(e.) Platinum Spoon.—When we require to fuse substances with the acid sulphate of potash, or to oxidize them by detonation with nitrate of potash, whereby we wish to preserve the oxide produced, we generally use a little spoon of platinum, about from nine to fifteen millimetres[1] in diameter, and shaped as represented in Fig. 7. The handle of this spoon is likewise of platinum, and should fit into a piece of cork, or be held with the forceps.
(f.) Platinum Forceps or Tongs.—We frequently are necessitated to examine small splinters of metals or minerals directly in the blowpipe flame. These pieces of metallic substances are held with the forceps or tongs represented as in Fig. 8, where ac is formed of steel, and aa are platinum bars inserted between the steel plates. At bb are knobs which by pressure so separate the platinum bars aa, that any small substance can be inserted between them.
FIG 8
(g.) Iron Spoons.—For a preliminary examination iron spoons are desirable. They may be made of sheet iron, about one-third of an inch in diameter, and are very useful in many examinations where the use of platinum would not be desirable.
(h.) Glass Tubes.—For the separation and recognition of volatile substances before the blowpipe flame, we use glass tubes. These should be about one-eighth of an inch in diameter, and cut into pieces about five or six inches in length. These tubes should have both ends open.
Tubes are of great value in the examination of volatile substances which require oxidizing or roasting, and heating with free access of air. Also to ascertain whether a substance under examination will sublimate volatile matter of a certain appearance. Such substances are selenium, sulphur, arsenic, antimony, and tellurium. These substances condense on a cool part of the tube, and they present characteristic appearances, or they may be recognized by their peculiar smell. These tubes must be made of the best kind of glass, white and difficult of fusion, and entirely free from lead. The substance to be examined must be put in the tube near one end, and exposed to the flame of the blowpipe. The end containing the substance must be held lower than the other end, and must be moved a little over the spirit-lamp before a draught of air is produced through the tube. It is a good plan to have a number of these tubes on hand. After having used a tube we cut off that end of it which contained the substance, with a file, and clean it from the sublimate, either by heating it over the spirit-lamp, or with a piece of paper wound around a wire. It sometimes happens that the substance falls out of the tube before it becomes sufficiently melted to adhere to the glass. To obviate this, we bend the tube not far from the end, at an obtuse angle, and place the substance in the angle, whereby the tube may be lowered as much as necessary. Fig. 9 will give the student a comprehension of the processes described, and of the manner of bending the tubes.
(i.) Glass Tubes closed at one End.—If we wish to expose volatile substances to heat, with the exclusion of air as much as possible, or to ascertain the contents of water, or other volatile fluids, or for the purpose of heating substances which will decrepitate, we use glass tubes closed at one end. These tubes must be about one-eighth of an inch wide, and from two to three inches in length. They should be made of white glass, difficult of fusion, and free from lead. They should be closed at one end, as figured in the margin, Fig. 10.
FIG 10
When a substance is to be examined for the purpose of ascertaining whether it contains combustible matter, as sulphur or arsenic, and where we wish to avoid oxidation, we use these tubes without extending the closed end, in order that there may be as little air admitted as possible, as is represented in tube B. But when a substance to be examined is to be tested for water, or other incombustible volatile matters, we employ tubes with little bulbs blown at one end, such as represented at tube A. Here there is room for a circulation of air at the bottom of the tube, by which the volatile matter rises more easily. In some cases, it is necessary to draw the closed end out to a fine point, as in the tubes C and D. Either one or the other of these tubes is employed, depending upon the nature of the substance used. The sublimates condense at the upper part of the tube a, and can be there examined and recognized. These tubes, before being used, must be thoroughly dried and cleaned. In experimenting with them, they should not be exposed at once to the hottest part of the flame, but should be submitted to the heat gradually. If the substance is of such a nature that it will sublime at a low heat, the tube should be held more horizontal, while a higher heat is attained by bringing the tube to a more vertical position.
Edulcorator or Washing Bottle.—Take a glass bottle of the capacity of about twelve ounces, and close the mouth of it very tight with a cork, through which a short glass tube is fitted airtight. The external end of this tube is drawn out to a point, with a very fine orifice. The bottle should be filled about half full of water. By blowing air into the bottle through the tube, and then turning it downwards, the compressed air will expel a fine stream of water through the fine orifice with considerable force. We use this washing bottle, Fig. 11, for the purpose of rinsing the small particles of coal from the reduced metals.
Agate Mortar and Pestle.—This mortar is used for the purpose of pulverizing hard substances, and for mixing fluxes. As this mortar will not yield to abrasion, there is no danger of any foreign matter becoming mixed with the substance pulverized in it. It should be cleaned after use with pumice stone. Steel mortars are very useful for the pulverization of hard bodies; but for all those substances which require great care in their analysis, and which can be obtained in very minute quantity, the agate mortar alone should be used.
A hammer made of steel is necessary. This should have the edge square.
A small anvil, polished on the surface, is also required. It is frequently used to test the malleability of metals.
A knife, for the purpose of ascertaining the hardness of minerals.
The student should also be provided with several three-edged files, and likewise with some flat ones.
A microscope, an instrument with two lenses, or with such a combination of lenses, that they may be used double or single, is frequently necessary for the examination of blowpipe experiments, or the reaction of the fluxes. Common lenses, howsoever cheap they may be, are certainly not recommended. A microscope with achromatic lenses can now be purchased so cheap that there is no longer any necessity of procuring one with the common lens. Besides, there is no reliability whatever to be placed in the revelations of the common lens; while on the contrary, the deceptive appearances which minute objects assume beneath such lenses are more injurious than otherwise. A small cheap set of magnifying glasses are all that is required for the purpose of blowpipe analysis, Fig. 12.
