A System of Instruction in the Practical Use of the Blowpipe Part 21

As bis.m.u.th occurs mostly in the metallic form, it is necessary to know how to distinguish it from metals similar to it. Its brittleness distinguishes it from lead, zinc and tin, as they are readily flattened by a stroke of the hammer, while bis.m.u.th is broken to pieces. Bis.m.u.th, in this latter respect, might perhaps be mistaken for antimony or tellurium; but, by the following examination, it is easy to separate bis.m.u.th from antimony or tellurium.

1. Neither bis.m.u.th nor antimony sublimates when heated in a gla.s.s tube closed at one end. At a temperature which is about to fuse the gla.s.s, tellurium yields a small quant.i.ty of a white vapor (some tellurium is oxidized to tellurous acid by the oxygen of the air in the tube).

After that, a grey metallic sublimate settles on the sides of the tube.

2. Heated in an open tube, antimony yields a white vapor, which coats the inside of the gla.s.s tube, and can be driven by heat from one part of the tube to another without leaving a residue. The metallic globule is covered with a considerable quant.i.ty of fused oxide. Tellurium produces, under the same circ.u.mstances, an intense vapor, and deposits on the gla.s.s a white powder, which melts by heat into globules that run over the gla.s.s. The metallic globules are covered by fused, transparent, and nearly colorless oxide, which becomes white while cooling. By a high temperature, and with little access of air, metallic tellurium sublimes with the deposition of a grey powder.

Bis.m.u.th produces, under similar treatment, scarcely any vapor, unless it is combined with sulphur. The metal is enveloped by fused oxide of a dark yellow color, which appears light yellow after being cooled. It acts upon the gla.s.s, and dissolves it.

3. Upon charcoal, exposed to the blowpipe flame, the three metals are volatilized, and yield a sublimate upon the charcoal. That of antimony is white, while those of bis.m.u.th and tellurium are dark yellow. By exposing them to the flame of reduction, the sublimate of tellurium disappears and communicates an intense green color to the flame. The antimony incrustation gives a feeble greenish-blue color, while the sublimate of bis.m.u.th gives no perceptible color in the light. It is, however, worthy of notice that if the operation takes place in the dark, a very pale blue flame will be seen with the bis.m.u.th.

(_c._) _Tin_ (Sn).--This metal does not occur in nature in the metallic state, very seldom in the sulphide, but chiefly in the oxide (tinstone). In the metallic state it is silver-white, possesses a very high l.u.s.tre, is soft (but harder than lead), ductile, but has not much tenacity, and it is very malleable. The metal when it is cast gives a peculiar creaking noise when twisted or bent, which proceeds from the crystalline structure of the metal. This crystallization is quite clearly manifested by attacking the surface of the metal, or that of tin plate, with acids.

Tin is very slightly tarnished by exposure to the air. It fuses at 442, and becomes grey, being a mixture of the oxide and the metal. At a high temperature even, tin is but little subject to pa.s.s off as vapor. It is soluble in aqua regia, and with the liberation of hydrogen, in hot sulphuric and hydrochloric acids, and in cold dilute nitric acid, without decomposing water, or the production of a gas, while nitrate of tin and nitrate of ammonia are formed. Concentrated nitric acid converts tin into insoluble tin acids.

([alpha].) _Protoxide of Tin_ (SnO) is a dark-grey powder. Its hydrate is white, and is soluble in caustic alkalies. When this solution is heated, anhydrous crystalline black protoxide is separated. The soluble neutral salts of tin-protoxide are decomposed by the addition of water, and converted into acid soluble, and basic insoluble salts.

When protoxide of tin is ignited with free access of air, it takes fire and is converted with considerable intensity into the acids, producing white vapors. This is likewise the case if it is touched by a spark of fire from steel. The hydrate of the protoxide of tin can be ignited by the flame of a candle, and glows like tinder.

([beta].) _Sesquioxide of Tin_ (Sn^{2}O^{3}) is a greyish-brown powder. Its hydrate is white, with a yellow tinge. It is soluble in aqua ammonia and in hydrochloric acid; this solution forms with solution of gold the "purple of Ca.s.sius."

