A System of Instruction in the Practical Use of the Blowpipe Part 23

(1.) _Water_ (HO).--Pure distilled water is composed of one volume of oxygen, and two volumes of hydrogen gases; or, by weight, of one part of hydrogen to eight parts of oxygen gases. Water is never found pure in nature, but possessing great solvent properties, it always is found with variable proportions of those substances it is most liable to meet with, dissolved in it. Thus it derives various designations depending upon the nature of the substance it may hold in solution, as lime-water, etc.

In taking cognizance of water in relation to blowpipe a.n.a.lysis, we regard it only as existing in minerals. The examination for water is generally performed thus: the substance may be placed in a dry tube, and then submitted to heat over a spirit-lamp. If the water exists in the mineral mechanically it will soon be driven off, but if it exists chemically combined, the heat will fail to drive it off, or if it does, it will only partially effect it. The water will condense upon the cool portions of the tube, where it can be readily discerned. If the water exists chemically combined, a much stronger heat must be applied in order to separate it.

Many substances may be perhaps mistaken for water by the beginner, such as the volatile acids, etc.

(2.) _Nitric Acid_ (NO^{5}).--Nitric acid occurs in nature in potash and soda saltpetre. These salts are generally impure, containing lime, as the sulphate, carbonate and nitrate, and also iron in small quant.i.ty. The soda saltpetre generally contains a quant.i.ty of the chloride of sodium. The salts containing nitric acid deflagrate when heated on charcoal. Substances containing nitric acid may be heated in a gla.s.s tube closed at one end, by which the characteristic red fumes of nitrous acid are eliminated. If the acid be in too minute a quant.i.ty to be thus distinguished, a portion of the substance may be intimately mixed with some bisulphate of potash, and treated as above.

The sulphuric acid of the bisulphate combines with the base, and liberates the nitric acid, while the tube contains the nitrous acid gas.

The nitrate of pota.s.sa, when heated in a gla.s.s tube, fuses to a clear gla.s.s, but gives off no water. When fused on platinum wire, it communicates to the external flame the characteristic violet color.

When fused and ignited on charcoal, its surface becomes frothy, indicating the nitric acid.

(3.) _Carbon_ (C).--Carbon is found in nature in the pure crystallized state as the diamond. It occurs likewise in several allotropic states as graphite, plumbago, charcoal, anthracite, etc. It exists in large quant.i.ties combined with oxygen as carbonic acid.

The diamond, although combustible, requires too high a heat for its combustion to enable us to burn it with the blowpipe. When excluded from the air, it may be heated to whiteness without undergoing fusion, but with the free access of air it burns at a temperature of 703 C, and is converted into carbonic acid. If mixed with nitre, the pota.s.sa retains the carbonic acid, and the carbon may be thus easily estimated. If a mineral containing carbonic acid is heated, the gas escapes with effervescence, or a strong mineral acid as the hydrochloric will expel the acid with the characteristic effervescence.

(4.) _Phosphorus, Phosphoric Acid _(PO^{6}).--This acid occurs in a variety of minerals, a.s.sociated with yttria, copper, uranium, iron, lead, manganese, etc. Phosphoric acid may be detected in minerals by pursuing the following process: dip a small piece of the mineral in sulphuric acid, and place it in the platinum tongs: this is heated at the point of the blue flame, when the outer flame will become colored of a greenish-blue hue. This color will not be mistaken for those of boracic acid, copper, or baryta. Some of the phosphoric minerals, when heated in the inner flame, will color the outer flame green.

If alumina be present with the phosphoric acid, the following wet method should be adopted for the detection of the latter: the substance should be powdered in the agate mortar with a mixture of six parts of soda, and one and a half parts of silica. The entire ma.s.s should now be placed on charcoal, and melted in the flame of oxidation. The residue should be treated with boiling water, which dissolves the phosphate and the excess of carbonate of soda, while the silicate of alumina, with some of the soda, is left. The clear liquor is now treated with acetic acid, and heated over the spirit-lamp, and a small portion of crystallized nitrate of silver added; a lemon-yellow precipitate of phosphate of silver is quickly developed.