A small magnet should be kept on hand, for the purpose of testing reduced metals.
Nippers, for the purpose of breaking off pieces of minerals for analysis, without injuring the entire piece, are indispensable, Fig 13.
FIG 13
A pair of scissors is required to trim the wick of the and for the trimming of the edge of platinum foil.
A small spatula should be kept for the purpose of mixing substances with fluxes.
Those substances which possess the property of acting upon other substances, in such a characteristic manner that they can be recognized, either by their color, or by their effervescence, or by the peculiar precipitation produced, are termed reagents. The phenomena thus produced is termed reaction. We use those reagents, or tests, for the purpose of ascertaining the presence or the absence of certain substances, through the peculiar phenomena produced when brought in contact with them.
The number of reagents employed in blowpipe analysis is not great, and therefore we shall here give a brief description of their preparation and use. It is indispensably necessary that they should be chemically pure, as every admixture of a foreign substance would only produce a false result. Some of them have a strong affinity for water, or are deliquescent, and consequently absorb it greedily from the air. These must be kept in glass bottles, with glass stoppers, fitted air-tight by grinding.
1. Carbonate of Soda.—(NaO, CO2) Wash the bicarbonate of soda (NaO, 2CO2) upon a filter, with cold water, until the filtrate ceases to give, after neutralization with diluted nitric acid (NO5), a precipitate with nitrate of baryta, (BaO, NO5), or nitrate of silver, (AgO, NO5). That left upon the filter we make red hot in a platinum, silver, or porcelain dish. One atom of carbonic acid is expelled, and the residue is carbonate of soda.
A solution of soda must not be changed by the addition of sulphide of ammonium. And when neutralized with hydrochloric acid, and evaporated to dryness, and again dissolved in water, there must be no residue left.
Carbonate of soda is an excellent agent in reduction, in consequence of its easy fusibility, whereby it causes the close contact of the oxides with the charcoal support, so that the blowpipe flame can reach every part of the substance under examination.
For the decomposition and determination of insoluble substances, particularly the silicates, carbonate of soda is indispensable. But for the latter purpose, we use with advantage a mixture of ten parts of soda and thirteen parts of dry carbonate of potash, which mixture fuses more easily than the carbonate of soda alone.
2. Hydrate of Baryta (BaO, HO).—This salt is used sometimes for the detection of alkalies in silicates. Mix one part of the substance with about four parts of the hydrate of baryta, and expose it to the blowpipe flame. The hydrate of baryta combines with the silicic acid, and forms the super-basic silicate of baryta, while the oxides become free. The fused mass must be dissolved in hydrochloric acid, which converts the oxides into chlorides. Evaporate to dryness, and dissolve the residue in water. The silicic acid remains insoluble.
The hydrate of baryta is prepared by mixing six parts of finely powdered heavy-spar (BaO, SO3) with one part of charcoal and one and a half parts of wheat flour, and exposing this mixture in a Hessian crucible with a cover to a strong and continuous red heat. The cooled chocolate-brown mass must be boiled with twenty parts of water, and, while boiling, there must be added the oxide of copper in sufficient quantity, or until the liquid will not impart a black color to a solution of acetate of lead (PbO, A). The liquid must be filtered while hot, and as it cools the hydrate of baryta appears in crystals. These crystals must be washed with a little cold water, and then heated at a low temperature in a porcelain dish until the crystal water is expelled. The hydrate of baryta melts by a low red heat without losing its water of hydration.
3. Bisulphate of Potassa (KO, 2S03).—At a red heat the half of the sulphuric acid of this salt becomes free, and thus separates and expels volatile substances, by which we can recognize lithium, boracic acid, nitric acid, fluoric acid, bromine, iodine, chlorine; or it decomposes and reveals some other compounds, as, for instance, the salts of the titanic, tantalic and tungstic acids. The bisulphate of potash is also used for the purpose of converting a substance into sulphate, or to free it at once from certain constituents. These sulphates are dissolved in water, by which we are enabled to effect the separation of its various constituents.
PREPARATION.—Two parts of coarsely powdered sulphate of potash are placed in a porcelain crucible, and one part of pure sulphuric acid is poured over it. Expose this to heat over the spirit-lamp, until the whole becomes a clear liquid. The cooled mass must be of a pure white color, and may be got out of the crucible by inverting it. It must be kept in a fine powder.
4. Oxalate of Potassa (KO, O).—Dissolve bioxalate of potash in water, and neutralize with carbonate of potash. Evaporate the solution at a low heat to dryness, stirring constantly towards the close of the operation. The dry residue is to be kept in the form of a powder.
The oxalate of potash, at a low red heat, eliminates a considerable quantity of carbonic oxide, which, having a strong affinity for oxygen, with which it forms carbonic acid, it is therefore a powerful agent of reduction. It is in many cases preferable to carbonate of soda.
5. Cyanide of Potassium (Cy, K).—In the dry method of analysis, this salt is one of the most efficient agents for the reduction of metallic oxides. It separates not only the metals from their oxygen compounds, but likewise from their sulphur compounds, while it is converted through the action of the oxygen into carbonate of potash, or, in the latter case, combines with the sulphur and forms the sulphureted cyanide of potassium. This separation is facilitated by its easy fusibility. But in many cases it melts too freely, and therefore it is better to mix it, for blowpipe analysis, with an equal quantity of soda. This mixture has great powers of reduction, and it is easily absorbed by the charcoal, while the globules of reduced metal are visible in the greatest purity.
