352

PROPERTIES OF THE METALLIC SULPHIDES.

sition of a metallic oxide may be ascertained, if it be decomposable by hydrogen, by heating the compound in a current of this gas, collecting and weighing the water produced, and determining the amount of reduced metal which a given weight of the oxide has yielded. In other cases their composition is determined synthetically, a given weight of the metal being converted into oxide, either by heating the metal in a current of air, or by converting it into a nitrate, and afterwards expelling the elements of nitric anhydride by the application of heat, and weighing the quantity of oxide which is left.

(535) SULPHIDES.-The combinations of sulphur with the metals are numerous; they are in many instances of great value, and form important ores. A large number of the native sulphides often exhibit a high metallic lustre, as is shown by the sulphides of iron, copper, lead, and antimony.

Sulphur frequently combines with the same metal in several proportions, and it usually happens that for each oxide a corresponding sulphide may be formed. Sometimes, as in the case of the metals of the alkalies and alkaline earths, the sulphides are more numerous than the oxides: for instance, three oxides of potassium and sodium, and two only of barium are known, but there are not fewer than five sulphides of each of these metals.

All the metallic sulphides are solid at ordinary temperatures. Most of them may be fused at a heat a little above redness, and if the air be excluded, the protosulphides undergo no change in composition; but many of the higher sulphides, such as the bisulphide of iron and bisulphide of tin, are decomposed, and give off the second atom of sulphur, whilst a lower sulphide of the metal is left. Sesquisulphide of arsenic, or orpiment (As,S), and sulphide of mercury, or cinnabar (HgS), may be sublimed if excluded from the air; that is to say, they may be converted into vapour, and recondensed in the solid form; indeed, these sulphides are usually purified by this operation.

The sulphides of all the metals are insoluble in water with the exception of those of the alkali-metals and of strontium and barium. Sulphides of calcium and magnesium, however, are sparingly soluble.

If solutions of the sulphides of the metals of the alkalies and alkaline earths be subjected to a current of gaseous sulphuretted hydrogen, they combine with it, and form soluble compounds which correspond to the hydrates of the oxides; for example

METALLIC SULPHIDES.

K2S+H2S=2 KHS
CaS+H,S=CaS,H,S.

353

The compounds thus formed have been termed sulph-hydrates. Those of calcium and magnesium are decomposed into hydrates by boiling with water, for example:

MgS,H,S+2 H2O=MgO,H2O+2 H2S.

Several of the sulph-hydrates may be obtained in crystals, if evaporated in vessels from which air is excluded.

The sulphides, like the oxides, may be subdivided into basic and acid sulphides, according to the nature of the metal and the number of atoms of sulphur with which each atom of metal is combined. These may be supposed to be formed on the type of one or more atoms of sulphuretted hydrogen, as the oxides are upon the type of one or more atoms of water. The protosulphides of the alkaline metals afford illustrations of basic sulphides, and they enter into combination with the higher sulphides of metals which, like antimony and arsenic, form acids with oxygen. Persulphide of arsenic, or arsenic sulphanhydride (As,S), in this way combines with sulphide of sodium, and forms a crystalline soluble compound (3 Na,S,As, 5, 15 H,→); and in like manner persulphide of antimony, or antimonic sulphanhydride (Sb,S), forms a soluble compound with sulphide of sodium (3 Na,S,SbS, 18 H2O, or Na,SbS, 9 H,O), which crystallizes in beautiful transparent tetrahedra. A large number of similar compounds may be formed with the sulphides of other metals, and these compounds are for the most part soluble in water.

2

In consequence of the tendency to the formation of these double sulphur salts, many of the sulphides which are insoluble in water are dissolved freely by solutions of sulphide of potassium or of sulphide of ammonium; and this circumstance is frequently taken advantage of in the laboratory during the progress of an analysis, for the purpose of separating certain metals the sulphides of which are soluble in solutions of the sulphides of the alkalifiable metals, from others which are not soluble in these compounds. The following sulphides may be dissolved by a solution of sulphide of ammonium, and by a solution of sulphide of potassium:

354

ACID AND BASIC SULPHIDES.

The double salts thus obtained are decomposed by the addition of an acid, such as the sulphuric or the hydrochloric, sulphuretted hydrogen being evolved, whilst the sulphide of the electronegative metal is precipitated; for example, 3 Na,S,Sb,S,+6 HCl become 6 NaCl + Sb2S;+3 H2S.

These electro-negative sulphides are often soluble in solutions of the alkalies, forming a mixture of a sulpho-salt with an oxysalt. Sesquisulphide of antimony, for instance, when dissolved in caustic potash, gives the following result:

:

Sb2S2+6 KHO=K ̧SbÐ ̧+K ̧SbSg+3 H2O.