Previous to the addition of the nitrate, the liquor should be well heated; otherwise, a white precipitate of dipyrophosphate of silver will be produced.

If the examination be of any of the metallic phosphides, the substances should be powdered in the agate mortar, and fused with nitrate of pota.s.sa on the platinum wire; the fused ma.s.s should be treated with soda in the same manner as any substance containing phosphoric acid. The metal and the phosphorus are oxidized, while the phosphate of pota.s.sa is fused, and the metallic oxide separates.

(5.) _Sulphur_ (S).--Sulphur is found native in crystals It is frequently found a.s.sociated with lime, iron, silica, carbon, etc., and combined extensively with metals.

The princ.i.p.al acid of sulphur (the sulphuric, SO^{3}) occurs combined with the earths, the alkalies, and the metallic oxides. Native sulphur is recognized, when heated upon charcoal, by its odor (sulphurous acid) and the blue color of its flame. The compounds of sulphur may be detected by several methods. If the substance is heated in a gla.s.s tube, closed at one end, the yellow sublimate of sulphur will subside upon the cool portions of the tube; if the substance should also contain a.r.s.enic, the sublimate will present itself as a light brown incrustation, consisting of the sulphide of a.r.s.enic.

If the a.s.say is heated in the open gla.s.s tube, sulphurous acid will thus be generated; but, if the gas is too little to be detected by the smell, a strip of moistened litmus paper will indicate the presence of the acid.

The a.s.say will give off sulphurous fumes if heated in the flame of oxidation.

If the powdered substance is fused with two parts of soda, and one part of borax, upon charcoal, the sulphide of sodium is formed. This salt, if moistened and applied to a polished silver surface, will blacken it. The borax serves no other purpose than to prevent the absorption of the formed sulphide of sodium by the charcoal. As selenium will blacken silver in the manner above indicated, the presence of this substance should be first ascertained, by heating the a.s.say; when, if it be present, the characteristic horse-radish odor will reveal the fact.

Sulphuric acid may be detected by fusing the substance with two parts of soda, and one part of borax, on charcoal, in the flame of reduction; the ma.s.s must now be wetted with water, and placed in contact with a surface of bright silver; when, if sulphuric acid be present, the silver will become blackened.

Or the substance may be fused with silicate of soda in the flame of reduction. In this case, the soda combines with a portion of the sulphuric acid, which is then reduced to the sulphide, while the bead becomes of an orange or red color, depending upon the amount of the sulphuric acid present. If the a.s.say should, however, be colored, then the previous treatment should be resorted to.

(6.) _Boron, Boracic Acid_ (BO^{3}).--This acid occurs in nature in several minerals combined with various bases, such as magnesia, lime, soda, alumina, etc. Combined with water, this acid exists in nature as the native boracic acid; this acid gives with test paper prepared from Brazil wood, when moistened with water, a characteristic reaction, for the paper becomes completely bleached. An alcohol solution turns curc.u.ma test paper brown. Heated on charcoal, it fuses to a clear bead; but, if the sulphate of lime be present, the bead becomes opaque upon cooling.

The following reaction is a certain one: the substance is pulverized and mixed with a flux of four and a half parts of bisulphate of pota.s.sa, and one part of pulverized fluoride of calcium. The whole is made into a paste with water, and the a.s.say is placed on the platinum wire, and submitted to the point of the blue flame. While the a.s.say is melting, fluoboric gas is disengaged, which tinges the outer flame green. If but a small portion of boracic acid is present, the color will be quite evanescent.

(7.) _Silica, Silicic Acid_ (SiO^{3}).--This acid exists in the greatest plenty, forming no inconsiderable portion of the solid part of this earth. It exists nearly pure in crystallized quartz, chalcedony, cornelian, flint, etc., the coloring ingredients of these minerals being generally iron or manganese.

With _microcosmic salt_, silica forms a bead in the flame of oxidation which, while hot, is clear, while the separated silica floats in it. A platinum wire is generally used for the purpose, the end of it being first dipped in the salt which is fused into a bead, after which the silica must be added, and then the bead submitted to the flame of oxidation.

