sulphide of ammonium, or, as the case may be, sulphide of $ 190. (Detection of the Metals of Group VI.: Arsenic, Antimony, Tin, Gold, Platinum.) If the precipitate consisting of the sulphides of Group VI. has a 118 PURE YELLOW COLOR, this indicates principally arsenic and tin; if it is distinctly ORANGE-YELLOW, antimony is present; if it is BROWN or BLACK, this denotes the presence of platinum or gold. Beyond these general indications, the color of the precipitate affords no safe guidance. It is therefore always advisable to test yellow precipitate also for antimony, gold, and platinum, since minute quantities of the sulphides of these metals are completely hid by a large quantity of bisulphide of tin or tersulphide of arsenic. Proceed accordingly as follows: Heat a little of the precipitate on the lid of a porcelain crucible, or on a fragment of porcelain or glass.† 1. Complete volatization ensues: probable presence of ARSENIC, 119 absence of the other metals of Group VI. Reduction of a portion of the precipitate with cyanide of potassium and carbonate of soda (§ 131, 12) will afford positive proof of the presence or absence of arsenic. Whether that metal was present in the form of arsenious acid or in that of arsenic acid, may be ascertained by the method described § 133. 2. A fixed residue is left. In that case all the metals of 120 Group VI. must be sought for. Dry the remainder of the precipitate thoroughly upon the filter, triturate it together with about 1 part of anhydrous carbonate of soda and 1 part of nitrate of soda, and transfer the mixture in small portions at a time to a little porcelain crucible, in which you have previously * If the residue suspended in the fluid containing sulphide of ammonium, and insoluble therein, subsides readily, it is not transferred to the filter, but washed in the tube by decantation. But if its subsidence proceeds slowly and with difficulty, it is transferred to the filter, and washed there; a hole is then made in the bottom of the filter, and the residue rinsed into a small porcelain basin by means of a washing-bottle; the application of a gentle heat will now materially aid the subsidence of the residue, and the supernatant water may then be decanted. That this preliminary examination may be omitted if the precipitate is not yellow, and that it can give a decisive result only if the sulphur precipitate submitted to the test has been thoroughly washed, is self-evident.. heated 2 parts of nitrate of soda to fusion.* As soon as complete oxidation is effected, pour the mass out on a piece of porcelain. After cooling, soak the fused mass (the portion still sticking to the inside of the crucible as well as the portion poured out on the porcelain) in cold water, filter the insoluble residuewhich will remain if the mass contained antimony, tin, gold, or platinum-and well wash with a mixture of about equal parts of water and alcohol. (The alcohol is added to prevent the solution of the antimonate of soda. The washings are not added to the filtrate.) The filtrate and the residue are now examined as follows: a. EXAMINATION OF THE FILTRATE FOR ARSENIC (which 121 must be present in it in the form of arsenate of soda). or Divide the filtrate into two portions, add highly dilute nitric acid cautiously to the one portion to slightly acid reaction, and apply heat. Add to the acidified solution some nitrate of silver (not too little), filter (in case some chloride of silver nitrite of silver should have separated), pour upon the filtrate, along the side of the tube held slanting, a layer of dilute solution of ammonia,-5 parts of water to 1 part of solution of ammonia-and let the mixture stand for some time without shaking. The formation of a reddish-brown precipitate, which appears hovering cloud-like between the two layers (and may be seen far more readily and distinctly by reflected than by transmitted light), denotes the presence of ARSENIC. If the arsenic is present in some quantity, and the free nitric acid of the solution is exactly saturated with ammonia, the fluid being stirred during this process, the precipitate of arsenate of silver which forms imparts a brownish-red tint to the entire fluid. To gain more positive information respecting the presence of 122 arsenic, precipitate the second portion of the filtrate with solution of neutral acetate of lead, filter the precipitate (which consists of sulphate, carbonate, and—if arsenic is really present— arsenate of lead), dry between blotting paper, and then expose on charcoal to the inner flame of the blowpipe. If arsenic is present, a globule of metallic lead containing arsenic will be produced, which will long continue to exhale the odor of * Should the amount of the precipitate be so minute that this operation cannot be conveniently performed, cut the filter, with the dried precipitate adhering to it, into small pieces, triturate