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The compound acid-radicals into the composition of which arsenic enters retain this variable basicity, just as the well known and investigated compound acid-radicals containing phosphorus.

THE HYDROGEN COMPOUND (H, As), or arseniuretted hydrogen, has been already spoken of at some length (p. 211); we will, however, here recapitulate its leading properties and characters.

It is produced when an alkaline arsenide is dissolved in water, or the arsenide of a metal insoluble in water is dissolved in an acid ; it is formed most commonly by acting with nascent hydrogen upon one of the oxygen compounds of arsenic. It is a colourless gas, of which the composition is H, As; it possesses a very offensive and peculiar odour, and is excessively poisonous when inhaled. The gas does not redden litmus. At a temperature of -40° C. it condenses to a colourless liquid ; but it has not yet been solidified. Its specific gravity is 2.695; water dissolves one-fifth of its volume; and the solution darkens solutions of many metallic salts. When heated, this gas is resolved into its elements, hydrogen and arsenic; and when a light is applied to it in contact with the air, it burns with a bluish white flame, forming the oxide of arsenic (As, 0) and water : if sufficient air is not present for the oxidation of the arsenic, the latter is deposited upon the sides of the containing vessel as a metallic film; and the same end may be compassed by depressing a cold porcelain surface upon a jet of the burning gas. These features have already been spoken of as tests for arsenic (p. 212).

In the decomposition of arsenide of potassium by water, a solid arseniuretted hydrogen is formed, which presents itself as a brown powder. Its formula appears to be H, As.

The salts of the two first subdivisions of basic radicals are not precipitated by the passage of arseniuretted hydrogen gas through their solutions ; those of iron also are not precipitated; those of MANGANESE, ZINC, and Tin are very slowly decomposed; but most salts of the fourth subdivision are precipitated, some in the form of arsenides, as COPPER and PLATINUM, whilst in other cases, as in those of SILVER and Gold, the metal is thrown down, and arsenious oxide formed, which remains in solution thus, when this gas is passed into nitrate of silver

12AgNO, +2H, As+51,0=12HNO, +12Ag+H, As, Og. Organic acid-radicals have some influence in preventing this precipitation, as is shown in the instances of the acetate of lead and of the tartrate of antimonyle and potassium (tartar-emetic) which are not acted upon by arseniuretted hydrogen.

SALTS OF ANTIMONY, OR ANTIMONIDES. The body antimony, as has been already stated, partakes almost more of the characters of a basic than of those of an acidradical, and in its combinations with metals it produces bodies which far more nearly resemble alloys (or combinations of metals with each other) than true saline bodies, in which no physical characteristics of the basic or acid-elements are perceptible. It is thus not an easy matter to obtain definite compounds, upon the formulæ of which an opinion may be safely pronounced.

THE HYDROGEN COMPOUND (H, Sb), or antimoniuretted hydrogen, is a very well defined body: it has been fully treated of at p. 200, and need be here but slightly noticed. It is obtained by the action of water on the antimonide of potassium, or by the action of nascent hydrogen on solutions of antimony salts. It is a gas which closely resembles H, As; its composition is H, Sb : when ignited it burns with formation of antimonious oxide (Sb,0z) and water; and if a cold porcelain surface be depressed upon the flame, a metallic spot is obtained : it is decomposed by a temperature below redness, into its elements antimony and hydrogen. This gas is not sensibly absorbed by water. A solid antimonide of hydrogen is also known.

When passed into alcoholic solutions of alkaline hydrates, a dark colour is produced, and, finally, brownish black flakes separate : this peculiarity distinguishes it from arseniuretted hydrogen. Solutions of salts of COPPER, SILVER, MERCURY, and PLATINUM are precipitated by it more or less readily, with formation of bodies of the formula M, Sb, thus 3AgNO3+H, Sb=3HNO; + Ag, Sb.

Section II.The nitrites, nitrates, hypophosphites, phosphites,

phosphates, arsenites, arseniates, antimoniates ; sulpharsenites,

sulpharseniates, sulphantimoniates. SALTS OF THE ACID-RADICALS WHICH CONTAIN NITROGEN,

PHOSPHORUS, ARSENIC, AND ANTIMONY COMBINED
WITH OXYGEN OR SULPHUR.

Of acid-radicals containing nitrogen combined with oxygen, two only are of sufficient importance to demand attention ; these are the nitrous and the nitric radicals (NO, and NO3): several other compounds of nitrogen and oxygen exist, but are of trivial importance in an analytical point of view.

SALTS OF THE NITROUS RADICAL, OR NITRITES.

