7. HEAVY METALS AND THEIR OXIDES. § 34. 1. ZINC (Zn). Select zinc of good quality and, above all, perfectly free from arsenic. The method described §§ 131, 10 will serve to detect the presence of the slightest trace of this substance. Fuse the metal, and pour the fused mass in a thin unbroken stream into a large vessel with water. Should the zinc contain arsenic, put the granulated metal into a Hessian crucible, in alternate layers, with part of nitrate of potassa, heat the crucible gradually until the reaction sets in, and the metal ultimately fuses; granulate now again (Meillet). Uses.-Zinc serves in qualitative analysis for the evolution of hydrogen, and also of arsenetted and antimonetted hydrogen gases (compare §§ 130 and 131); it is occasionally used also to precipitate some metals from their solutions; in which process the zinc simply displaces the other metal (Cu O, SO, + Zn = Zn O, S 0, + Cu). 3 § 35. 2. IRON (Fe). Iron reduces many metals, and precipitates them from their solutions in the metallic state. We use it especially for the detection of copper, which precipitates upon it with its characteristic color. Any clean surface of iron, such as a knife-blade, a needle, a piece of wire, &c., will serve for this purpose. § 36. 3. COPPER (Cu). We use copper exclusively to effect the reduction of mercury, which precipitates upon it as a white coating shining with silvery lustre when rubbed. A copper coin scoured with fine sand, or in fact any clean surface of copper, may be employed for this purpose. $37. 4. HYDRATE OF TEROXIDE OF BISMUTH (Bi O,, H 0).* Preparation.-Dissolve bismuth, freed from arsenic by fusion with hepar sulphuris or nitrate of potassa, in dilute nitric acid; dilute the solution as much as is practicable without producing a permanent precipitate; filter, and evaporate the filtrate to crystallization. Wash the crystals with water containing nitric acid, triturate them with water, add ammonia in excess, and let the mixture digest for some time; then filter, wash, and dry the white precipitate, and keep it for use. Tests.-Hydrosulphuric acid must throw down from a solution of this reagent in dilute nitric acid, a precipitate insoluble in ammonia and sulphide of ammonium; and, accordingly, the fluid filtered off from the precipitate treated with ammonia must remain perfectly clear upon addition of hydrochloric acid, whilst in that filtered off from the precipitate * The basic nitrate of teroxide of bismuth of commerce, if perfectly free from arsenic and antimony, may also be used instead of the hydrate of teroxide. treated with sulphide of ammonium, that acid must only produce a pure white turbidity (sulphur). Uses.-Teroxide of bismuth, when boiled with alkaline solutions of metallic sulphides, decomposes with the latter, giving rise to the formation of metallic oxides and sulphide of bismuth. It is better adapted to effect decompositions of this kind than oxide of copper, since it enables the operator to judge immediately upon the addition of a fresh portion whether the decomposition is complete or not. It has still another advantage over oxide of copper, viz., it does not, like the latter, dissolve in the alkaline fluid in presence of organic substances; nor does it act as a reducing agent upon reducible oxygen compounds. We use it principally to convert tersulphide and pentasulphide of arsenic into arsenious and arsenic acids, for which purpose oxide of copper is altogether inapplicable, since it converts the arsenious acid immediately into arsenic acid, being itself reduced to the state of suboxide. b. SULPHUR BASES. $ 38. 1. SULPHIDE OF AMMONIUM (NHS). We use in analysis— a. Colorless proto-sulphide of ammonium. Preparation.-Transmit hydrosulphuric acid gas through 3 parts of solution of oxide of ammonium until no further absorption takes place; then add 2 parts more of the same solution of oxide of ammonium. The action of hydrosulphuric acid upon oxide of ammonium gives rise to the formation, first, of N H,, S (NH, O and HS = NHS and HO), then of N HS, HS; upon addition of the same quantity of solution of ammonia as has been saturated, the oxide of ammonium decomposes with the double sulphide of ammonium and hydrogen, or, as it is commonly called, the hydrosulphate of sulphide of ammonium, and simple or proto-sulphide of ammonium is formed (N H, S, HS+NH 0=2 (NHS) + HO. The rule, however, is to add only two-thirds of the quantity of solution of ammonia, as it is better the preparation should contain a little hydrosulphate of sulphide of ammonium, than that free ammonia should be present. To employ, as has usually been the case hitherto, hydrosulphate of sulphide of ammonium instead of the simple proto-sulphide, is unnecessary, and simply tends to increase the smell of sulphuretted