PREPARATION.—Deprive the ferrocyanide of potassium (2KCy + FeCy) of its water by heating it over the spirit-lamp in a porcelain dish. Mix eight parts of this anhydrous salt with three parts of dry carbonate of potash, and fuse the mixture by a low red heat in a Hessian, or still better, in an iron crucible with a cover, until the mass flows quiet and clear, and a sample taken up with an iron spatula appears perfectly white. Pour the clear mass out into a china or porcelain dish or an iron plate, but with caution that the fine iron particles which have settled to the bottom, do not mix with it. The white fused mass must be powdered, and kept from the air. The cyanide of potassium thus prepared, contains some of the cyanate of potassa, but the admixture does not deteriorate it for blowpipe use. It must be perfectly white, free from iron, charcoal, and sulphide of potassium. The solution of it in water must give a white precipitate with a solution of lead, and when neutralized with hydrochloric acid, and evaporated to dryness, it must not give an insoluble residue by dissolving it again in water.
6. Nitrate of Potassa, Saltpetre (KO, NO5).—Saturate boiling water with commercial saltpetre, filter while hot in a beaker glass, which is to be placed in cold water, and stir while the solution is cooling. The greater part of the saltpetre will crystallize in very fine crystals. Place these crystals upon a filter, and wash them with a little cold water, until a solution of nitrate of silver ceases to exhibit any reaction upon the filtrate. These crystals must be dried and powdered.
Saltpetre, when heated with substances easy of oxidation, yields its oxygen quite readily, and is, therefore, a powerful means of oxidation. In blowpipe analysis, we use it particularly to convert sulphides (as those of arsenic, antimony, &c.) into oxides and acids. We furthermore use saltpetre for the purpose of producing a complete oxidation of small quantities of metallic oxides, which oxidize with difficulty in the oxidation flame, so that the color of the bead, in its highest state of oxidation, shall be visible, as for instance, manganese dissolved in the microcosmic salt.
7. Biborate of soda, borax—(NaO + 2BO3).—Commercial borax is seldom pure enough for a reagent. A solution of borax must not give a precipitate with carbonate of potassa; or, after the addition of dilute nitric acid, it must remain clear upon the addition of nitrate of silver, or nitrate of baryta. Or a small piece of the dry salt, fused upon a platinum wire, must give a clear and uncolored glass, as well in the oxidation flame as in the reduction flame. If these tests indicate a foreign admixture, the borax must be purified by re-crystallization. These crystals are washed upon a filter, dried, and heated, to expel the crystal water, or until the mass ceases to swell up, and it is reduced to powder.
Boracic acid is incombustible, and has a strong affinity for oxides when fused with them; therefore, it not only directly combines with oxides, but it expels, by fusion, all other volatile acids from their salts. Furthermore, boracic acid promotes the oxidation of metals and sulphur, and induces haloid compounds, in the oxidation flame, to combine with the rising oxides. Borates thus made, melt generally by themselves; but admixed with borate of soda, they fuse much more readily, give a clear bead. Borax acts either as a flux, or through the formation of double salts.
In borax, we have the action of free boracic acid, as well as borate of soda, and for that reason it is an excellent reagent for blowpipe analysis.
All experiments in which borax is employed should be effected upon platinum wire. The hook of the wire should be heated red hot, and then dipped into the powdered borax. This should be exposed to the oxidation flame, when it will be fused to a bead, which adheres to the hook. This should be then dipped into the powdered substance, which will adhere to it if it is hot; but if the bead is cool, it must be previously moistened. Expose this bead to the oxidation flame until it ceases to change, then allow it to cool, when it should be exposed to the reduction flame. Look for the following in the oxidation flame:
(1.) Whether the heated substance is fused to a clear bead or not, and whether the bead remains transparent after cooling. The beads of some substances, for instance those of the alkaline earths, are clear while hot; but upon cooling, are milk-white and enamelled. Some substances give a clear bead when heated and when cold, but appear enamelled when heated intermittingly or with a flame which changes often from oxidation to reduction, or with an unsteady flame produced by too strong a blast. The reason is an incomplete fusion, while from the basic borate compound a part of the base is separated. As the boracic acid is capable of dissolving more in the heat, a bead will be clear while hot, enamelled when cold, as a part in the latter instance will become separated.
(2.) Whether the substance dissolves easily or not, and whether it intumesces from arising gases.
(3.) Whether the bead, when exposed to the oxidation flame, exhibits any color, and whether the color remains after the bead shall have cooled, or whether the color fades.
(4.) Whether the bead exhibits any other reaction in the reduction flame.
The bead should not be overcharged with the substance under examination, or it will become colored so deeply as not to present any transparency, or the color light enough to discern its hue.
8. Microcosmic Salt—Phosphate of Soda and Ammonia—(NaO, NH4O + PO5).—Dissolve six parts of phosphate of soda (2NaO, HO, PO5), and one part of pure chloride of Ammonium (NH4Cl.), in two parts of boiling water, and allow it to cool. The greatest part of the formed double salt crystallizes, while the mother-liquid contains chloride of sodium, and some of the double salt. The crystals must be dissolved in as little boiling water as possible, and re-crystallized. These crystals must be dried and powdered.
When this double salt is heated, the water and the ammonia escape, while the incombustible residue has a composition similar to borax, viz., a free acid and an easily fusible salt. The effect of it is, therefore, similar to the borax. The free phosphoric acid expels, likewise, most other acids from their combinations, and combines with metallic oxides.
For supports, the platinum wire may be used, but the hook must be smaller than when borax is used, or the bead will not adhere. As for all the other experiments with this salt, the microscosmic salt is used the same as borax.
