The distillation of zinc in the laboratory being a troublesome and costly operation, chemists generally use the raw product of the reduction of the ore by distillation, as this contains, at least in many cases, only such trifling traces of iron that it may be safely used for reducing solutions of sesquioxide of iron. The zinc of commerce, which is prepared by fusing the reduced metal in iron pots, contains much more iron, as melted zinc has the property of slowly dissolving that metal. Of the several sorts of unrefined zinc which I have had occasion to examine, Silesian zinc contains the least admixture of iron. To procure zinc which leaves no residue upon solution in dilute sulphuric acid, it is indispensable to have recourse to re-distillation of the commercial article. This is effected in a retort made of the material of Hessian or blacklead crucibles. The operation is conducted in a wind-furnace with good draught. The neck of the retort must hang down as perpendicular as possible. Under the neck is placed a basin or small tub, filled with water. The distillation begins as soon as the retort is at a bright red heat. As the neck of the retort is very liable to become choked up with zinc, or oxide of zinc, it is necessary to keep it constantly free by means of a piece of iron wire. The zinc obtained by this re-distillation is quite free from lead, or, at least, nearly so, but it still contains perceptible traces of iron (from the wire used to keep the neck of the retort free). : Tests. The following is the simplest way of testing the purity of zinc dissolve the metal in dilute sulphuric acid in a small flask provided with a gas-evolution tube, place the outer limb of the tube under water, and, when the solution is completed, let the water entirely or partly recede into the flask; after cooling, add to the fluid, drop by drop, a sufficiently dilute solution of permanganate of potassa. If a drop of that solution imparts the same red tint to the zine solution, as to an equal volume of water, the zinc may be considered free from iron. I prefer this way of testing the purity of zinc to other methods, as it affords, at the same time, an approximate, or, if the zinc has been weighed, and the chameleon solution (which, in that case, must be considerably diluted) measured, an accurate and precise knowledge of the quantity of iron present. If lead or copper are present, these metals remain undissolved upon solution of the zinc. 2. COPPER. The copper of commerce, with the exception of the Japanese, which is not always easy to procure, is rarely sufficiently pure for analytical purposes. The following is the most convenient mode of preparing pure copper precipitate the metal from a solution of the sulphate by a clean iron plate, free the precipitated copper from the iron by boiling with hydrochloric acid; wash, dry, fuse, and roll into thin sheets (Fuchs). Tests.-Pure copper must dissolve completely in nitric acid, and addition of ammonia in excess to this solution must not, even after long standing, produce in it the faintest trace of a precipitate (iron, lead, &c.); neither should the addition of hydrochloric acid render the solution turbid (silver). Sulphuretted hydrogen must completely precipitate the copper from the solution. Uses. This metal serves us occasionally in indirect analysis; thus it is used, for instauce, to determine the amount of copper present in a fluid, and also the amount of protoxide of iron existing in presence of the sesquioxide of that metal, &c. However, since the introduction of volumetrical methods, it is but rarely used in quantitative analysis. 3. OXIDE OF LEAD. Precipitate pure nitrate or acetate of lead with carbonate of ammonia, wash the precipitate, dry, and ignite gently to complete decomposition. Oxide of lead is often used to fix an acid, so that it is not expelled even at a red heat. 4. OXIDE OF MERCURY. Preparation.-Pour a solution of chloride of mercury into a hot moderately dilute solution of soda, taking care to leave the solution of soda always in excess, and wash the yellow precipitate thoroughly by decantation. Tests.-Oxide of mercury must leave no residue when heated to redness in a platinum crucible. Uses. This reagent serves, in quantitative investigations, principally to decompose chloride of magnesium in the process of separating magnesia from the alkalies. b. Sulphur Bases. 