TESTS FOR ARSENIC.

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TRIHYDRIDE OF ARSENICUM OR ARSENIURETTED HYDROGEN.

[H2As.]

Is a gas of great importance as a test for arsenic; it corresponds to the trihydrides of antimony and of phosphorus.

The gas is produced whenever Zn, H2SO, and a salt of As are mixed together. Like the antimony compound, it burns with flame, and if the tube through which it is passed be heated at one place, the gas is decomposed and a mirror of metallic arsenic deposited inside the tube near that place. If a stream of H2S be now passed through the same tube when cold, the As is converted into yellow As2S. If now a stream of dry HCl gas be passed through the tube, the yellow sulphide is not acted upon. But if it were antimony sulphide it would be. This reaction is one means of distinguishing Sb from As. Spots of metallic As are also deposited on a cold body, as a piece of porcelain, for example, if held in the flame of the gas. These spots are of a lighter colour than those produced by Sb under similar circumstances. (See p. 144.)

TESTS FOR ARSENIC.

THE metal is volatile, and gives a garlic-like smell when heated before the blowpipe flame.

As2O, is volatile, and the vapour free from smell.

3

H2S produces a bright yellow precipitate in salts of As"" and As, in presence of free HCl (see p. 150); the precipitate with As" is but slowly produced, so that the As is usually reduced to As'" by means of SO, and then tested by H2S. (See p. 149.)

2

Copper sulphate produces a green precipitate, called Scheele's green. (See p. 148.)

Silver nitrate gives a yellow precipitate with salts of

As" (see p. 148), and a red-brown with salts of As (see p. 149). For delicate testing an ammonio-sulphate of copper and an ammonio-nitrate of silver are used to test for arsenic. They are formed by precipitating either normal salt with solution of ammonia, and adding ammonia until the precipitate is nearly dissolved. The clear solution is used as a test. The solution of As to be tested must be free from any excess of H salt.

Marsh's Test.-By arseniuretted hydrogen (see p. 151, and also antimoniuretted hydrogen, p. 144).

Reinsch's Test. When bright Cu is boiled with free HCl, the copper remains bright, the HCl having no action on it (see p. 130). But if a solution containing As be boiled with Cu as above, the As is precipitated in the form of a dark grey film of metal on the bright Cu. If such coated copper be carefully dried and heated in a narrow glass tube open at both ends and held in a slanting position, the As oxidises to As,O,, and condenses, forming a ring of white octahedral crystals in the cold part of the tube.

A similar action takes place with Sb, but the oxide of Sb formed is less volatile, and amorphous. (See p. 145.)

GROUP D.-NOBLE METALS.

FIVE metals will be treated of in this group, namely:—

There are also four other metals generally associated together in the ores of platinum; they belong to this group, but are neither plentiful nor very useful, namely :—

NOBLE METALS.-MERCURY.

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There is little family relationship between the metals of this group beyond the fact that in the air they do not. tarnish or oxidise; the oxides of the first five are reduced to the metallic state by heat alone, on which account they have received the name of noble metals. They do not decompose water under any circumstances, and possess a more powerful affinity for S and Cl than for O.

MERCURY.

[Symbol, Hg (Hydrargyrum); Atomic Proportion, 200; S.g. 13·6.]

OCCURRENCE. This metal is found in only few localities. It occurs native, but mostly as red sulphide, called cinnabar (HgS); occasionally in union with the non-metal selenium (Se), and very rarely as native chloride (HgCl). The chief mines of Hg are at Almaden in Spain and Idria in Transylvania; it is also found in California, Peru, China, and Japan.

HISTORY.-It has been known from very remote ages; the alchemists gave it the name mercury (); also it is called quicksilver, or live silver; its name hydrargyrum is derived from üdwp, water, and äpyupos, silver.

PREPARATION.-Generally from the native sulphide.

1. By heating it in an iron retort with Fe or CaO, when the Hg volatilises and is condensed.

2. By roasting cinnabar (HgS) on a grating over a fire; SO, escapes, and the fumes of Hg are condensed, either in long tubes or chambers.

2

PROPERTIES.-It is the only metal which is liquid at the ordinary temperature; it is silver-white, and slowly evaporates even without being heated. The vapour of Hg in large quantity is poisonous; care must be taken not to spill the metal in dwelling-rooms or breathe its vapours during experiments. It freezes at 39°C., and is then

malleable. It boils at 350° C. Its vapour is colourless. When pure, Hg runs in globules, and does not adhere to glass, porcelain, or paper; but if it contains other metals, it leaves a tail or streak behind it. When pure, it does not tarnish in the air.

Hydric chloride has no action on Hg.

Dihydric sulphate has no action when cold, but boiling hot it attacks Hg with formation of white insoluble mercury sulphate and SO2, which escapes (see C. N. p. 83).

Hydric nitrate, even when cold, attacks Hg, forming a soluble nitrate.

Uses.-Mercury is used to extract gold and silver from their ores by forming an amalgam with them, which is soluble in an excess of Hg. It is used to fill barometer and thermometer tubes; alloyed with tin, to cover the backs of looking-glasses; to make detonating powder, and the colour vermilion. Also Hg and its salts are employed as medicines.

MERCURY WITH OXYGEN.

MERCURY forms two well-known compounds with O, representing two classes of salts, which the student must be careful not to confound together, the one containing univalent mercury (Hg), the other divalent mercury (Hgʻ′), namely―

Black Mercurous oxide or Dimercury oxide
Red Mercuric oxide or Mercury oxide

Hg'2O.
Hg"O.

The black oxide (Hg,O) is a powerful basylous radical, and may be formed by adding a solution of alkali to mercury chloride (HgCl').

The red oxide (Hg"O") is sold as a bright red crystalline powder, made by heating Hg, exposed to the air, to nearly its boiling point; O is slowly absorbed, a red powder (HgO) forming on the surface. At a little higher heat, it is decomposed into Hg and O (see C. N. p. 50). It may also be formed by decomposing the corresponding nitrate

NITRATES AND SULPHIDES OF MERCURY.

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2(Hg2NO,).H2O by heat. It is also precipitated as a yellow amorphous solid by adding a solution of alkali to one of Hg", as the dichloride (HgCl2). It is slightly soluble in water.

THE NITRATES OF MERCURY

ARE numerous, the exact composition of the salt formed depending upon the strength of the solution of hydric nitrate used to act upon the metal. It will be sufficient here to state that when Hg is acted upon by cold dilute solution of HNO, a nitrate is formed containing Hg; and when a strong solution of HNO3 and heat are employed, a nitrate results containing Hg".

NORMAL MERCUROUS OR MERCURY NITRATE.

[2(Hg (NO)').H2O.]

THIS salt crystallises out in colourless soluble crystals, when Hg is acted upon by dilute cold HNO,. It decomposes when largely diluted with water into an oxysalt.

MERCURIC NITRATE OR MERCURY DINITRATE.

[2(Hg"2(NO3)').H2O.]

THIS nitrate may be formed by dissolving the red oxide (Hg"O") in HNO3, or by boiling Hg with strong solution of HNO3. The normal salt is difficult to crystallise; if largely diluted, a basic insoluble salt is thrown down, from which all the nitrate may be washed by boiling water, leaving Hg'O.

MERCURY WITH SULPHUR.

THERE are two sulphides of Hg, corresponding to the two oxides, namely—

The former is precipitated, when H2S is added to a solution containing Hg.