9. Nitrate of Cobalt.—(CoO, NO5).—This salt can be prepared by dissolving pure oxide of cobalt in diluted nitric acid, and evaporating to dryness with a low heat. The dry residue should be dissolved in ten parts of water, and filtered. The filtrate is now ready for use, and should be kept in a bottle with a glass stopper. If the pure oxide of cobalt cannot be procured, then it may be prepared by mixing two parts of finely powdered glance of cobalt with four parts of saltpetre, and one part of dry carbonate of potassa with one part of water free from carbonate of soda. This mixture should be added in successive portions into a red-hot Hessian crucible, and the heat continued until the mass is fused, or at least greatly diminished in volume. The cooled mass must be triturated with hot water, and then heated with hydrochloric acid until it is dissolved and forms a dark green solution, which generally presents a gelatinous appearance, occasioned by separated silica. The solution is to be evaporated to dryness, the dry residue moistened with hydrochloric acid, boiled with water, filtered and neutralized while hot with carbonate of ammonia, until it ceases to give an acid reaction with test-paper. This must now be filtered again, and carbonate of potassa added to the filtrate as long as a precipitate is produced. This precipitate is brought upon a filter and washed thoroughly, and then dissolved in diluted nitric acid. This is evaporated to dryness, and one part of it is dissolved in ten parts of water for use.
The oxide of cobalt combines, with strong heat in the oxidation flame, with various earths and infusible metallic oxides, and thus produces peculiarly colored compounds, and is therefore used for their detection; (alumina, magnesia, oxide of zinc, oxide of tin, etc.) Some of the powdered substance is heated upon charcoal in the flame of oxidation, and moistened with a drop of the solution of the nitrate of cobalt, when the oxidation flame is thrown upon it. Alumina gives a pure blue color, the oxide of zinc a bright green, magnesia a light red, and the oxide of tin a bluish-green color; but the latter is only distinctly visible after cooling.
The dropping bottle, is the most useful apparatus for the purpose of getting small quantities of fluid. It is composed of a glass tube, drawn out to a point, with a small orifice. This tube passes through the cork of the bottle. By pressing in the cork into the neck of the bottle, the air within will be compressed, and the liquid will rise in the tube. If now we draw the cork out, with the tube filled with the fluid, and pressing the finger upon the upper orifice, the fluid can be forced out in the smallest quantity, even to a fraction of a drop.
10. Tin.—This metal is used in the form of foil, cut into strips about half an inch wide. Tin is very susceptible of oxidation, and therefore deprives oxidized substances of their oxygen very quickly, when heated in contact with them. It is employed in blowpipe analysis, for the purpose of producing in glass beads a lower degree of oxidation, particularly if the substance under examination contains only a small portion of such oxide. These oxides give a characteristic color to the bead, and thus are detected. The bead is heated upon charcoal in the reduction flame, with a small portion of the tin, whereby some of the tin is melted and mixes with the bead. The bead should be reduced quickly in the reduction flame, for by continuing the blast too great a while, the oxide of tin separates the other oxides in the reduced or metallic state, while we only require that they shall only be converted into a sub-oxide, in order that its peculiar color may be recognized in the bead. The addition of too much tin causes the bead to present an unclean appearance, and prevents the required reaction.
11. Silica (SiO3).—This acid does not even expel carbonic acid in the wet way, but in a glowing heat it expels the strongest volatile acids. In blowpipe analysis, we use it fused with carbonate of soda to a bead, as a test for sulphuric acid, and in some cases for phosphoric acid. Also with carbonate of soda and borax, for the purpose of separating tin from copper.
Finely powdered quartz will answer these purposes. If it cannot be procured, take well washed white sand and mix it with two parts of carbonate of soda and two parts of carbonate of potassa. Melt the materials together, pound up the cooled mass, dissolve in hot water, filter, add to the filtrate hydrochloric acid, and evaporate to dryness. Moisten the dry residue with hydrochloric acid, and boil in water. The silica remains insoluble. It should be washed well, dried, and heated, and then reduced to powder.
12. TEST-PAPERS.—(a.) Blue, Litmus Paper.—Dissolve one part of litmus in six or eight parts of water, and filter. Divide the filtrate into two parts. In one of the parts neutralize the free alkali by stirring it with a glass rod dipped in diluted sulphuric acid, until the fluid appears slightly red. Then mix the two parts together, and draw slips of unsized paper, free from alkali, such as fine filtering paper. Hang these strips on a line to dry, in the shade and free from floating dust. If the litmus solution is too light, it will not give sufficient characteristic indications, and if too dark it is not sensitive enough. The blue color of the paper should be changed to red, when brought in contact with a solution containing the minutest trace of free acid; but it should be recollected that the neutral salts of the heavy metals produce the same change.
(b.) Red Litmus Paper.—The preparation of the red litmus paper is similar to the above, the acid being added until a red color is obtained. Reddened litmus paper is a very sensitive reagent for free alkalies, the carbonates of the alkalies, alkaline earths, sulphides of the alkalies and of the alkaline earths, and alkaline salts with weak acids, such as boracic acid. These substances restore the original blue color of the litmus.
(c.) Logwood Paper.—Take bruised logwood, boil it in water, filter, and proceed as above. Logwood paper is a very delicate test for free alkalies, which impart a violet tint to it. It is sometimes used to detect hydrofluoric acid, which changes its color to yellow.
All the test-papers are to be cut into narrow strips, and preserved in closely stopped vials. The especial employment of the test-papers we shall allude to in another place.
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.
Qualitative analysis refers to those examinations which relate simply to the presence or the absence of certain substances, irrespective of their quantities. But before we take cognizance of special examinations, it would facilitate the progress of the student to pass through a course of Initiatory Exercises. These at once lead into the special analysis of all those substances susceptible of examination by the blowpipe. The Initiatory Analysis is best studied by adopting the following arrangement:
The glass of which the bulb is made should be entirely free from lead, otherwise fictitious results will ensue. If the bulb be of flint glass, then by heating it, there is a slightly iridescent film caused upon the surface of the glass, which may easily be mistaken for arsenic. Besides, this kind of glass is easily fusible in the oxidating flame of the blowpipe, while, in the reducing flame, its ready decomposition would preclude its use entirely. The tube should be composed of the potash or hard Bohemian glass, should be perfectly white, and very thin, or the heat will crack it.