This radical is usually obtained in combination with potassium by heating the nitrate of potassium (KNO) in a crucible, by which means the nitrite (KNO,) and oxygen are produced. Nitrites detonate when heated with combustible bodies; they are either colourless or yellow, and generally crystallizable: the nitrite of silver is almost the only insoluble salt. This radical appears to have the property of assuming oxygen and transforming itself into monobasic, bibasio, and tribasic radicals, after the manner of the phosphoric acid-radical.

THE HYDROGEN SALT (HNO2), or nitrous acid, is known only in solution in water, forming a blue liquid ; its anhydride (N,0) is obtained as a gas when 1 part of starch is heated with 8 parts of nitric acid of specific gravity 1.25: if the vapour is first dried by chloride of calcium, and then passed through a tube cooled to

-20° C., a very volatile liquid is obtained, green at the ordinary temperature, but colourless when exposed to extreme cold ; the vapour of this body is yellowish red.

THE POTASSIUM, SODIUM, BARIUM, STRONTIUM, CALCIUM, MAGNESIUM, IRON, and COPPER SAlts of this radical are soluble.

The Silver Salt is obtained by adding nitrate of silver to a soluble nitrite (as the potassium salt); it is a yellowish white crystalline precipitate, the formula of which is probably AgNO,.

It requires 300 parts of cold water for its solution, but is more soluble in hot water; and from its hot solution it crystallizes on cooling. It is insoluble in alcohol.

THE MERCURIC Salts are of doubtful existence; and the LEAD Salt is soluble.

This radical is also detected by the following experiments:

a. By its liberation, even by weak acids (as acetic acid), in the form of the hydrogen compound, and the instantaneous decomposition of the latter, with formation of nitric oxide gas, which reddens on contact with air.

B. By the addition of a solution of ferrous sulphate and sulphuric acid to the solution of a nitrite, a deep red liquid is obtained.

y. Nitrites, when added to solutions of auric chloride or mercurous nitrate, cause the precipitation of gold or mercury; from solutions of manganous or ferrous salts, they precipitate manganic or ferric hydrate.

d. When to a solution containing pure sulphuric acid, iodide of potassium, and starch, a minute portion of a nitrite is added, the characteristic blue iodide of starch is at once produced.

SALTS OF THE NITRIC RADICAL, OR NITRATES.

The abundant forms in which this radical occurs are its potassium, sodium, and calcium salts; they are produced whenever nitrogenized organic matter is allowed to decompose in the presence of the hydrates of those basic radicals. Nitrates in general have a peculiar cooling taste ; they are all decomposed at a red heat: some, as the alkaline nitrates, yield nitrites in the first place; but ultimately all are converted into oxides, with evolution of nitrogen and oxygen. Nitrate of ammonium (NH, NOx) undergoes a peculiar decomposition under these circumstances, water and the gas called nitrous oxide (N,O) or laughing-gas being formed:

NH, NO,=2H,0+N,0. Nitrates when fused on charcoal deflagrate, their oxygen uniting with the carbon to form carbonic acid gas, which then

takes oxygen, and converts the metal into a carbonate, if that salt is capable of withstanding the temperature to which it is exposed. When heated before the blowpipe with carbon and sulphur, with carbon and phosphorus, or with cyanide of potassium, a very violent and dangerous detonation ensues.

THE HYDROGEN COMPOUND (HNO,), or nitric acid, is an exceedingly corrosive liquid, and forms a most powerful solvent for metals, by reason of its ready decomposability. When acting as a solvent, however, it differs from most acids hitherto considered, which simply part with their hydrogen and receive the replacing molecule of metal : nitric acid undergoes a more radical change ; for so comparatively feeble is the affinity which the nitrogen has for its 3 equivalents of oxygen, that the hydrogen of the acid in escaping decomposes a part of the acid-radical with the formation of water and the evolution of one of the gaseous lower oxides of nitrogen. Thus, when it acts upon copper8HNO, +6Cu=6CUNO, +1,0,+46,0.

nitric oxide. This property it is which imparts such a peculiar energy to the mixture of nitric and hydrochloric acids known by the names nitrohydrochloric acid and aqua regia : the oxygen of the nitric radical unites with the hydrogen of the hydrochloric acid; and among other products of decomposition, chlorine is set free, and, like all other nascent elements, exerts upon the body submitted to its action the most powerful effect :

2HNO, +2H01=2H,0+N, 0,+201. In this way it is that this mixture of acids will dissolve the precious metals gold and platinum, which are attacked by chlorine, although not affected by either hydrochloric or nitric acids separately. The same reaction of course takes place when either hydrochloric acid is added in excess to a nitrate, or nitric acid in excess to a chloride ; for

2MNO, +4HC1 =2MCI +24,0+1,0,+201

or 2MC1 +4HNO,=2MNO,+2H,0+1,0,+201 ; and it should be remembered that we always have it in our power to convert one of these salts into the other by adding

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