The silicates dissolve in soda but partially, and then with effervescence. If the oxygen of the acid be twice that of the base, a clear bead will be obtained that will retain its transparency when cold. If the soda be added in small quant.i.ty, the bead will then be opaque. In the first instance, a part of the base which separates is re-dissolved, and, therefore, the transparency of the gla.s.s; but, if too large a quant.i.ty of the soda is added, the separation of the base is sufficient to render the a.s.say infusible.

(8.) _Chlorine_ (Cl).--Chlorine exists in nature always in combination, as the chlorides of sodium, pota.s.sium, calcium, ammonium, magnesia, silver, mercury, lead, copper, etc.

The chlorine existing in metallic chlorides may be detected as follows: the wet way may be accomplished in the following manner. If the substance is insoluble, it must be melted with soda to render it soluble; if it be already soluble it must be dissolved in pure water, and nitrate of silver added, when the one ten-thousandth part of chlorine will manifest its presence by imparting a milky hue to the fluid.

By the blowpipe, chlorine may be detected in the following manner: Oxide of copper is dissolved in microcosmic salt on the platinum wire in the flame of oxidation, and a clear bead is obtained. The substance containing the chlorine is now added, and heat is applied. The a.s.say will soon be enveloped by a blue or purplish flame. As none of the acids that occur in the mineral kingdom give this reaction, chlorine cannot be confounded with them, for those which impart a color to the flame, when mixed with a copper salt, will not do so when tested in the microcosmic salt bead as above indicated.

If the a.s.say is soluble in water, the following method may be followed: a small quant.i.ty of sulphate of copper or iron is dissolved; a few drops of the solution is placed upon a bright surface of silver, and the metallic chloride added; when, if chlorine is present, the silver is blackened. If the chloride is insoluble in water, it must be rendered soluble by fusion upon a platinum wire with soda, and then treated as above.[2]

[2] Plattner.

(9.) _Bromine_ (Br).--The bromide of magnesium and sodium exists in many salt springs, and it is from these that the bromine of commerce is obtained. The metallic bromides give the same reactions on silver with the microcosmic bead and copper salt as the metallic chlorides.

The purplish color which, however, characterizes the chlorides, is more inclined to greenish with the bromides. If the substance be placed in a flask or gla.s.s tube, and fused with bisulphate of pota.s.sa, over the spirit-lamp, sulphurous gas and bromine will be eliminated.

Bromine will be readily detected by its yellow color and its smell.

Bromine may be readily detected by pa.s.sing a current of chlorine through the fluid, after which ether is added and the whole is agitated. The ether rises to the top, carrying with it the bromine in solution; after being withdrawn, this ether is mixed with pota.s.sa, by which the bromide and bromate of pota.s.sa are formed. The solution is evaporated to dryness, the residue is fused in a platinum vessel, the bromate is decomposed, while the bromide remains; this must be distilled with sulphuric acid and the binoxide of manganese. A red or brown vapor will then appear, indicating the presence of bromine; this vapor will color starch paste--which may be put in the receiver on purpose--of a deep orange color.

If, to a solution containing a bromide, concentrated sulphuric or nitric acid be added, the bromine is liberated and colors the solution yellow or red. The hypochlorites act in the same manner. The bromine salts are coming into use extensively in photography, in consequence of their greater sensitiveness to the action of light than the chlorides alone.

(10.) _Iodine_ (I).--This element occurs in salt-springs, generally combined with sodium; it also exists in rock-salt; it has likewise been found in sea-water, also in a mineral from Mexico, in combination with silver, and in one from Silesia, in combination with zinc. As sea-water contains iodine, we would consequently expect to find it existing in the sea-weeds, and it is generally from the ashes of these that it is obtained in commerce.

When the metallic iodides are fused with the microcosmic salt and copper, as previously indicated, they impart a green color to the flame. This color cannot be mistaken for the color imparted to the flame by copper alone. When the metallic iodides are fused in a gla.s.s tube, closed at one end, with the bisulphate of pota.s.sa, the vapor of iodine is liberated, and may be recognized by its characteristic color. Those mineral waters containing iodine can be treated the same as for bromine, as previously indicated, while the violet-colored vapor of the iodine can be easily discerned. The nitrate of silver is the best test for iodine, the yellow color of the iodide of silver being not easily mistaken, while its almost insolubility in ammonia will confirm its ident.i.ty. The chloride of silver, on the contrary, dissolves in ammonia with the greatest facility.