The tube should be perfectly clean, which can be easily attained by wrapping a clean cotton rag around a small stick, and inserting it in the tube. Before using the tube, see also that it is perfectly dry.
The quantity of the substance put into the tube for examination should be small. From one to three grains is quite sufficient, as a general rule, but circumstances vary the quantity. The sides of the tube should not catch any of the substance as it is being placed at the bottom of the tube, or into the bulb. If any of the powder, however, should adhere, it should be pushed down with a roll of clean paper, or the clean cotton rag referred to above.
In submitting the tube to the flame, it should be heated at first very gently, the heat being increased until the glass begins to soften, when the observations of what is ensuing within it may be made.
If the substance be of an organic nature, a peculiar empyreumatic odor will be given off. If the substance chars, then it may be inferred that it is of an organic nature. The matters which are given off and cause the empyreumatic odor, are a peculiar oil, ammonia, carbonic acid, acetic acid, water, cyanogen, and frequently other compounds. If a piece of paper is heated in the bulb, a dark colored oil condenses upon the sides of the tube, which has a strong empyreumatic odor. A piece of litmus paper indicates that this oil is acid, as it is quickly changed to red by contact with it. A black residue is now left in the tube, and upon examination we will find that it is charcoal. If, instead of the paper, a piece of animal substance is placed in the bulb, the reddened litmus paper will be converted into its original blue color, while charcoal will be left at the bottom of the tube.
A changing of the substance, however, to a dark color, should not be accepted as an invariable indication of charcoal, as some inorganic bodies thus change color, but the dark substance will not be likely to be mistaken for charcoal. By igniting the suspected substance with nitrate of potassa, it can quickly be ascertained whether it is organic or not, for if the latter, the vivid deflagration will indicate it.
If the substance contains water, it will condense upon the cold portion of the tube, and may be there examined as to whether it is acid or alkaline. If the former, the matter under examination is, perhaps, vegetable; if the latter, it is of an animal nature. The water may be that fluid absorbed, or it may form a portion of its constitution,
If the substance contain sulphur, the sublimate upon the cold part of the tube may be recognized by its characteristic appearance, especially if the substance should be a sulphide of tin, copper, antimony, or iron. The hyposulphites, and several other sulphides, also give off sulphur when heated. The volatile metals, mercury and arsenic, will, however, sublime without undergoing decomposition. As the sulphide of arsenic may be mistaken, from its color and appearance, for sulphur, it must be examined especially for the purpose of determining that point.
Selenium will likewise sublime by heat as does sulphur. This is the case if selenides are present. Selenium gives off the smell of decayed horse-radish.
When the persalts are heated they are reduced to protosalts, with the elimination of a part of their acid. This will be indicated by the blue litmus paper.
If some of the neutral salts containing a volatile acid be present, they will become decomposed. For instance, the red nitrous acid water of the nitrates will indicate the decomposition of the salt, especially if it be the nitrate of a metallic oxide.
If there is an odor of sulphur, then it is quite probable, if no free sulphur be present, that a hyposulphite is decomposed.
If an oxalate be present, it is decomposed with the evolution of carbonic oxide, which may be inflamed at the mouth of the tube; but there are oxalates that give off carbonic acid gas, which, of course, will not burn. A cyanide will become decomposed and eliminate nitrogen gas, while the residue is charred. Some cyanides are, however, not thus decomposed, as the dry cyanides of the earths and alkalies.
There are several oxides of metals which will sublime, and may be thus examined in the tube. Arsenious acid sublimes with great ease in minute octohedral crystals. The oxides of tellurium and antimony will sublime, the latter in minute glittering needles.
There are several metals which will sublime, and may be examined in the cold portion of the tube. Mercury condenses upon the tube in minute globules. These often do not present the metallic appearance until they are disturbed with a glass rod, when they attract each other, and adhere as small globules. Place in the tube about a grain of red precipitate of the drug stores and apply heat, when the oxide will become decomposed, its oxygen will escape while the vaporized mercury will condense upon the cold portion of the tube, and may there be examined with a magnifying glass.
Arsenic, when vaporized, may be known by its peculiar alliaceous odor. Arsenic is vaporized from its metallic state, and likewise from its alloys. Several compounds which contain arsenic will also sublime, such as the arsenical cobalt. Place in the bulb a small piece of arsenical cobalt or "fly-stone," and apply heat. The sulphide of arsenic will first rise, but soon the arsenic will adhere to the sides of the tube.
The metals tellurium and cadmium are susceptible of solution, but the heat required is a high one. This is best done upon charcoal.
The perchloride of mercury sublimes undecomposed in the bulb, previously undergoing fusion.
The protochloride of mercury likewise sublimes, but it does not undergo fusion first, as is the case with the corrosive sublimate.
The ammoniacal salts all are susceptible of sublimation, which they do without leaving a residue. There are, however, several which contain fixed acids, which latter are left in the bulb. This is particularly the case with the phosphates and borates. A piece of red litmus paper will readily detect the escaping ammonia, while its odor will indicate its presence with great certainty. The halogen compounds of mercury, we should have mentioned, also sublime, the red iodide giving a yellow sublimate.
The bulb is also a convenient little instrument for the purpose of heating those substances which phosphoresce, and likewise those salts that decrepitate.
