the excess of gas, collect the precipitate, dry it, and reduce it by hydrogen gas in the following manner. Put the sulphuret in a little horizontal tube, transmit hydrogen through the tube by means of the apparatus represented in Figure 9, and when all the air of the apparatus is expelled, apply heat to the sulphuret with a spirit-lamp. Hydrosulphuric acid gas is evolved, and metallic antimony is left, if the current of hydrogen be gentle, or it is sublimed if the current be rapid.—When there is much animal or vegetable matter present in the sulphuret, the metal is not always distinctly visible. In that case, dissolve the antimony by the action of nitric acid on the mixed material and broken fragments of the tube, and throw down the orange sulphuret again from the neutralized solution by hydrosulphuric acid.
2. If hydrosulphuric acid do not distinctly affect the liquid, or if no precipitate be separated after boiling, or so small a quantity as cannot well be collected,—evaporate the liquid to dryness, char it by means of nitric acid and chlorate of potash, as directed for copper (p. [357]), boil the carbonaceous mass for half an hour in a mixture of eight parts of hydrochloric acid and one of nitric acid, and introduce the filtered solution into the modification of Marsh’s apparatus for detecting arsenic described in page [204], but without the tube e h. Kindle the gas at e, and try whether a black, dull stain, not removable by solution of chloride of lime, be produced on a surface of porcelain held across the flame. If no stain be produced, there was no antimony in the liquid under examination. If the porcelain be stained, apply the heat of a spirit-lamp flame to the tube d e. Antimony will be deposited within the tube where the heat is applied. In order to ascertain its nature, break the tube, heat the portion containing the crust with nitro-hydrochloric acid, evaporate to dryness, dissolve the residue in hydrochloric acid, decompose a part of this solution with water, and subject the rest to a stream of hydrosulphuric acid gas, which will produce the usual orange sulphuret of antimony.
3. If antimony be not indicated in either of these ways in the fluid part of the subject of analysis, the solid portion may next be subjected to the second process; but success will very seldom attend the search when the previous steps have failed.