The reactions of iodine are similar to those of bromine with concentrated sulphuric acid and binoxide of manganese, and with nitric acid: The iodine is released and, if the quant.i.ty be not too great, colors the liquid brown. If there be a considerable quant.i.ty of iodine present, it is precipitated as a dark colored powder. Either of these, when heated, gives out the violet-color of the iodine.

With starch paste free iodine combines, producing a deep blue compound. If, however, the iodine be in very minute quant.i.ty, the color, instead of being blue, will be light violet or rose color.

If to a solution of the sulphate of copper, to which a small portion of sulphurous acid has been added, a liquid containing iodine and bromine is poured in, a dirty, white precipitate of the subiodide of copper is produced, and the bromine remains in the solution. The latter may then be tested for the bromine by strong sulphuric acid.

(11.) _Fluorine_ (Fl).--This element exists combined with sodium, calcium, lithium, aluminium, magnesium, yttrium, and cerium. Fluorine also exists in the enamel of the teeth, and in the bones of some animals. This element has a strong affinity for hydrogen, and, therefore, we find it frequently in the form of hydrofluoric acid.

Brazil-wood paper is the most delicate test for hydrofluoric acid, which it tinges of a light yellow color. Phosphoric acid likewise colors Brazil paper yellow, but as this acid is not volatile at a heat sufficient to examine hydrofluoric acid, there can be no mistake. If the substance is supposed to contain this acid, it should be placed on a slip of gla.s.s, and moistened with hydrochloric acid, when the test paper may be applied, and the characteristic yellow color will indicate the presence of the fluorine.

As hydrofluoric acid acts upon gla.s.s, this property may be used for its detection. The substance may be put into a gla.s.s tube, and sulphuric acid poured upon it in sufficient quant.i.ty to moisten it; a slight heat applied to the tube will develop the acid, which will act upon the gla.s.s of the tube. If the acid is retained in the mineral by a feeble affinity, and water be present, a piece of it may be put in the tube and heated, when the acid gas will be eliminated. The test paper will indicate its presence, even before it has time to act upon the gla.s.s. If the temperature be too high, fluosilicic acid is generated, and will form a silicious incrustation upon the cool portion of the tube.

If the fluorine is too minute to produce either of the above reactions, then the following process, recommended by Plattner, should be followed: the a.s.say should be mixed with metaphosphate of soda, formed by heating the microcosmic salt to dull redness. The ma.s.s must then be placed in an open gla.s.s tube, in such a position that there will be an access of hot air from the flame. Thus aqueous hydrofluoric acid is formed, which can be recognized by its smell being more suffocating than chlorine, and also by the etching produced by the condensation of vapor in the tube. Moist Brazil paper, applied to the extremity of the tube, will be instantly colored yellow.

Merlet's method for the detection of this acid is the following:[3]

Pulverize the substance for examination, then triturate it to an impalpable powder, and mix it with an equal part of bisulphate of pota.s.sa. Heat the ma.s.s gradually in a moderately wide test-tube. The judicious application of heat must be strictly observed, for if the operator first heats the part of the tube where the a.s.say rests, the whole may be lost on account of the gla.s.s being shattered. The spirit-flame must be first applied to the fore part of the tube, and then made to recede slowly until it fuses the a.s.say. After the mixture has been for some time kept in a molten state, the lamp must be withdrawn, and the part containing the a.s.say severed with a file. The fore part of the tube must then be well washed, and afterwards dried with bibulous paper. Should the fluorine contained in the substance be appreciable, the gla.s.s tube, when held up to the light, will be found to have lost its transparency, and to be very rough to the touch.

[3] Quoted by Plattner.

Great care should be observed not to allow this very corrosive acid to come into contact with the skin, as an ulcer will be the consequence that will be extremely difficult to heal.

When hydrofluoric acid comes in contact with any silicious substance, hydrofluosilicic acid gas is always formed.