Should the above reactions not be readily discerned, it should not be considered as an indication that the substances are not present, for they are frequently expelled in such combinations that the above reactions will not take place. This is often the case with sulphur, selenium, arsenic, and tellurium. It frequently happens, likewise, that these substances are in such combinations that heat alone will not sublime them; or else two or more of them may arise together, and thus complicate the sublimate, so that the eye cannot readily detect either substance. Sometimes sulphur and arsenic will coat the tube with a metal-like appearance, which is deceptive. This coating presents a metallic lustre at its lower portion, but changing, as it progresses upward, to a dark brown, light brown, orange or yellow; this sublimate being due to combinations of arsenic and sulphur, which compounds are volatilized at a lower temperature than metallic arsenic.
If certain reagents are mixed with many substances, changes are effected which would not ensue with heat alone. Formiate of soda possesses the property of readily reducing metallic oxides. When this salt is heated, it gives off a quantity of carbonic oxide gas. This gas, when in the presence of a metallic oxide, easily reduces the metal, by withdrawing its oxygen from it, and being changed into carbonic oxide. If a little fly-stone is mixed with some formiate of soda, and heated in the bulb, the arsenic is reduced, volatilized, and condenses in the cool portion of the tube. By this method, the smallest portion of a grain of the arsenical compound may be thus examined with the greatest readiness. If the residue is now washed, by which the soda is got rid of, the metallic arsenic may be obtained in small spangles. If the compound examined be the sulphide of antimony, the one-thousandth part can be readily detected, and hence this method is admirably adapted to the examination of medicinal antimonial compounds. The arsenites of silver and copper are reduced by the formiate of soda to their metals, mixed with metallic arsenic. The mercurial salts are all reduced with the metal plainly visible as a bright silvery ring on the cool portion of the tube. The chloride and nitrate of silver are completely reduced, and may be obtained after working out the soda, as bright metallic spangles. The salts of antimony and zinc are thus reduced; also the sulphate of cadmium. The sublimate of the latter, although in appearance not unlike that of arsenic, can easily be distinguished by its brighter color. It is, in fact, the rich yellow of this sublimate which has led artists to adopt it as one of their most valued pigments.
The substance to be operated upon should be placed in the tube, about half an inch from the end, and the flame applied at first very cautiously, increasing gradually to the required temperature. The tube, in all these roasting operations, as they are termed, should be held in an inclined position. The nearer perpendicular the tube is held, the stronger is the draught of air that passes through it. If but little heat is required in the open tube operation, the spirit-lamp is the best method of applying the heat. But if a greater temperature is required, then recourse must be had to the blowpipe. Upon the angle of inclination of the tube depends the amount of air that passes through it, and therefore, the rapidity of the draught may be easily regulated at the will of the operator. The inclination of the tube may, as a general rule, be about the angle represented in Fig. 14.
FIG 14
The length of the tube must be about six inches, so that the portion upon which the substance rested in a previous examination may be cut off. The portion of the tube left will answer for several similar operations.
When the substance is under examination, we should devote our attention to the nature of the sublimates, and to that of the odors of the gases. If sulphur be in the substance experimented upon, the characteristic odor of sulphurous acid gas will readily indicate the sulphur. If metallic sulphides, for instance, are experimented upon, the sulphurous acid gas eliminated will readily reveal their presence. As it is a property of this gas to bleach, a piece of Brazil-wood test paper should be held in the mouth of the tube, when its loss of color will indicate the presence of the sulphurous acid. It often happens, too, that a slight deposition of sulphur will be observed upon the cool portion of the tube. This is particularly the case with those sulphides, which yield sublimates of sulphur when heated in the bulb.
Selenium undergoes but slight oxidation, but it becomes readily volatilized, and may be observed on the cool portion of the tube. At the same time the nose, if applied close to the end of the tube, will detect the characteristic odor of rotten horse-radish. Arsenic also gives its peculiar alliaceous odor, which is so characteristic that it can be easily detected. A few of the arsenides produce this odor. The sublimates should be carefully observed, as they indicate often with great certainty the presence of certain substances; for instance, that of arsenic. The sublimate, in this case, presents itself as the arsenious acid, or the metallic arsenic itself. If it be the former, it may be discerned by aid of the magnifying glass as beautiful glittering octohedral crystals. If the latter, the metallic lustre will reveal it.
But it will be observed that while some of the arsenides are sublimed at a comparatively low temperature, others require a very high one.
Antimony gives a white sublimate when its salts are roasted, as the sulphide, or the antimonides themselves, or the oxide of this metal. This white sublimate is not antimonious acid, but there is mixed with it the oxide of antimony with which the acid is sublimed. As is the case with arsenious acid, the antimonious acid may, by dexterous heating, be driven from one portion of the tube to another.
Tellurium, or its acid and oxide, may be got as a sublimate in the tube. The tellurious acid, unlike the arsenious and antimonious acids, cannot be driven from one portion of the tube to another, but, on the contrary, it fuses into small clear globules, visible to the naked eye sometimes, but quite so with the aid of the magnifying glass.
Lead, or its chloride, sublimes like tellurium, and, like that substance, fuses into globules or drops.
Bismuth, or its sulphide, sublimes into an orange or brownish globules, when it is melted, as directed above, for tellurium. The color of the bismuth and lead oxides are somewhat similar, although that of the latter is paler.
If any mineral containing fluorine, is fused, first with the microcosmic salt bead, then put into the tube, and the flame of the blowpipe be directed into the tube upon the bead, hydrofluoric acid is disengaged and attacks the inside of the tube. The fluoride of calcium, or fluorspar, may be used for this experiment.
During the roasting, a brisk current of air should be allowed to pass through the tube, whereby unoxidized matter may be prevented from volatilization, and the clogging up of the substance under examination be prevented.
