b. By Ignition. a. Compounds like the carbonate or nitrate of teroxide of bismuth are ignited in a porcelain crucible until their weight remains constant. B. Compounds of teroxide of bismuth with organic acids are treated like the corresponding compounds of oxide of copper (§ 119, 1, d.). c. By Precipitation as Tersulphide of Bismuth. Dilute the solution with water slightly acidulated with acetic acid (to prevent the precipitation of a basic salt), and precipitate with sulphuretted hydrogen water or gas; allow the precipitate to subside, and test a portion of the supernatant fluid with sulphuretted hydrogen water; if it remains clear, which is a sign that the bismuth is completely precipitated, filter, and wash the precipitate. Or mix with ammonia until the free acid is neutralized, and then add sulphide of ammonium in excess. The tersulphide of bismuth may be weighed in that form, if the operator is convinced that no free sulphur has been thrown down in conjunction with it. But, as in the case of compounds of bismuth we have mostly to deal with very acid solutions, and which contain nitric acid, there is generally reason to apprehend a partial decomposition of the sulphuretted hydrogen, attended with separation of free sulphur, on which account it is always safest to convert the tersulphide into teroxide. To this end the filter, with the washed and still moist precipitate on it, is put into a beaker, and heated with moderately strong nitric acid until complete decomposition is effected; the solution is then diluted with water slightly acidulated with acetic or nitric acid, and filtered, the filter being washed with the acidulated water; the filtrate is then finally precipitated as directed in a. 2. Determination of Bismuth as Chromate of Teroxide (J. Löwe's method, "Journ. f. prakt. Chem.," 67, 464). Pour the solution of teroxide of bismuth, which must be as neutral as possible, and must, if necessary, be first freed from the excess of nitric acid by evaporation on the water-bath, into a warm solution of pure bichromate of potassa, in a porcelain dish, with stirring, and take care to leave the alkaline chromate slightly in excess. Rinse the vessel which contained the solution of bismuth with water containing nitric acid, and add the rinsings to the contents of the porcelain dish. The precipitate formed must be orange-yellow, and dense throughout; if it is flocculent, and has the color of the yolk of an egg, this is a sign that there is a deficiency of chromate of potassa; in which case add a fresh quantity of this salt, taking care, however, to guard against too great an excess, and boil until the precipitate presents the proper appearance. Boil the contents of the dish for ten minutes, with stirring; then wash the precipitate, first by repeated boiling with water and decantation on to a weighed filter, at last thoroughly on the latter with boiling water, dry at about 248° F., and weigh. For the properties and composition of the precipitate, see § 86.* Results very satisfactory. * Pearson (“ Phil. Mag.,❞ xi. 204) gives Bi 0,, Cr O, as the formula of the precipitate, and, at the same time, 1 grm. of bismuth in 1-491 of chromate of teroxide, which involves a contradiction. Both statements are erroneous. His volumetrical estimation is based upon the mistaken assumption that chromate of teroxide of bismuth is insoluble in dilute nitric acid, whilst it is only so in presence of a sufficient excess of chromate of potassa. 216 OXIDE OF CADMIUM-PROTOXIDE OF PALLADIUM. [§§ 121, 122. § 121. 7. OXIDE OF CADMIUM. a. Solution. Cadmium, its oxide, and all the other compounds insoluble in water, are dissolved in hydrochloric acid or in nitric acid. b. Determination. Cadmium is weighed either in the form of oxide, or in that of sulphide (§ 87). We may convert into 1. OXIDE OF CADMIUM. a. By Precipitation. The compounds of cadmium which are soluble in water; the insoluble compounds, the acid of which is removed upon solution in hydrochloric acid; salts of cadmium with organic acids. b. By Ignition. Salts of cadmium with readily volatile or easily decomposable inorganic oxygen acids. 2. SULPHIDE OF CADMIUM. All compounds of cadmium without exception. 1. Determination as Oxide of Cadmium. a. By Precipitation. Precipitate with carbonate of soda or potassa, wash the precipitated carbonate of cadmium, and reduce it, by ignition, to the state of pure oxide. For the details of the method of proceeding, and the accuracy of the results, I refer to § 108 (Zinc). For the properties of the precipitate and residue, see § 87. b. By Ignition. See § 108, c (Zinc). 