hydrogen in the laboratory, as the preparation allows that gas to escape when in contact with metallic sulphur acids. Sulphide of ammonium should be kept in small well-stoppered bottles. It is colourless at first, and deposits no sulphur upon addition of acids. Upon exposure to the air, however, it acquires a yellow tint, owing to the formation of bisulphide of ammonium, which is attended also with formation of ammonia and water: 2 (NHS) + 0 = NH ̧ S2+ NH ̧ + H O. 4 2 Continued action of the oxygen of the air upon the sulphide of ammonium tends at first to the formation of still higher sulphides; but afterwards the fluid deposits sulphur, and, in the end, there remains nothing in solution but pure ammonia, the whole of the sulphur having separated. The sulphide of ammonium, which has turned yellow by exposure to the air, may be used for all purposes requiring the employment of yellow sulphide of ammonium: the yellow sulphide may also be expeditiously prepared by digesting the proto-sulphide with some sulphur. All kinds of yellow sulphide of ammonium deposit sulphur, and look turbid and milky on being mixed with acids. Tests. Sulphide of ammonium must strongly emit the odor peculiar to it; with acids it must evolve abundance of sulphuretted hydrogen; the evolution of gas may be attended by the separation of a pure white deposit, but no other precipitate must be formed. Upon evaporation and exposure to a red heat on a platinum dish, it must leave no residue. It must not precipitate, nor even render turbid, solution of magnesia or solution of lime (carbonate of ammonia or free ammonia). Uses. Sulphide of ammonium is one of the most frequently employed reagents. It serves (a) to effect the precipitation of those metals which hydrosulphuric acid fails to throw down from acid solutions, e. g. of iron, cobalt, &c. (NH,S+ Fe O, SO, Fe S+NH, O, SO,), (b) to separate the metallic sulphides thrown down from acid solutions by hydrosulphuric acid, as it dissolves some of them to sulphur salts, as, for instance, the sulphides of arsenic and antimony, &c. (N H, S, As S,, &c.), whilst leaving others undissolved-for instance, sulphide of lead, sulphide of cadmium, &c. The sulphide of ammonium used for this purpose must contain an excess of sulphur, if the metallic sulphides to be dissolved will dissolve only as higher sulphides, as, for instance, Sn S, which dissolves with ease only as Sn S2. 3 From solutions of salts of alumina and sesquioxide of chromium, sulphide of ammonium precipitates hydrates of these oxides, with escape of sulphuretted hydrogen, as the sulphur compounds corresponding to these oxides cannot form in the humid way. [AI, 0, 3S 0, +3 NH, S= Al, 0, 3 HO+ 3 (N H, O, SO,) ]. Salts insoluble in water are thrown down by sulphide of ammonium unaltered from their solutions in acids; thus, for instance, phosphate of lime is precipitated unaltered from its solution in hydrochloric acid. 4 $ 39. 2. SULPHIDE OF SODIUM (Na S). Preparation. Same as sulphide of ammonium, except that solution of soda is substituted for solution of ammonia. Keep the fluid obtained in well-stoppered bottles. If required to contain some higher sulphide of sodium, digest it with powdered sulphur. Uses. Sulphide of sodium is substituted for sulphide of ammonium to effect the separation of sulphide of copper from sulphur compounds soluble in alkaline sulphides, e. g., from proto-sulphide of tin, as sulphide of copper is not quite insoluble in sulphide of ammonium. IV. SALTS. Of the many salts employed as reagents, those of potassa, soda, and ammonia are used principally on account of their acids; salts of soda may, therefore, often be substituted for the corresponding potassa salts, &c. Thus it is almost always a matter of perfect indifference whether we use carbonate of soda or carbonate of potassa, ferrocyanide of potassium or ferrocyanide of sodium, &c. I have, accordingly, here classified the salts of the alkalies by their acids. With the salts of the alkaline earths and those of the oxides of the heavy metals the case is different ; these are not used for their acid, but for their base; we may, therefore, often substitute for one salt of a base another similar one, as e.g., nitrate or acetate of baryta for chloride of barium, &c. For this reason I have classified the salts of the alkaline earths and of the heavy metals by their bases. a. SALTS OF THE ALKALIES. § 40. 1. SULPHATE OF POTASSA (K O, S 0). Preparation.-Purify sulphate of potassa of commerce by recrystallization, and dissolve 1 part of the pure salt in 12 parts of water. Uses.-Sulphate of potassa serves to detect and separate baryta and strontia. It is in many cases used in preference to dilute sulphuric acid, which is employed for the same purpose, as it does not, like the latter reagent, disturb the neutrality of the solution. $ 41. 