In making examinations upon charcoal, it is quite necessary that the student should make himself familiar with the different and characteristic appearances of the deposits upon the charcoal. In this case I have found the advice given by Dr. Sherer to be the best; that is, to begin with the examination of the pure materials first, until the eye becomes familiarized with the appearances of their incrustations upon charcoal.
The greater part of the metals fuse when submitted to the heat of the blowpipe, and if exposed to the outer flame, they oxidize. These metals, termed the noble metals, do not oxidize, but they fuse. The metals platinum, iridium, rhodium, osmium and palladium do not fuse. The metal osmium, if exposed to the flame of oxidation, fuses and is finally dissipated as osmic acid. In the latter flame, the salts of the noble metals are reduced to the metallic state, and the charcoal is covered with the bright metal.
We shall give a brief description of the appearance of the principal elementary bodies upon being fused with charcoal. This plan is that deemed the most conducive to the progress of the student, by Berzelius, Plattner, and Sherer. Experience has taught us that this method is the most efficient that could have been devised as an initiatory exercise for the student, ere he commences a more concise and methodical method of analysis. In these reactions upon charcoal, we shall follow nearly the language of Plattner and Sherer.
SELENIUM is not difficult of fusion, and gives off brown fumes in either the oxidation or reduction flame. The deposit upon the charcoal is of a steel-grey color, with a slightly metallic lustre. The deposit however that fuses outside of this steel-grey one is of a dull violet color, shading off to a light brown. Under the flame of oxidation this deposit is easily driven from one portion of the charcoal to another, while the application of the reducing flame volatilizes it with the evolution of a beautiful blue light. The characteristic odor of decayed horse-radish distinguishes the volatilization of this metal.
TELLURIUM.—This metal fuses with the greatest readiness, and is reduced to vapor under both flames with fumes, and coats the charcoal with a deposit of tellurous acid. This deposit is white near the centre, and is of a dark yellow near the edges. It may be driven from place to place by the flame of oxidation, while that of reduction volatilizes it with a green flame. If there be a mixture of selenium present, then the color of the flame is bluish-green.
ARSENIC.—This metal is volatilized without fusing, and covers the charcoal both in the oxidizing and reducing flames with a deposit of arsenious acid. This coating is white in the centre, and grey towards the edges, and is found some distance from the assay. By the most gentle application of the flame, it is immediately volatilized, and if touched for a moment with the reducing flame, it disappears, tinging the flame pale blue. During volatilization a strong garlic odor is distinctly perceptible, very characteristic of arsenic, and by which its presence in any compound may be immediately recognized.
ANTIMONY.—This metal fuses readily, and coats the charcoal under both flames with antimonious acid. This incrustation is of a white color where thick, but of a bluish tint where it is thin, and is found nearer to the assay than that of arsenic. When greatly heated by the flame of oxidation, it is driven from place to place without coloring the flame, but when volatilized by the flame of reduction, it tinges the flame blue. As antimonious acid is not so volatile as arsenious acid, they may thus be easily distinguished from one another.
When metallic antimony is fused upon charcoal, and the metallic bead raised to a red heat, if the blast be suspended, the fluid bead remains for some time at this temperature, giving off opaque white fumes, which are at first deposited on the surrounding charcoal, and then upon the bead itself, covering it with white, pearly crystals. The phenomenon is dependent upon the fact, that the heated button of antimony, in absorbing oxygen from the air, developes sufficient heat to maintain the metal in a fluid state, until it becomes entirely covered with crystals of antimonious acid so formed.
BISMUTH.—This metal fuses with ease, and under both flames covers the charcoal with a coating of oxide, which, while hot, is of an orange-yellow color, and after cooling, of a lemon-yellow color, passing, at the edges, into a bluish white. This white coating consists of the carbonate of bismuth. The sublimate from bismuth is formed at a less distance from the assay than is the case with antimony. It may be driven from place to place by the application of either flame; but in so doing, the oxide is first reduced by the heated charcoal, and the metallic bismuth so formed is volatilized and reoxidized. The flame is uncolored.
LEAD.—This metal readily fuses under either flame, and incrusts the charcoal with oxide at about the same distance from the assay as is the case with bismuth. The oxide is, while hot, of a dark lemon-yellow color, but upon cooling, becomes of a sulphur yellow. The carbonate which is formed upon the charcoal, beyond the oxide, is of a bluish-white color. If the yellow incrustation of the oxide be heated with the flame of oxidation, it disappears, undergoing changes similar to those of bismuth above mentioned. Under the flame of reduction, it, however, disappears, tinging the flame blue.
CADMIUM.—This metal fuses with ease, and, in the flame of oxidation, takes fire, and burns with a deep yellow color, giving off brown fumes, which coat the charcoal, to within a small distance of the assay, with oxide of cadmium. This coating exhibits its characteristic reddish-brown color most clearly when cold. Where the coating is very thin, it passes to an orange color. As oxide of cadmium is easily reduced, and the metal very volatile, the coating of oxide may be driven from place to place by the application of either flame, to neither of which does it impart any color. Around the deposit of oxide, the charcoal has occasionally a variegated tarnish.
ZINC.—This metal fuses with ease, and takes fire in the flame of oxidation, burning with a brilliant greenish-white light, and forming thick white fumes of oxide of zinc, which coat the charcoal round the assay. This coating is yellow while hot, but when perfectly cooled, becomes white. If heated with the flame of oxidation, it shines brilliantly, but is not volatilized, since the heated charcoal is, under these circumstances, insufficient to effect its reduction. Even under the reducing flame, it disappears very slowly.