([gamma].) _Stannic Acid_ (peroxide, SnO^{2}).--This acid occurs in nature crystallized in quadro-octahedrons, of a brown or an intense black color, and of great hardness (tinstone). Artificially prepared, it is a white or yellowish-white powder. It exists in two distinct or isomeric modifications, one of which is insoluble in acids (natural tin-acid) while the other (tin-acid prepared in the wet way) is soluble in acids. By ignition the soluble acid is converted into the insoluble. Both modifications form hydrates.

_Reactions before the Blowpipe._--Metallic tin melts easily. It is covered in the flame of oxidation into a yellowish-white oxide, which is carried off sometimes by the stream of air which propels the flame.

In the reduction flame, and upon charcoal, melting tin retains its metallic l.u.s.tre, while a thin sublimate is produced upon the charcoal.

This sublimate is light-yellow while hot, and gives a strong light in the flame of oxidation, and turns white while cooling. This sublimate is found near to the metal, and cannot be volatilized in the oxidation flame. In the flame of reduction it is reduced to metallic tin.

Sometimes this incrustation is so imperceptible that it can scarcely be distinguished from the ashes of the charcoal. If such be the case, moisten it with a solution of cobalt, and expose it to the flame of oxidation, when the sublimate will exhibit, after cooling, a bluish-green color.

Protoxide of tin takes fire in the flame of oxidation, and burns with flame and some white vapor into tin acid, or stannic acid. In a strong and continued reduction flame, it may be reduced to metal, when the same sublimate above mentioned is visible. The sesquioxide of tin behaves as the above.

Stannic acid, heated in the flame of oxidation, does not melt and is not volatilized, but produces a strong light, and appears yellowish while hot, but changing as it cools to a dirty-yellow white color. In a strong and continued flame of reduction, it may be reduced likewise to the metallic state, with the production of the same sublimate as the above.

_Borax_ dissolves tin compounds in the flame of oxidation, and upon platinum wire, very tardily, and in small quant.i.ty, to a transparent colorless bead, which remains clear after cooling, and also when heated intermittingly. But if a saturated bead, after being completely cool, is exposed again to the flame of oxidation, at a low red heat, the bead while cooling is opaque, loses its globular form, and exhibits an indistinct crystallization. This is the case too in the flame of reduction, but if the bead is highly saturated, a part of the oxide is reduced.

_Microcosmic Salt_ dissolves the oxides in the flame of reduction very tardily in a small quant.i.ty to a transparent colorless bead, which remains clear while cooling. If to this bead sesquioxide of iron is added in proper proportion, the sesquioxide loses its property of coloring the bead, but of course an excess of the iron salt will communicate to the bead its own characteristic color. In the flame of reduction no further alteration is visible.

Tin-oxides combine with carbonate of soda, in the flame of oxidation upon platinum wire, with intumescence to a bulky and confused ma.s.s, which is insoluble in more soda. Upon charcoal, in the reduction flame, it is easily reduced to a metallic globule. Certain compounds of tin-oxides, particularly if they contain tantalum, are by fusion with carbonate of soda reduced with difficulty; but by the addition of some borax, the reduction to the metallic state is easily effected.

Tin-oxides exposed to the oxidation flame, then moistened with a solution of cobalt, and exposed again to the flame of oxidation, will exhibit, after having completely cooled, a bluish-green color.

EIGHTH GROUP.--MERCURY, a.r.s.eNIC.

These two metals are volatilized at a temperature lower than that of a red heat, and produce, therefore, no reactions with borax and microcosmic salt. Their oxides are easily reduced to the metallic state.

(_a._) _Mercury_ (Hg).--This metal occurs in nature chiefly combined with sulphur as a bisulphide.

It occurs still more rarely in the metallic form, or combined with silver, selenium, or chlorine.

Mercury, in the metallic state, has a strong l.u.s.tre, and is liquid at ordinary temperatures, whereby it is distinguished from any other metal. It freezes at 40 and boils at 620, but it evaporates at common temperatures. Pure mercury is unalterable. Upon being exposed to the air, it tarnishes only by admixture with other metals, turns grey on the surface, and loses its l.u.s.tre. It is soluble in cold nitric acid and in concentrated hot sulphuric acid, but not in hydrochloric acid.

([chi].) _Protoxide of Mercury_ (Hg^{2}O).--It is a black powder, which is decomposed by ignition into metallic mercury and oxygen. By digestion with certain acids, and particularly with caustic alkalies, it is converted into metallic mercury and peroxide. Some neutral salts of the protoxide are only partly soluble in water, as they are converted into basic insoluble and acid soluble salts.