these together with some carbonate of soda and nitrate of soda, and project both the powder and the paper into the fusing nitrate of soda. It is preferable, however, in such cases, to procure at once, if practicable, a sufficiently large amount of the precipitate, as otherwise there will be but little hope of effecting the positive detection of all the metals of Group VI. Supposing all the metallic sulphides of the sixth group to have been present, the fused mass would consist of antimonate and arsenate of soda, binoxide of tin, metallic gold and platinum, sulphate, carbonate, nitrate, and some nitrite of soda. Compare also § 133, 1. + In some cases where a somewhat larger proportion of carbonate of soda had been used, or a very strong heat applied, a trifling precipitate (hydrated binoxide of tin) may separate upon the acidification of the filtrate with nitric acid. This may be filtered off, and then treated in the same manner as the undissolved residue. Chloride of silver will separate if the reagents were not perfectly pure, and the precipitate has not been thoroughly washed. garlic whenever the inner flame of the blowpipe is made to act be ascer b. EXAMINATION OF THE RESIDUE FOR ANTIMONY, TIN, GOLD, 123 PLATINUM. (As the antimony, if present in the residue, must exist as white, pulverulent antimonate of soda, the tin as white, flocculent binoxide, the gold and platinum in the metallic state, the appearance of the residue is in itself indicative of its nature.) Dry the residue on the filter. If there is much of it, separate it from the filter, and fuse it together with about 4 parts of cyanide of potassium, either in a small porcelain crucible or in a somewhat wide glass tube sealed at one end. If the quantity is small, insert the filter together with the precipitate in a platinum wire twisted into a spiral coil, and incinerate in the outer mantle of a small spirit-lamp or gas-flame; transfer the residue together with the filter-ash to a narrow glass tube sealed at one end, and fuse it in this together with 4 parts of cyanide of potassium; insert the ignited part of the tube, whilst still red-hot, into a test-tube filled with cold water, which will cause the part containing the fused mass to crack off in splinters. In either case remove the soluble salts completely by repeated warming with water and decantation, pour hydrochloric acid over the metallic residue, heat to boiling, and test the solution, which-in case of the presence of TIN-must contain protochloride of tin, with chloride of mercury (§ 128, 8). Boil the residue once more with hydrochloric acid, decant the fluid, then dissolve the residue still remaining in hydrochloric acid with addition of the least possible amount of nitric acid to effect solution, mix this with solution of hydrosulphuric acid, and heat to incipient boiling. If a precipitate of a distinct orange color forms, ANTIMONY is 124 present. In presence of much tin, the color is usually more or less concealed by an admixture of bisulphide of tin (for it is not easy to effect a complete separation of the two metals by boiling with hydrochloric acid); the precipitate may also contain bisulphide of platinum and metallic gold (resulting from the reduction by heat of precipitated tersulphide of gold). Let the precipitate, therefore, subside, wash it several times by decantation, and finally heat it with hydrochloric acid, to dissolve the sulphide of antimony present. If the precipitate did contain gold or bisulphide of platinum, a black pulverulent residue will remain. Test now in the first place the hydrochloric acid solution for ANTIMONY, by means of zinc and platinum (§ 130, 8), then dissolve the residue, if any, in some nitrohydrochloric acid, and test a portion of the solution for GOLD, with protochloride of tin, another portion for the same metal with sulphate of protoxide of iron (§ 125). Evaporate the remainder of the solution with addition of some chloride of potassium, and add to the residue a mixture of equal parts of alcohol and water. The formation of a yellow precipitate indicates the presence of PLATINUM. $191. (Detection of the Metallic Oxides of Group V. 2nd Section :-Oxide of Lead. Teroxide of Bismuth. Oxide of Copper. Oxide of Cadmium. Oxide of Mercury.) WASH THE PRECIPITATE WHICH HAS NOT BEEN DISSOLVED BY SUL- 125 PHIDE OF AMMONIUM, AND BOIL WITH NITRIC ACID. This operation is performed best in a small porcelain dish; the boiling mass must be constantly stirred with a glass rod during the process. A great excess of acid must be avoided. 