TIN.—This metal fuses readily, and, in the flame of oxidation, becomes covered with oxide, which, by a strong blast, may be easily blown off. In the reducing flame, the fused metal assumes a white surface, and the charcoal becomes covered with oxide. This oxide is of a pale yellow color while hot, and is quite brilliant when the flame of oxidation is directed upon it. After cooling, it becomes white. It is found immediately around the assay, and cannot be volatilized by the application of either flame.
MOLYBDENUM.—This metal, in powder, is infusible before the blowpipe. If heated in the outer flame, it becomes gradually oxidized, and incrusts the charcoal, at a small distance from the assay, with molybdic acid, which, near the assay, forms transparent crystalline scales, and is elsewhere deposited as a fine powder. The incrustation, while hot, is of a yellow color, but becomes white after cooling. It may be volatilized by heating with either flame, and leaves the surface of the charcoal, when perfectly cooled, of a dark-red copper color, with a metallic lustre, due to the oxide of molybdenum, which has been formed by the reducing action of the charcoal upon the molybdic acid. In the reducing flame, metallic molybdenum remains unchanged.
SILVER.—This metal, when fused alone, and kept in this state for some time, under a strong oxidizing flame, covers the charcoal with a thin film of dark reddish-brown oxide. If the silver be alloyed with lead, a yellow incrustation of the oxide of that metal is first formed, and afterwards, as the silver becomes more pure, a dark red deposit is formed on the charcoal beyond. If the silver contains a small quantity of antimony, a white incrustation of antimonious acid is formed, which becomes red on the surface if the blast be continued. And if lead and antimony are both present in the silver, after the greater part of these metals have been volatilized, a beautiful crimson incrustation is produced upon the charcoal. This result is sometimes obtained in fusing rich silver ores on charcoal.
In blowpipe experiments, it rarely occurs that we have to deal with pure metals, which, if not absolutely non-volatile, are recognized by the incrustation they form upon charcoal. Some compound substances, when heated upon charcoal, form white incrustations, resembling that formed by antimony, and which, when heated, may, in like manner, be driven from place to place. Among these are certain sulphides, as sulphide of potassium, and sulphide of sodium, which are formed by the action of the reducing flame upon the sulphates of potassa and soda, and are, when volatilized, reconverted into those sulphates, and as such deposited on the charcoal. No incrustation is, however, formed, until the whole of the alkaline sulphate has been absorbed into the charcoal, and has parted with its oxygen. As sulphide of potassium is more volatile than sulphide of sodium, an incrustation is formed from the former sooner than from the latter of these salts, and is considerably thicker in the former case. If the potash incrustation be touched with the reducing flame, it disappears with a violet-colored flame; and if a soda incrustation be treated in like manner, an orange-yellow flame is produced.
Sulphide of lithium, formed by heating the sulphate in the reducing flame, is volatilized in similar manner by a strong blast, although less readily than the sulphide of sodium. It affords a greyish white film, which disappears with a crimson flame when submitted to the reducing flame.
Besides the above, the sulphides of bismuth and lead give, when heated in either flame, two different incrustations, of which the more volatile is of a white color, and consists in the one case of sulphate of lead, and in the other of sulphate of bismuth. If either of these be heated under the reducing flame, it disappears in the former case with a bluish flame, in the latter unaccompanied by any visible flame. The incrustation formed nearest to the assay consists of the oxide of lead or bismuth, and is easily recognized by its color when hot and after cooling. There are many other metallic sulphides, which, when heated by the blowpipe flame, cover the charcoal with a white incrustation, as sulphide of antimony, sulphide of zinc, and sulphide of tin. In all these cases, however, the incrustation consists of the metallic oxide alone, and either volatilizes or remains unchanged, when submitted to the oxidizing flame.
Of the metallic chlorides there are many which, when heated on charcoal with the blowpipe flame, are volatilized and redeposited as a white incrustation. Among these are the chlorides of potassium, sodium, and lithium, which volatilize and cover the charcoal immediately around the assay with a thin white film, after they have been fused and absorbed into the charcoal, chloride of potassium forms the thickest deposit, and chloride of lithium the thinnest, the latter being moreover of a greyish-white color. The chlorides of ammonium, mercury, and antimony volatilize without fusing.
The chlorides of zinc, cadmium, lead, bismuth, and tin first fuse and then cover the charcoal with two different incrustations, one of which is a white volatile chloride, and the other a less volatile oxide of the metal.
Some of the incrustations formed by metallic chlorides disappear with a colored flame when heated with the reducing flame; thus chloride of potassium affords a violet flame, chloride of sodium an orange one, chloride of lithium a crimson flame, and chloride of lead a blue one. The other metals mentioned above volatilize without coloring the flame.
The chloride of copper fuses and colors the flame of a beautiful blue. Moreover, if a continuous blast be directed upon the salt, a part of it is driven off in the form of white fumes which smell strongly of chlorine, and the charcoal is covered with incrustations of three different colors. That which is formed nearest to the assay is of a dark grey color, the next, a dark yellow passing into brown, and the most distant of a bluish white color. If this incrustation be heated under the reducing flame, it disappears with a blue flame.
Metallic iodides and bromides behave upon charcoal in a similar manner to the chlorides. Those principally deserving of mention are the bromides and iodides of potassium and sodium. These fuse upon charcoal, are absorbed into its pores, and volatilize in the form of white fumes, which are deposited upon the charcoal at some distance from the assay. When the saline films so formed are submitted to the reducing flame, they disappear, coloring the flame in the same manner as the corresponding chlorides.
Before the student attempts to make an examination in the platinum forceps or tongs, he should first ascertain whether or not it will act upon the platinum. If the substance to be examined shall act chemically upon the platinum, then it should be examined on the charcoal, and the color of the flame ascertained as rigidly as possible. The following list of substances produce the color attached to them.