Protoxide of mercury is completely insoluble in hydrochloric acid. Its neutral salts change blue litmus paper to red.

([beta].) _Peroxide of Mercury_ (HgO).--This oxide exists in two allotropic modifications. One is of a brick-red color, and the other is orange. Being exposed to heat, they turn black, but regain their respective colors upon cooling. They are decomposed at a high temperature into metallic mercury and oxygen. They yield with acids their own peculiar salts.

Mercury, in the metallic form, can never be mistaken for any other metal in consequence of its fluid condition at ordinary temperatures.

Exposed to the blowpipe flame, it is instantly volatilized. This is also the case with it when combined with other metals. The oxides of mercury are, in the oxidation and reduction flames, instantly reduced and volatilized. They do not produce any alteration with fluxes, as they are volatilized before the bead melts. Heated with carbonate of soda in a gla.s.s tube closed at one end, they are reduced to metallic mercury, which is volatilized, and condenses upon a cool portion of the tube as a grey powder. By cautious knocking against the tube, or by rubbing with a gla.s.s rod, this sublimate can be brought together into one globule of metallic mercury. Compounds of mercury can be most completely reduced by a mixture of neutral oxalate of pota.s.sa and cyanide of pota.s.sium. If the substance under examination contains such a small quant.i.ty of mercury that it cannot be distinguished by volatilization, a strip of gold leaf may be attached to an iron wire, and introduced during the experiment in the gla.s.s tube. The smallest trace of mercury will whiten the gold leaf in spots.

(_b._) _a.r.s.enic_ (As).--This metal occurs in considerable quant.i.ty in nature, chiefly combined with sulphur or metals.

a.r.s.enic, in the metallic state, is of a whitish-grey color, high l.u.s.tre, and is crystalline, of a foliated structure, and is so brittle that it can be pulverized. It does not melt, but is volatilized at 356. Its vapor has a strong alliaceous odor. a.r.s.enic sublimes in irregular crystals. By exposure to the air it soon tarnishes, and is coated black. Being mixed with nitrate of pota.s.sa and inflamed, it detonates with vehemence. Mixed with carbonate of pota.s.sa, it is inflamed by a stroke of the hammer, and detonates violently.

Heated in oxygen gas, it is inflamed, and burns with a pale blue flame to a.r.s.enious acid.

([beta].) _a.r.s.enious Acid_ (AsO^{3}).--This acid crystallizes in octahedrons, or, when fused, forms a colorless gla.s.s, which finally becomes opaque and enamel-like, or forms a white powder. It sublimes without change or decomposition. When heated for a longer while below the temperature of sublimation, it melts into a transparent, colorless, tough gla.s.s. The opaque acid is sparingly soluble in cold water, and still more soluble in hot water. It is converted, by continued boiling, into the transparent acid, which is much more soluble in water. a.r.s.enious acid is easily dissolved by caustic pota.s.sa. It is also soluble in hydrochloric acid. This acid occurs a.s.sociated with antimonious acid, protoxide of tin, protoxide of lead, and oxide of copper. It occurs likewise in very small quant.i.ty in ferruginous mineral springs.

([gamma].)_a.r.s.enic Acid_ (AsO^{5}) is a white ma.s.s, which readily absorbs moisture and dissolves. It will not volatilize at a low red heat, nor will it decompose. Exposed to a strong heat, it is decomposed, yielding oxygen, and pa.s.sing into a.r.s.enious acid.

_Reactions before the Blowpipe._

Metallic a.r.s.enic, heated in a gla.s.s tube closed at one end, yields a black sublimate of a metallic l.u.s.tre, and at the same time gives out the characteristic alliaceous odor. This is the case too with alloys of a.r.s.enic, if there is a maximum quant.i.ty of a.r.s.enic present.