1. SULPHIDE OF AMMONIUM (see “Qual. Anal.”). We require both the colorless simple sulphide, and the higher yellow sulphide. 2. SULPHIDE OF SODIUM (see "Qual. Anal."). IV. SALTS. a. Salts of the Alkalies. $61. 1. SULPHATE OF POTASSA (see "Qual. Anal."). 2. PHOSPHATE OF AMMONIA. Preparation.—Mix pure dilute phosphoric acid (prepared from phosphorus) of 113 sp. gr. with an equal quantity of water, add pure ammonia to the mixture until it shows a strongly alkaline reaction, let it stand some time, filter, if necessary, and keep for use. Tests.-Phosphate of ammonia must be free from any admixture of arsenic acid, nitric acid, and sulphuric acid, but more particularly of potassa, or soda. The presence of either of these alkalies may be detected by adding solution of pure acetate of lead, until the formation of a precipitate ceases, filtering, precipitating the excess of lead with sulphuretted hydrogen, filtering again, evaporating to dryness, and igniting the residue. If there now remains a residue soluble in water, and of alkaline reaction, this may be considered a conclusive proof of the presence of soda or potassa. In most cases phosphate of soda (see "Qual. Anal.") may be substituted for phosphate of ammonia. 3. OXALATE OF AMMONIA (see "Qual. Anal.”). 4. ACETATE OF AMMONIA (see "Qual. Anal.”). Preparation.-Saturate succinic acid, which has been purified by re crystallization from its solution in nitric acid, with dilute ammonia. The reaction of the new compound should be rather slightly alkaline than acid. Uses. This reagent serves occasionally to separate sesquioxide of iron from other metallic oxides. 6. CARBONATE of Soda. This reagent is required both in solution and in pure crystals; in the latter form to neutralize an excess of acid in a fluid which it is desirable not to dilute overmuch. 7. CARBONATE OF AMMONIA (see "Qual. Anal."). b. Salts of the Alkaline Earths. § 62. 1. CHLORIDE OF BARIUM (see "Qual. Anal.”). The following process, which I have of late devised, gives a very pure chloride of barium, free from lime and strontia :-Transmit through a concentrated solution of impure chloride of barium hydrochloric gas, as long as a precipitate continues to form. Nearly the whole of the chloride of barium present is by this means separated from the solution, in form of a crystalline powder. Collect this on a filter, let the adhering liquid drain off, wash the powder repeatedly with small quantities of pure hydrochloric acid, until a sample of the washings, diluted with water, and precipitated with sulphuric acid, gives a filtrate which, upon evaporation in a platinum dish, leaves no residue. The hydrochloric mother-liquor serves to dissolve fresh portious of witherite. I make use of the chloride of barium so obtained, principally for the preparation of perfectly pure carbonate of baryta, which is often required in quantitative analyses. 2. ACETATE Of Baryta. Preparation.-Dissolve pure carbonate of baryta in moderately dilute acetic acid, filter, and evaporate the filtrate to crystallization. Tests.-Dilute solution of acetate of baryta must not be troubled by solution of nitrate of silver. See also "Qual. Anal.," Chloride of barium, the same tests being also used to ascertain the purity of the acetate. Uses.-Acetate of baryta is used instead of chloride of barium, to effect the precipitation of sulphuric acid, in cases where it is desirable to avoid the introduction of a metallic chloride into the solution, or to convert the base into an acetate. As the reagent is seldom required, it is best kept in crystals. 3. CARBONATE OF BARYTA (see "Qual. Anal."). 4. CHLORIDE OF STRONTIUM. Preparation.-Chloride of strontium is prepared from strontianite or celestine, by the same process as chloride of barium from witherite. The pure crystals obtained are dissolved in spirit of wine of 96 per cent., the solution is filtered, and the filtrate kept for use. Uses.-The alcoholic solution of chloride of strontium is used to effect the conversion of alkaline sulphates into metallic chlorides, in cases where it is desirable to avoid the introduction into the fluid of a salt insoluble in spirit of wine. 5. CHLORIDE OF CALCIUM (see "Qual. Anal."). 