2. Determination as Sulphide of Cadmium. Precipitate the solution, if neutral or acid, with sulphuretted hydrogen water or gas-if alkaline, with sulphide of ammonium; collect the precipitate on a weighed filter, wash, dry at 212° F., and weigh. For the properties of the precipitate, see § 87. The results are accurate. Should the precipitated sulphide of cadmium contain an admixture of free sulphur, it must be converted into nitrate or chloride of cadmium, and subsequently precipitated as carbonate of cadmium, in the manner directed, § 108 (Zinc). The precipitation of sulphur along with the sulphide of cadmium may often be prevented, by adding to the solution of cadmium cyanide of potassium until the precipitate formed is redissolved, and then precipitating the solution with sulphuretted hydrogen. Supplement to the Fifth Group. 8. PROTOXIDE OF PALLADIUM. Protoxide of palladium is reduced to the metallic state for the purpose of effecting its quantitative estimation. This may be done in several ways. a. Neutralize the solution of protochloride of palladium almost completely with carbonate of soda, mix with a solution of cyanide of mercury; and digest the mixture for some time. A yellowish-white precipitate of protocyanide of palladium will subside, from dilute solutions, only after the lapse of some time. Wash this precipitate, dry, and ignite; weigh the reduced metal obtained. If the solution contains nitrate of protoxide, evaporate it first with hydrochloric acid to dryness; as the precipitate obtained from a solution of the kind deflagrates upon ignition (Wollaston). b. Mix the solution of the protochloride or nitrate of protoxide of palladium with formate of soda or potassa, and warm until no more carbonic acid escapes. The palladium precipitates in brilliant scales (Döbereiner). c. Precipitate the acid solution of palladium with sulphuretted hydrogen, filter, wash with boiling water, roast, and either convert the basic sulphate of protoxide of palladium formed into pure metal, by ignition over the blast gas-lamp, or dissolve it in hydrochloric acid, and precipitate as in a. Exposed to a moderate red heat metallic palladium becomes covered with a film varying from violet to blue, but at a higher temperature it recovers its lustre; this tarnishing and recovery of the metallic lustre is not attended with any perceptible difference of weight. Palladium requires the very highest degree of heat for its fusion. It dissolves readily in nitrohydrochloric acid, with difficulty in pure nitric acid, more easily in nitric acid containing nitrous acid, with difficulty in boiling hydrated sulphuric acid. SIXTH GROUP OF THE BASES. OF TIN-ARSENIOUS AND TEROXIDE OF GOLD-BINOXIDE OF PLATINUM-TEROXIDE OF ANTIMONY-BINOXIDE OF TIN-PROTOXIDE ARSENIC ACIDS-(MOLYBDIC ACID). Metallic gold, and all compounds of gold insoluble in water, are warmed with hydrochloric acid, and nitric acid is gradually added until complete solution is effected; or they are repeatedly digested with strong chlorine water. The latter method is resorted to more especially in cases where the quantity of gold to be dissolved is small, and mixed up with other oxides, which it is wished to leave undissolved. b. Determination. Gold is always weighed in the metallic state, to which its compounds are reduced by direct precipitation, or by ignition, preceded, in some cases, by precipitation as tersulphide of gold. We may convert into 218 c. By Precipitation as Tersulphide of Gold. This method serves to effect the separation of gold from certain other metals which may be mixed with it in a solution. Determination as Metallic Gold. a. By Ignition. Heat the compound, in a covered porcelain or platinum crucible, very gently at first, but finally to redness, and weigh the residuary pure gold. For properties of the residue, see § 88. The results are most accurate. b. By Precipitation as Metallic Gold. a. The solution is free from Nitric Acid. Mix the solution with a little hydrochloric acid, if it does not already contain some of that acid in the free state, and add a clear solution of sulphate of protoxide of iron in excess; heat gently for a few hours until the precipitated fine gold powder has completely subsided; filter, wash, dry, and ignite (§ 52). A porcelain dish is a more appropriate vessel to effect the precipitation in than a beaker, as the heavy fine gold powder is more readily rinsed out of the former than out of the latter. The results are accurate. α. B. The solution of Gold contains Nitric Acid. Evaporate the solution, on a water-bath, to the consistence of syrup, adding from time to time hydrochloric acid; dissolve the residue in water containing hydrochloric acid, and treat the