upon their dilution or not. The dilute solution is then mixed with carbonate of ammonia in excess, heated for a short time, nearly to ebullition, and the fluid subsequently filtered off from the precipitated carbonate of bismuth, which latter is collected upon the filter, dried, and ignited, exactly as directed § 87. (Ignition of carbonate of lead.) The process of ignition converts the carbonate of bismuth into the pure oxide. For the properties of the precipitate and residue, vide § 60. This method, if properly executed, yields very accurate results, although a very trifling loss occurs in most instances, owing to the circumstance that carbonate of bismuth is not absolutely insoluble in carbonate of ammonia. This method, as already stated, is inapplicable to solutions of salts of bismuth, which, besides the solvent, (nitric acid,) contain other acids. If it were attempted, for instance, to precipitate in this manner, a solution of bismuth containing sulphuric acid, or hydrochloric acid, the precipitate would contain, in the former case, an admixture of basic sulphate; in the latter, of basic chloride of bismuth. Were the precipitate filtered off, without previous application of heat, part of the basic carbonate of bismuth would remain in solution, and thus a considerable loss would be incurred. (Compare Experiment No. 62.) b. By ignition. a. Compounds like carbonate or nitrate of bismuth are ignited in a porcelain crucible until their weight ceases to diminish. B. Compounds of bismuth with organic acids are treated like the correponding compound of oxide of copper, vide § 90, 1 b, B. e. By precipitation as sulphuret of bismuth. The solution of the compound under examination is diluted with water, slightly mixed with acetic acid, (to prevent the subsequent precipitation of basic salt,) and precipitated with sulphuretted hydrogen either as gas or in solution; the fluid is filtered off from the precipitated sulphuret of bismuth, and the latter carefully washed. Another method of precipitating bismuth as sulphuret is, to mix the diluted solution with ammonia to complete neutralization of the free acid present, and then to add sulphuret of ammonium in excess. The precipitated sulphuret of bismuth may be weighed as such if the operator has every reason to be perfectly convinced that no free sulphur has been precipitated in conjunction with the sulphuret. But as the solutions of the salts of bismuth contain generally much free nitric acid, which may readily cause the decomposition of part of the sulphuretted hydrogen used as precipitant, it is invariably the safest way to convert the sulphuret into oxide. For this purpose the filter, with the washed and still moist precipitate, is introduced into a beaker-glass, moderately concentrated nitric acid poured over it, and heat applied until the sulphuret of bismuth is completely decomposed; the solution is then diluted with water slightly acidulated with acetic or nitric acid, and filtered; the filter is washed with the same kind of acidulated water, and the filtrate finally precipitated as directed sub. a. § 92. 7. OXIDE OF CADMIUM. a. Solution. Cadmium, oxide of cadmium, and all those of its compounds which are insoluble in water, are dissolved in hydrochloric or nitric acid. b. Determination. Cadmium is weighed either as OXIDE, or as SULPHURET OF CADMIUM, (§ 61.) We may convert into 1. OXIDE OF CADMIUM. a. By precipitation. All the compounds of cad b. By ignition. Salts of cadmium with readily mium soluble in water, and those of the insoluble compounds, the acid of which is removed upon solution in hydrochloric acid; likewise salts of cadmium with organic acids. volatile or easily decomposable inorganic oxygen acids. 2. SULPHURET OF CADMIUM. All compounds of cadmium without exception. 1. Determination as oxide of cadmium. a. By precipitation. The solution of the compound under examination is precipitated with carbonate of potass, and the precipitated carbonate of cadmium washed, dried, and ignited. The process of ignition converts the carbonate into the pure oxide of cadmium. For the details, accuracy, &c., of the method, vide § 80, (zinc.) For the properties of the precipitate and residue, vide § 61. b. By ignition. The same method is pursued as described § 80 c. (zinc.) 2. Determination as sulphuret of cadmium. Neutral or acid solutions are precipitated with sulphuretted hydrogen gas, or with sulphuretted hydrogen water; alkaline solutions are precipitated with sulphuret of ammonium. The precipitate is collected upon a weighed filter, washed, dried at 212°, and weighed. For the properties of the precipitate, vide § 61. The results are accurate. Should the precipitated sulphuret of cadmium contain an admixture of free sulphur, it must be converted into nitrate or chloride of cadmium, and subsequently precipitated as carbonate. For the details, vide § 80, b. (zinc.) SIXTH GROUP. PEROXIDE OF GOLD-PEROXIDE OF PLATINUM-OXIDE OF ANTI Metallic gold and all those compounds of gold which are insoluble in water, are heated with hydrochloric acid, and nitric acid gradually added until complete solution ensues. b. Determination. Gold is invariably weighed in its pure metallic state, to which its compounds are reduced by ignition, or by precipitation as metallic gold, or lastly, by precipitation as sulphuret of gold with subsequent ignition. a. By ignition. METALLIC GOLD. All those combinations of gold, which contain no fixed acid. b. By precipitation as metallic gold. All compounds of gold without exception.-This method is applied in all cases where the application of a. is inadmissible. c. By precipitation as sulphuret of gold. This method serves to separate gold in solution from certain other metals. Determination as metallic gold. a. By ignition. The compound under examination is heated in a covered porcelain or platinum crucible, gently at first, finally to redness, and the residuary pure gold weighed. For the properties of this, vide § 62. The results are exceedingly accurate. b. By precipitation as metallic gold. a. The solution of gold is free from nitric acid. The solution is mixed with some hydrochloric acid,-should it not already contain this acid in a free state, and a clear solution of protosulphate of iron added in excess. A gentle heat is then applied, and maintained for several hours, until the precipitated fine gold powder has completely subsided; the fluid is then filtered off from the gold, and the latter washed, dried, and ignited. (§ 32.) The precipitation is most advantageously effected in a porcelain dish, as a vessel of this description permits the removal of the heavy and fine gold powder by rinsing more readily than a beaker-glass. This method, if properly executed, yields very accurate results. B. The solution of gold contains nitric acid. The solution is evaporated in the water-bath to a syrupy consistence, adding from time to time, during the evaporation, a fresh amount of hydrochloric acid; the residue is dissolved in water acidulated with hydrochloric acid, and the solution treated as directed sub. a. It may happen that during the process part of the perchloride of gold is decomposed into protochloride and metallic gold; the latter will remain undissolved when the residue is treated with the acidulated water; this, however, does not require the slightest modification of the method. y. In cases where it is desirable to avoid the presence of iron in the filtrate, the analysed compound of gold may be reduced with oxalic acid. For this purpose, the solution-(being previously freed from nitric acid, if necessary-vide B.)—is introduced into a beaker-glass, and mixed with oxalate of ammonia in excess; some hydrochloric acid is added, should this not be present already,-and the vessel, covered with a glass plate, kept standing for two days in a moderately warm place; after the lapse of this time, the whole of the gold present will be found to have separated in small yellow -- |