6. FLUORIDE OF CALCIUM. This substance is used in quantitative analysis to displace and determine boracic acid; none but perfectly pure fluor-spar (like that of Derbyshire), free from any admixture of silicic acid, is applicable for this purpose. The hydrofluoric acid used in analytical investigations for the decomposition of silicates may be prepared from a less pure kind of fluor-spar. Tests. The best way of testing the purity of fluor spar, in case of doubt, is to pulverize it finely, and to treat a weighed amount of the powder with pure concentrated sulphuric acid, in a platinum crucible, at a gentle heat, which is gradually increased to redness; the same operation being repeated until the weight of the residue remains constant. The fluor-spar may be considered pure if the calculated amount of sulphate of lime is obtained by this process. 7. SULPHATE OF MAGNESIA (see "Qual. Anal."). This reagent is principally used to precipitate phosphoric acid from aqueous solutions. The solution required for this purpose should be kept ready prepared; it is made by dissolving 1 part of crystallized sulphate of magnesia and 1 part of pure chloride of ammonium in 8 parts of water and 4 parts of solution of ammonia, allowing the fluid to stand at rest for several days, and then filtering. c. Salts of the Oxides of the Heavy Metals. § 63. 1. SULPHATE OF PROTOXIDE OF IRON (see "Qual. Anal."). 2. SESQUICHLORIDE OF IRON (see "Qual. Anal."). 3. ACETATE OF SESQUIOXIDE OF URANIUM. Heat finely powdered pitchblende with dilute nitric acid, filter the fluid from the undissolved portion, and treat the filtrate with hydrosulphuric acid, to remove the lead, copper, and arsenic; filter again, evaporate the filtrate to dryness, extract the residue with water, and filter the solution from the oxides of iron, cobalt and manganesc. Nitrate of sesquioxide of uranium crystallizes from the filtrate; purify this by recrystallization, and then heat the crystals until a small portion of the sesquioxide of uranium is reduced. Warm the yellowish-red mass thus obtained with acetic acid, filter and let the filtrate crystallize. The crystals are acetate of sesquioxide of uranium, and the mother-liquor contains the remainder of the nitrate (Wertheim). Tests.-Solution of acetate of sesquioxide of uranium must not be altered by hydrosulphuric acid; carbonate of ammonia must produce in it a precipitate, soluble to a clear fluid in an excess of the precipitant. Uses.-Acetate of sesquioxide of uranium may serve, in many cases, to effect the separation and quantitative determination of phosphoric acid. 4. NITRATE OF SILVER (see "Qual. Anal.”). 6. Chloride of MERCURY (see "Qual. Anal.”). 9. SODIO-PROTOCHLORIDE OF PALLADIUM (see "Qual. Anal.”). B. REAGENTS FOR GRAVIMETRICAL ANALYSIS IN 1. CARBONATE OF SODA, pure anhydrous (see “Qual. Anal.”). 4. NITRATE OF POTASSA (see "Qual. Anal."). Preparation. Heat crystallized borax (see “Qual. Anal.") in a platinum or porcelain dish, until there is no further intumescence; reduce the porous mass to powder, and heat this in a platinum crucible until it is fused to a transparent mass. Pour the semi-fluid, viscid mass upon a fragment of porcelain. A better way is to fuse the borax in a net of platinum gauze, by making the gas blowpipe-flame act upon it. The drops are collected in a platinum dish. The vitrified borax obtained is kept in a well stoppered bottle. But as it is always necessary to heat the vitrified borax previous to use, to make quite sure that it is perfectly anhydrous, the best way is to prepare it only when required. Uses.-Vitrified borax is used to effect the expulsion of carbonic acid and other volatile acids, at a red heat. 7. BISULPHATE OF POTASSA. Preparation. Mix 87 parts of neutral sulphate of potassa (see “Qual. Anal."), in a platinum crucible, with 49 parts of concentrated pure sulphuric acid, and heat to gentle redness, until the mass is in a state of uniform and limpid fusion. Pour the fused salt on a fragment of porcelain, or into a platinum dish standing in cold water. After cooling. break the mass into pieces, and keep for use. Uses. This reagent serves as a flux for certain native compounds of alumina and sesquioxide of chromium. Bisulphate of potassa is used also, as we have already had occasion to state, for the cleansing of platinum crucibles; for this latter purpose, however, the salt which is obtained in the preparation of nitric acid, will be found sufficiently Preparation. (See "Qual. Anal.")-This reagent serves to convert the bisulphates of the alkalies into neutral salts. It must completely volatilize when heated in a platinum dish. 9. NITRATE OF AMMONIA. |