1. THE PRECIPITATE DISSOLVES, AND THERE REMAINS FLOAting in 126 THE FLUID ONLY THE SEPARATED, LIGHT, FLOCCULENT, YELLOW SUL PHUR; this indicates the absence of mercury. CADMIUM, COPPER, Filter the fluid from the separated sulphur, and treat the filtrate as follows (should there be too much nitric acid present, the greater part of this must first be driven off by evaporation): add to a portion of the filtrate dilute sulphuric acid in moderate quantity, heat gently, and let the fluid stand some time. a. No PRECIPITATE FORMS; absence of lead. Mix the re- 127 mainder of the filtrate with ammonia in excess, and gently heat. a. No precipitate is formed; absence of BISMUTH. If the 128 liquid is blue, COPPER is present; very minute traces of copper, however, might be overlooked, if the color of the ammoniated fluid alone were consulted. To be quite safe, and also to test for cadmium, evaporate the ammoniated solution nearly to dryness, add a little acetic acid, and, if necessary, some water, and aa. Test a small portion of the fluid for copper with 129 ferrocyanide of potassium. The formation of a reddishbrown precipitate, or a light brownish-red turbidity, indicates the presence of COPPER (in the latter case only to a very trifling amount). bb. Mix the remainder of the fluid with solution of 130 hydrosulphuric acid in excess. The formation of a yellow precipitate denotes CADMIUM. If, on account of the presence of copper, the sulphide of cadmium cannot be distinctly recognised, allow the precipitate produced by the hydrosulphuric acid to subside, decant the supernatant fluid, and add to the precipitate solution of cyanide of potassium until the sulphide of copper is dissolved. If a yellow residue is left undissolved, CADMIUM is present; in the contrary case, not. B. A precipitate is formed. BISMUTH is present. Filter 131 the fluid, and test the filtrate for copper and cadmium, as directed in a (128). To test the washed precipitate more fully for bismuth, slightly dry the filter containing it between blotting-paper, remove the still moist precipitate with a platinum spatula, dissolve in a watch-glass in the least possible quantity of hydrochloric acid, and then add a sufficient quantity of water. The appearance of a milky b. A PRECIPITATE IS FORMED. Presence of LEAD. Treat the 132 entire fluid the same as you have treated the sample, filter off the precipitate of sulphate of lead, and test the filtrate for bismuth, copper, and cadmium, as directed in a (127).* Test the precipitate, after washing, by pouring solution of hydrosulphuric acid over it. 2. THE PRECIPITATE OF THE METALLIC SULPHIDES DOES NOT COM-133 PLETELY DISSOLVE IN THE BOILING NITRIC ACID, BUT LEAVES A RESIDUE, BESIDES THE LIGHT FLAKES OF SULPHUR THAT FLOAT IN THE FLUID. Probable presence of OXIDE OF MERCURY (which may be pronounced almost certain, if the precipitate is heavy and black). Allow the precipitate to subside, filter off the fluid, which must still be tested for CADMIUM, COPPER, LEAD, and BISMUTH; mix a small portion of the filtrate with a large amount of solution of hydrosulphuric acid, and should a precipitate form or a coloration become visible, treat the remainder according to the directions of § 191, 1 (126). Wash the residue (which, besides sulphide of mercury, may also contain sulphate of lead, formed by the action of nitric acid upon sulphide of lead, and also binoxide of tin, as the complete separation of bisulphide of tin from many of the sulphides of the metals of Group V. is rather difficult), and examine one half of it for mercury,+ by dissolving it in some hydrochloric acid, with addition of a very small proportion of nitric acid, and testing the solution with copper, or protochloride of tin (§ 118); fuse the other half with cyanide of potassium and carbonate of soda. If you obtain metallic grains, wash, heat with nitric acid, and test the solution obtained with sulphuric acid for lead, the residue, which may be left, for tin, according to the directions of § 190, 2, b (123). $192. (Precipitation with Sulphide of Ammonium, Separation and Detection of the Oxides of Groups III. and IV.: Alumina, Sesquioxide of Chromium; -Oxide of Zinc, Protoxide of Manganese, Protoxide of Nickel, Protoxide of Cobalt, Proto- and Sesquioxide of Iron; and also of those Salts of the Alkaline Earths which are precipitated by Ammonia from their Solution in Hydrochloric Acid; Phosphates, Borates, Oxalates, Silicates, and Fluorides.) PUT A small portion OF THE FLUID IN WHICH SOLUTION OF HYDRO- 134 SULPHURIC ACID HAS FAILED TO PRODUCE A PRECIPITATE (§ 188, 1, [110]), OR OF THE FLUID WHICH HAS BEEN FILTERED FROM THE PRECI * For another method of distinguishing cadmium, copper, lead, and bismuth from each other, I refer to the Third Section (additions and remarks to § 191) page 274. + If you have an aqueous solution, or a solution in very dilute hydrochloric acid, the oxide of mercury formed was present in the original substance in that form; but if the solution has been prepared by boiling with concentrated hydrochloric acid, or by heating with nitric acid, the mercury may most likely have been originally present in the form of suboxide, and may have been converted into oxide in the process. |