Читать книгу A System of Instruction in the Practical Use of the Blowpipe - Unknown - Страница 7

13. Fused Boracic Acid (BO³).—The commercial article is sufficiently pure for blowpipe analysis. It is employed in some cases to detect phosphoric acid, and also minute traces of copper in lead compounds.

14. Fluorspar (CaFl²).—This substance should be pounded fine and strongly heated. Fluorspar is often mixed with boracic acid, which renders it unfit for analytical purposes. Such an admixture can be detected if it be mixed with bisulphate of potassa, and exposed upon platinum wire to the interior or blue flame. It is soon fused, the boracic acid is reduced and evaporated, and by passing through the external flame it is reoxidized, and colors the flame green. We use fluorspar mixed with bisulphate of potassa as a test for lithia and boracic acid in complicated compounds.

15. Oxalate of Nickel (NiO, O).—It is prepared by dissolving the pure oxide of nickel in diluted hydrochloric acid. Evaporate to dryness, dissolve in water, and precipitate with oxalate of ammonia. The precipitate must be washed with caution upon a filter, and then dried. It is employed in blowpipe analysis to detect salts of potassa in the presence of sodium and lithium.

16. Oxide of Copper (CuO).—Pure metallic copper is dissolved in nitric acid. The solution is evaporated in a porcelain dish to dryness, and gradually heated over a spirit-lamp, until the blue color of the salt has disappeared and the mass presents a uniform black color. The oxide of copper so prepared must be powdered, and preserved in a vial. It serves to detect, in complicated compounds, minute traces of chlorine.

17. Antimoniate of Potassa (KO, SbO⁶).—Mix four parts of the bruised metal of antimony, with nine parts of saltpetre. Throw this mixture, in small portions, into a red-hot Hessian crucible, and keep it at a glowing heat for awhile after all the mixture is added. Boil the cooled mass with water, and dry the residue. Take two parts of this, and mix it with one part of dry carbonate of potassa, and expose this to a red heat for about half an hour. Then wash the mass in cold water, and boil the residue in water; filter, evaporate the filtrate to dryness, and then, with a strong heat, render it free of water. Powder it while it is warm, and preserve it in closed vials. It is used for the detection of small quantities of charcoal in compound substances, as it shares its oxygen with the carbonaceous matter, the antimony becomes separated, and carbonate of potassa is produced, which restores red litmus paper to blue, and effervesces with acids.

18. Silver Foil.—A small piece of silver foil is used for the purpose of detecting sulphur and the sulphides of the metals, which impart a dark stain to it. If no silver foil is at hand, strips of filtering paper, impregnated with acetate of lead, will answer in many cases.

19. Nitroprusside of Sodium (Fe²Cy⁵, NO⁵, 2Na).—This is a very delicate test for sulphur, and was discovered by Dr. Playfair. This test has lately been examined with considerable ability by Prof. J.W. Bailey, of West Point. If any sulphate or sulphide is heated by the blowpipe upon charcoal with the carbonate of soda, and the fused mass is placed on a watch-glass, with a little water, and a small piece of the nitroprusside of sodium is added, there will be produced a splendid purple color. This color, or reaction, will be produced from any substance containing sulphur, such as the parings of the nails, hair, albumen, etc. In regard to these latter substances, the carbonate of soda should be mixed with a little starch, which will prevent the loss of any of the sulphur by oxidation. Coil a piece of hair around a platinum wire, moisten it, and dip it into a mixture of carbonate of soda, to which a little starch has been added, and then heat it with the blowpipe, when the fused mass will give with the nitroprusside of sodium the characteristic purple reaction, indicative of the presence of sulphur. With the proper delicacy of manipulation, a piece of hair, half an inch in length, will give distinct indications of sulphur.

Preparation.—The nitroprussides of sodium and potassium (for either salt will give the above reactions), are prepared as follows: One atom (422 grains) of pulverized ferrocyanide of potassium is mixed with five atoms of commercial nitric acid, diluted with an equal quantity of water. One-fifth of this quantity (one atom) of the acid is sufficient to transfer the ferrocyanide into nitroprusside; but the use of a larger quantity is found to give the best results. The acid is poured all at once upon the ferrocyanide, the cold produced by the mixing being sufficient to moderate the action. The mixture first assumes a milky appearance, but after a little while, the salt dissolves, forming a coffee-colored solution, and gases are disengaged in abundance. When the salt is completely dissolved, the solution is found to contain ferrocyanide (red prussiate) of potassium, mixed with nitroprusside and nitrate of the same base. It is then immediately decanted into a large flask, and heated over the water-bath. It continues to evolve gas, and after awhile, no longer yields a dark blue precipitate with ferrous salts, but a dark green or slate-colored precipitate. It is then removed from the fire, and left to crystallize, whereupon it yields a large quantity of crystals of nitre, and more or less oxamide. The strongly-colored mother liquid is then neutralized with carbonate of potash or soda, according to the salt to be prepared, and the solution is boiled, whereupon it generally deposits a green or brown precipitate, which must be separated by filtration. The liquid then contains nothing but nitroprusside and nitrate of potash or soda. The nitrates being the least soluble, are first crystallized, and the remaining liquid, on farther evaporation, yields crystals of the nitroprusside. The sodium salt crystallizes most easily.—(PLAYFAIR.)

As some substances, particularly in complicated compounds, are not detected with sufficient nicety in the dry way of analysis, it will often be necessary to resort to the wet way. It is therefore necessary to have prepared the reagents required for such testing, as every person, before he can become an expert blowpipe analyst, must be acquainted with the characteristic tests as applied in the wet way.