2. PHOSPHATE OF SODA (2 Na O, HO, PO, +24 aq.). 5 Preparation.-Purify phosphate of soda of commerce by recrystallization, and dissolve one part of the pure salt in 10 parts of water for use. Tests.-Solution of phosphate of soda must not become turbid when heated with ammonia. The precipitates which solution of nitrate of baryta and solution of nitrate of silver produce in it, must completely, and without effervescence, redissolve upon addition of dilute nitric acid. Uses.-Phosphate of soda precipitates the alkaline earths, and all metallic oxides, by double affinity. It serves, in the course of analysis, after the separation of the oxides of the heavy metals, as a test for alkaline earths in general; and, after the separation of baryta, strontia, and lime, as a special test for the detection of magnesia; for which latter purpose it is used in conjunction with ammonia, the magnesia precipitating, under these circumstances, as basic phosphate of magnesia and ammonia. § 42. 3. OXALATE OF AMMONIA (N H, O, C, O,+ aq., or NH ̧O, O + aq.). 4 2 Preparation.-Dissolve commercial oxalic acid in 20 parts of water, add caustic ammonia, or carbonate of ammonia, until the fluid begins to manifest a slight alkaline reaction; let it stand at rest for 12 hours, filter, and evaporate to crystallization. Purify by recrystallization. Dissolve 1 part of the pure salt in 24 parts of water for use. Tests.-The solution of oxalate of ammonia must not be precipitated nor rendered turbid by hydrosulphuric acid, nor by sulphide of ammonium. Uses.-Oxalic acid forms with lime, strontia, baryta, oxide of lead, and other metallic oxides, insoluble or very difficultly soluble compounds; oxalate of ammonia produces, therefore, in the aqueous solutions of the salts of these bases precipitates of the corresponding oxalates. In analysis it serves principally for the detection of lime. $ 43. 4. ACETATE OF SODA (Na O, C, H, 0, + 6 aq., or Na O, à + 6 aq.). 4 Preparation. Dissolve crystallized carbonate of soda in a little water, add to the solution acetic acid to slight excess, evaporate to crystallization, and purify the salt by recrystallization. For use dissolve 1 part of the salt in 10 parts of water. Tests.-Acetate of soda when boiled with a solution of molybdate of ammonia mixed with hydrochloric acid must not impart a yellow color to it. Uses.-The stronger acids in the free state decompose acetate of soda, combining with the base, and setting the acetic acid free. In the course of analysis, acetate of soda is used principally to precipitate phosphate of sesquioxide of iron (which is insoluble in acetic acid) from its solution in hydrochloric acid. Preparation.-Take bicarbonate of soda of commerce, put the powder into a funnel stopped loosely with some cotton, make the surface even, cover it with a disc of difficultly permeable paper with turned-up edges, and wash by pouring small quantities of water on the paper disc, until the filtrate, when acidified with nitric acid, is not rendered turbid by solution of nitrate of silver, nor by solution of chloride of barium. Let the salt dry, and then convert it by gentle ignition into the simple carbonate. This is effected best in a crucible or dish of silver or platinum; but it may be done also in a perfectly clean vessel of cast iron, or, on a small scale, in a porcelain dish. Pure carbonate of soda may be obtained also by repeated recrystallization of carbonate of soda of comFor use, dissolve 1 part of the anhydrous salt or 2·7 parts of the crystallized salt in 5 parts of water. merce. Tests.-Carbonate of soda intended for analytical purposes must be perfectly white. Its solution, when supersaturated with nitric acid, must not be rendered turbid by chloride of barium nor by nitrate of silver ; nor must addition of sulphocyanide of potassium impart a red, or boiling with molybdate of ammonia a yellow tint to it; the residue which remains upon evaporating its solution to dryness, after previous supersaturation with hydrochloric acid, must leave no residue (silicic acid) when redissolved in water. Uses. With the exception of the alkalies, carbonate of soda precipitates the whole of the bases, most of them as carbonates, but some also as hydrated oxides. Those bases which are soluble in water as bicarbonates require boiling for their complete precipitation from acid solutions. Many of the precipitates produced by the action of carbonate of soda exhibit a characteristic color, which may lead to the detection of the individual metals which they respectively contain. Solution of carbonate of soda serves, moreover, for the decomposition of many insoluble salts of the alkaline earths or of the metals, more particularly of those |