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SECTION XIV.

OF ARSENIC ACID.

Experiment 1. Take two ounces of arsenious acid in powder, put it into a tubulated retort, pour on it six or seven ounces of muriatic acid, and apply the heat of a lamp until the arsenious acid is dissolved. Then add three or four ounces of nitric acid, and heat it again gradually. An intestine motion now takes place, and much red vapour or nitrous gas is extricated. As soon as, in the progress of the operation, the red va. pours have ceased, an ounce of finely powdered arsenious acid is to be again added, and the solution effected as before, by a gentle ebullition; to this two ounces of nitric acid must be added, which will pro. duce à second effervescence and discharge of red vapours; the distillation must now be continued to dryness, and the fire must be urged towards the end, to such a degree, as to make the residual mass thoroughly red-hot. This mass is arsenic acid, which may either be preserved in that form, or be dissolved in boiling distilled water.

Remark. All the preparations of arsenic are deadly poison. The hydro-sulphurets are the best antidotes. A diluted solution of hydro-sulphuret of potash, soda, or lime, is therefore administered with success to persons who have been poisoned by arsenic. For the same reason sulphurous mineral waters may be given in such cases; oil, milk, butter, &c. which are too often resorted to, should never be employed, if a sulphuret or hydro-sulphuret can possibly be procured.

Arsenic is capable of combining with two doses of oxygen, and of forming two acids, namely, the arsenious, commonly called the white oxyd of arsenic, and the arsenic, which was discovered by Scheele. Arsenic acid is composed of about 65 arsenic and 35 oxygen in the hundred.*

Thomson.

Arsenic acid is capable of existing in the solid state, but it is not crystallizable. It appears in the form of a white pulverulent matter. It attracts humidity from the air. It has an acid caustic taste. It is soluble in six parts of its weight of water. This solution possesses a considerable acid taste. It is not volatile, but may

be evaporated to dryness and even converted into glass. It is decomposable by all combustible bodies and by many oxyds. It is soluble in some acids, but without change or intimate combination. Its specific gravity is 3.391. See Arsenic..

SECTION XV.

OF TUNGSTIC ACID.

Experiment 1. Let one part of powdered tungstate of lime be digested in three of nitric acid, till it ac-quires a yellow colour. Decant the acid, and let the remaining yellow powder, after being washed in distilled water, be digested in liquid ammonia till it is rendered considerably whiter. This ammoniacal solution is then poured off, and the residual undecomposed part of the tungstate is once more treated with nitric acid as before. The acid being again separated, it is again digested with liquid ammonia, and so on alternately till it is totally decomposed.

The ammoniacal solution is then decomposed by the addition of nitric acid; the tungstic acid becomes precipitated in the form of a white powder, which, after being washed, dried, and exposed to heat, assumes a yellow colour.

Rationale. On adding nitric acid to the ammoniacal solution, nitrate of ammonia is formed, and the tungstic acid becomes separated; but this last is not perfectly free from ammonia, it is therefore necessary to heat it, or to wash it in diluted nitric acid, by which means the adhering ammonia is either volatilized or neutralized,

which then may be washed away by means of distilled water.

Remark. The tungstic acid may also be obtained from the mineral called wolfram, in which it is combined with iron and manganese, and frequently with silica, in the state of a yellow oxyd, as directed before.

Tungstic acid does not exist in nature in an uncombined state, but always united to lime (tungstate of lime) or with iron and manganese (wolfram.) See Tung

sten.

SECTION XVI.

OF MOLYBDIC. ACID.

Experiment 1. If sulphuret of molybdenum, reduced to powder, be introduced into a retort with a mixture of nitric and muriatic acids, and heat applied, it will be converted into molyhdic acid, which appears in the form of a white powder: if this be washed with water, the remainder will be pure molybdic acid.

Rationale. As in other cases, the nitric acid acidifies the metal, as well as a portion of the sulphur, forming the molybdic and sulphuric acids; the latter, together with the muriatic acid, is washed off, leaving the molybdic acid in a state of purity.

Remark. This acid was discovered by Scheele, and more accurately investigated by Bucholz. It exists in the form of a white powder, whose specific gravity is 3.46. It melts and crystallizes in close vessels; but, in the open air, it sublimes, and may be collected. It is soluble in water. The solution reddens litmus. It is decomposed by sulphur and charcoal, and several of the metals. It is soluble in sulphuric and muriatic acid. It unites with salifiable bases, forming a class of salts called molybdates. It contains 33 per cent. of oxygen.

SECTION XVII.

OF CHROMIC ACID.

Experiment 1. If chromate of lead (red lead ore of Siberiu) be introduced into a vessel with a solution of carbonate of soda, and the whole boiled for a short time; the solution then filtered, and sulphuric acid added to saturation, a red precipitate will be obtained, which is chromic acid.

Rationale. The carbonate of soda decomposes the chromate of lead; chromate of soda remains in solution, and carbonate of lead' is precipitated. From the former, the chromic acid is separated by the sulphuric acid; sulphate of soda being formed, whilst the chromic acid is precipitated.

Remark. This acid was discovered by Vauquelin. It is of a red or orange yellow colour, and has an acrid and metallic taste. It is soluble in water, and susceptible of crystallization. Added to filings of tin and muriatic acid, it becomes at first yellowish brown, and afterwards a beautiful green colour.

If paper be dipped in chromic acid, and exposed for some days to the rays of the sun, it assumes a green colour, which does not change in the dark. Muriatic acid, heated in a retort with this acid, produces a strong effervescence, and oxygenated' muriatic acid gas is evolved.

Chromic acid, mixed with a solution of the hydrosulphuret of potash, is precipitated in the form of greenish brown flakes. Tannin precipitates it in flakes of a fawn colour. Heated by the blow pipe on charcoal, it boils up, and leaves an infusible green matter.

Fused with phosphoric acid, or with borax, a vitreous globule is obtained, of an emerald-green colour. Sulphuric acid; while cold, produces no effect upon.it; but when heated it makes it assume a bluish green colour.

With a solution of nitrate of mercury, it gives a precipitate of a dark, cinnabar red. With a solution of nitrate of sila

ver, it gives a precipitate, which, the moment it is formed, appears of a beautiful carmine red, but becomes purple by exposure to light. It unites with the alkalies, and forms crystallizable salts of a beautiful orange colour.

SECTION XVIII.

OF COLUMBIC ACID.

This acid was discovered by Hatchett in an ore brought from the United States. Its existence, however, has been questioned. It was obtained by fusing the ore with potash, dissolving the potash in water, and adding nitric acid to the solution. The columbic acid precipitates in flakes.

A summary of its general properties, may be found in Thomson's Chemistry.

COMBUSTIBLE ACIDS.

The acids considered under this head, comprehending the vegetable and animal are combustible; hence some chemists have treated them as such. The following properties will serve to characterize them:

1. If combined with potash and distilled, they are decomposed; charcoal is formed, and carburetted hydrogen gas is disengaged.

2. They contain, as a base, at least two simple combustibles, viz. hydrogen and carbon. Azote, sometimes enters into their

composition. Oxygen is a constituent part of them, though perhaps not always.

3. They are not susceptible of union with different doses of oxygen, without changing the other constituent parts.

4. The more powerful acid supporters decompose them, converting them into other combustible acids or oxyds.

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