A System of Instruction in the Practical Use of the Blowpipe Part 3

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 pota.s.sa. Melt the materials together, pound up the cooled ma.s.s, 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 gla.s.s 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.

B. ESPECIAL REAGENTS.

13. _Fused Boracic Acid_ (BO^{3}).--The commercial article is sufficiently pure for blowpipe a.n.a.lysis. It is employed in some cases to detect phosphoric acid, and also minute traces of copper in lead compounds.

14. _Fluorspar_ (CaFl^{2}).--This substance should be pounded fine and strongly heated. Fluorspar is often mixed with boracic acid, which renders it unfit for a.n.a.lytical purposes. Such an admixture can be detected if it be mixed with bisulphate of pota.s.sa, and exposed upon platinum wire to the interior or blue flame. It is soon fused, the boracic acid is reduced and evaporated, and by pa.s.sing through the external flame it is reoxidized, and colors the flame green. We use fluorspar mixed with bisulphate of pota.s.sa 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 a.n.a.lysis to detect salts of pota.s.sa 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 ma.s.s 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 Pota.s.sa_ (KO, SbO^{6}).--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 ma.s.s with water, and dry the residue. Take two parts of this, and mix it with one part of dry carbonate of pota.s.sa, and expose this to a red heat for about half an hour. Then wash the ma.s.s 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 quant.i.ties of charcoal in compound substances, as it shares its oxygen with the carbonaceous matter, the antimony becomes separated, and carbonate of pota.s.sa 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_ (Fe^{2}Cy^{5}, NO^{5}, 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 ma.s.s is placed on a watch-gla.s.s, 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, alb.u.men, 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 ma.s.s 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 pota.s.sium (for either salt will give the above reactions), are prepared as follows: One atom (422 grains) of pulverized ferrocyanide of pota.s.sium is mixed with five atoms of commercial nitric acid, diluted with an equal quant.i.ty of water. One-fifth of this quant.i.ty (one atom) of the acid is sufficient to transfer the ferrocyanide into nitroprusside; but the use of a larger quant.i.ty 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 a.s.sumes 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 pota.s.sium, 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 quant.i.ty 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 a.n.a.lysis, 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 a.n.a.lyst, must be acquainted with the characteristic tests as applied in the wet way.

Part II.

INITIATORY a.n.a.lYSIS.

Qualitative a.n.a.lysis refers to those examinations which relate simply to the presence or the absence of certain substances, irrespective of their quant.i.ties. But before we take cognizance of special examinations, it would facilitate the progress of the student to pa.s.s through a course of Initiatory Exercises. These at once lead into the special a.n.a.lysis of all those substances susceptible of examination by the blowpipe. The Initiatory a.n.a.lysis is best studied by adopting the following arrangement:

1. EXAMINATIONS WITH THE GLa.s.s BULB.

The gla.s.s of which the bulb is made should be entirely free from lead, otherwise fict.i.tious results will ensue. If the bulb be of flint gla.s.s, then by heating it, there is a slightly iridescent film caused upon the surface of the gla.s.s, which may easily be mistaken for a.r.s.enic. Besides, this kind of gla.s.s 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 gla.s.s, 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 quant.i.ty 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 circ.u.mstances vary the quant.i.ty. 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 gla.s.s 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 pota.s.sa, 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 const.i.tution,

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 a.r.s.enic, will, however, sublime without undergoing decomposition. As the sulphide of a.r.s.enic 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. _a.r.s.enious 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 gla.s.s 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 gla.s.s.

_a.r.s.enic_, when vaporized, may be known by its peculiar alliaceous odor. a.r.s.enic is vaporized from its metallic state, and likewise from its alloys. Several compounds which contain a.r.s.enic will also sublime, such as the a.r.s.enical cobalt. Place in the bulb a small piece of a.r.s.enical cobalt or "fly-stone," and apply heat. The sulphide of a.r.s.enic will first rise, but soon the a.r.s.enic 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.