solution as directed in It will sometimes happen that the residue does not dissolve to a clear fluid, in consequence of a partial decomposition of the terchloride of gold into protochloride and metallic gold; however, this is a matter of perfect indifference. y. In cases where it is wished to avoid the presence of iron in the filtrate, the gold may be reduced by means of oxalic acid. the solution-freed previously, if necessary, from nitric acid, in the To this end, manner directed in ẞ-is mixed, in a beaker, with oxalate of ammonia in excess, some hydrochloric acid added (if that acid is not already present in the free state), and the vessel, covered with a glass plate, is kept standing for two days in a moderately warm place. of that time, the whole of the gold will be found to have separated in At the end small yellow scales; the supernatant fluid is filtered off from the gold, which is then washed, dried, and ignited. c. By Precipitation as Tersulphide of Gold. Sulphuretted hydrogen gas is transmitted in excess through the dilute solution of the compound under examination; the precipitate formed is speedily filtered off, without heating, washed, dried, and ignited in a porcelain or platinum crucible. For the properties of the precipitate, see § 88. The results are accurate. a. Solution. $124. 2. BINOXIDE OF PLATINUM. Metallic platinum, and the compounds of platinum which are insoluble in water, are dissolved by digestion, at a gentle heat, with nitrohydrochloric acid. b. Determination. Platinum is invariably weighed in the pure metallic state, to which its compounds are reduced, either by precipitation as bichloride of platinum and chloride of ammonium, or as bichloride of platinum and chloride of potassium, or as bisulphide of platinum, or by ignition, or by precipitation with reducing agents. All compounds of platinum, without exception, may, in most cases, be reduced to the metallic state by either of these methods. Which is the most advantageous process to be pursued in special instances, depends entirely upon the circumstances of particular cases. The reduction of compounds of platinum to the metallic state by simple ignition is preferable to the other methods, in all cases where its application is admissible. The precipitation as bisulphide of platinum is resorted to exclusively to effect the separation of platinum from other metals. Determination as Metallic Platinum. a. By Precipitation as Bichloride of Platinum and Chloride of Am monium. Mix the solution, in a beaker, with ammonia until the excess of acid (that is, supposing an excess of acid to be present) is nearly saturated; add chloride of ammonium in excess, and mix the fluid with a pretty large quantity of absolute alcohol. Should your solution of platinum be very dilute, you must concentrate it previously to the addition of the ammonia. Cover the vessel now with a glass plate, and let it stand for twentyfour hours, after which filter on an unweighed filter, wash the precipitate thoroughly with spirit of wine of about 80 per cent., and dry carefully. Introduce the dry precipitate, wrapped up in the filter, into a weighed porcelain or platinum crucible, put on the lid, and apply a very gentle heat until no more fumes of chloride of ammonium escape; now remove the lid, place the crucible obliquely (§ 41), and incinerate the filter. Apply finally an intense heat for some time, and then weigh the residue. For the properties of the precipitate and residue, see § 89. The results are satisfactory, though generally a little too low, as the bichloride of platinum and chloride of ammonium is not altogether insoluble in spirit of wine (Experiment No. 15); and as the fumes of chloride of ammonium evolved during the first stage of the process of ignition are liable to carry away traces of the yet undecomposed double chloride, if the application of heat is not conducted with the greatest possible care. If the precipitated bichloride of platinum and chloride of ammonium were weighed in that form, the results would be inaccurate, since, as I have convinced myself by direct experiments, it is impossible to completely free the double chloride, by washing with spirit of wine, from all traces of the chloride of ammonium thrown down in conjunction with it, without dissolving, at the same time, a considerable portion of the double chloride. As a general rule, however, the results obtained by weighing the bichloride of platinum and chloride of ammonium in that form are one or two per cent. too high. b. By Precipitation as Bichloride of Platinum and Chloride of Potas sium. Mix the solution of the compound under examination, in a beaker, |