When heated in a gla.s.s tube open at both ends, metallic a.r.s.enic is oxidized to a.r.s.enious acid, which appears as a white crystalline sublimate on the sides of the gla.s.s tube. This deposit will occur at some distance from the a.s.say, in consequence of the great volatility of the a.r.s.enic. The sublimate can be driven from one place upon the tube to another, by a very low heat. Alloys of a.r.s.enic are converted into basic a.r.s.eniates of metal oxides, while surplus a.r.s.enic is converted into a.r.s.enious acid, which sublimes on the tube. If too much a.r.s.enic is used for this experiment, a dark-brown incrustation will sublime upon the sides of the tube which will give an alliaceous smell. If this sublimate should be deposited near the a.s.say, then it resembles the white sublimate of a.r.s.enious acid.

Heated upon charcoal, metallic a.r.s.enic is volatilized before it melts, and incrusts the charcoal in the flame of oxidation as a white deposit of a.r.s.enious acid. This sublimate appears sometimes of a greyish color, and takes place at some distance from the a.s.say. When heated slightly with the blowpipe flame, this sublimate is instantly driven away, and being heated rapidly in the reduction flame, it disappears with a light blue tinge, while the usual alliaceous or garlic smell may be discerned.

a.r.s.enious acid sublimes in both gla.s.s tubes very readily, as a white crystalline sublimate. These crystals appear to be regular octahedrons when observed under the microscope. Upon charcoal it instantly volatilizes, and when heated, the characteristic garlic smell may be observed.

a.r.s.enic acid yields, heated strongly in a gla.s.s tube closed at one end, oxygen and a.r.s.enious acid, the latter of which sublimes in the cool portions of the tube. Compounds of a.r.s.enic produce, in consequence of their volatility, no reactions with fluxes. Being heated upon charcoal with carbonate of soda, they are reduced to metallic a.r.s.enic which may be detected by the alliaceous odor peculiar to all the a.r.s.enic compounds when volatilized.

NINTH GROUP.--COPPER, SILVER, GOLD.

These metals are not volatile, neither are their oxides. They are reduced to the metallic state, by fusion with carbonate of soda, when they melt to a metallic grain. The oxides of silver and gold are reduced _per se_ to the metallic state by ignition. In the reduction of the oxides of this group, no sublimate is visible upon the charcoal.

(_a._) _Copper_ (Cu).--This metal occurs in the metallic state, also as the protoxide, and as oxides combined with acids in different salts (carbonate of copper as malachite, etc.) The sulphide of copper is the princ.i.p.al ore of copper occurring in nature. In the metallic state, copper is of a red color, has great l.u.s.tre and tenacity, is ductile and malleable, and crystallizes in octahedrons and cubes. It melts at a bright red heat, is more difficult than silver to fuse, but fuses more readily than gold. It absorbs oxygen while melting. There arises from its surface a fine dust of metallic globules, which are covered with the protoxide. The surface of the metal is likewise covered with the protoxide. Copper exposed to moist air tarnishes, and is converted into hydratic carbonate of copper. When ignited in the open air, it is soon covered with the brownish-red protoxide.

([chi].) _Protoxide of Copper_ (Cu^{2}O).--This oxide occurs in nature, crystallized in octahedrons of a ruby-red color, of a lamellar structure, and transparent. Artificially prepared, it forms a powder of the same color. It is decomposed by dilute acids into salts of peroxide and metal. It is converted by ignition, with free access of air, into peroxide.

([beta].) _Oxide of Copper_ (CuO).--This oxide is a dark-brown or black powder. It is dissolved by acids, with a blue or green-colored solution. It is soluble in aqua ammonia, and the solution is of a dark blue color.

_Reactions before the Blowpipe._--Oxide of copper exposed upon platinum wire to the inmost flame (the blue flame), communicates to the external flame a green color. Heated upon charcoal in the oxidation flame, it melts to a black ball, soon spreads over the charcoal, and is partially reduced.

Exposed to the reduction flame, at a temperature which will not melt copper, it is reduced with a bright metallic l.u.s.tre, but as soon as the blast ceases, the surface of the metal becomes oxidized, and appears dark brown or black. If the temperature is continued still higher, it melts to a metallic grain.

_Borax_ dissolves the oxide of copper in the flame of oxidation to a clear green-colored bead, even if the quant.i.ty of oxide be quite small, but by cooling, the bead becomes blue. In the flame of reduction upon platinum wire, the bead soon becomes colorless, but while cooling presents a red color (protoxide of copper). This bead is opaque, but, if too much of the oxide is added, a part of it is reduced to metal, which is visible by breaking the metallic grain.