Decompositions.-If free oxygen or atmospheric air be allowed. access to the heated sulphides, they are all decomposed; the sulphur becomes oxidized, and passes off as sulphurous anhydride, whilst the metal, in most cases, as occurs with tin, antimony, and molybdenum, remains in combination with oxygen. The sulphides of the metals of the alkalies and of the alkaline earths become converted into sulphates of the metal, and the same thing occurs less completely with many of the metals which have a strong attraction for oxygen; the sulphides of iron, lead, and copper, are partially converted into sulphates, but by a stronger heat these sulphates afterwards lose their acid, and the oxide of the metal only is left. The sulphides of the noble metals, when roasted in a current of air, lose their sulphur, which burns off in the form of sulphurous anhydride, while the pure metal remains behind, though in the case of silver a portion of sulphate of silver is commonly formed.

Many of the hydrated sulphides become oxidized by exposure to the air, and generally are converted into sulphates. Hydrated sulphide of iron, however, furnishes hydrated sesquioxide of the metal, whilst sulphur is liberated: whilst the sulphides of the metals of the alkalies and alkaline earths become converted into hyposulphites; for instance, 2 €aS+ H2O + CO2 + 2 Ð ̧=¤a§ ̧Н ̧Ð ̧ + Єa¤Ð ̧.

2

A large number of the sulphides, especially those of the more oxidizable metals, such as those of iron, zinc, and manganese, are dissolved by diluted hydrochloric acid when cold, and still more readily when heated,—a chloride of the metal and hydrosulphuric acid being formed. Others, such as those of nickel, cobalt, and lead, require boiling with the concentrated acid: it is in this way that hydrochloric acid acts upon the sesquisulphide of antimony; Sb,S+ 6.HCl becoming 2 SbCl ̧ + 3 H2S. Sulphuric acid, when diluted, acts in a similar manner upon the sulphides of the more oxidizable metals, though less readily than hydrochloric acid.

DECOMPOSITION AND PREPARATION OF THE SULPHIDES.

355 The sulphides are all decomposed when heated in a current of chlorine gas, chloride of sulphur and chloride of the metal being formed. This property is sometimes made use of in the analysis of ores consisting chiefly of sulphides, or of sulphides and arsenides of the metals; the volatile metallic chlorides are in this way separated from the more fixed ones. Aqua regia attacks and decomposes the sulphides as readily as gaseous chlorine; and a mixture of hydrochloric acid and chlorate of potassium is equally effectual in decomposing them. With the exception of sulphide of mercury, they are also decomposed by nitric acid, sulphuric acid and nitrate of the metal being formed; during this operation part of the sulphur is often separated in the form of tough elastic masses, which, if the heat be continued, collect into yellow globules, and can be oxidized only by prolonged digestion in the acid. When the sulphides are fused with the alkaline carbonates or with the hydrated alkalies, they are partially decomposed, and the mass contains a variable mixture of alkaline sulphide with oxide of the metal, and different oxysalts of sulphur.

Before the blowpipe the sulphides are easily recognized by the odour of sulphurous anhydride which they emit, either when heated in a glass tube open at both ends, or when roasted upon charcoal. Some other particulars relating to the sulphides have been already mentioned (428).

Preparation. Many methods for preparing the sulphides may be adopted. 1.-Sulphur may be heated with the metallic oxides, many of which it decomposes: with the alkalies and alkaline earths, a sulphate, and a sulphide with variable proportions of sulphur, are obtained: but when definite and pure sulphides are required, other means should be adopted. 2.-The lowest sulphides of the metals of the alkalies and alkaline earths may be procured by decomposing their sulphates by igniting them in closed vessels with charcoal; oxygen is removed, carbonic oxide formed, and the remaining sulphide may be dissolved in water and freed from the excess of charcoal; K2SO4 + 4€ = K2S + 4 €0. 3.-Hydrogen is sometimes employed for preparing the sulphides from the sulphates, which are to be placed in a tube and ignited in a current of the gas. In this manner the protosulphides of the alkalifiable metals are easily obtained, but the sulphates of the other metals frequently lose a portion of the sulphur, as well as all their oxygen, and subsulphides are procured. 4. Many of the metals combine directly with sulphur, if heated with it, and form sulphides; but the compounds thus obtained often contain sulphur dissolved in, or disseminated

356 PREPARATION AND ESTIMATION OF METALLIC SULPHIDES.

through, the mass. Sulphide of iron is usually prepared in this manner. Indeed, sulphur, though itself combustible, supports the combustion of many metallic bodies, which burn vividly when heated in its vapour. 5.-In other cases the sulphides may be formed by heating the metal in a current of sulphuretted hydrogen, or in the vapour of bisulphide of carbon. This latter method is the plan commonly adopted in procuring sulphide of titanium from titanic acid; TiO,+CS, TiS,+CO2. 6.— Hydrated sulphides of the metals of the last three groups may also be procured by passing a stream of sulphuretted hydrogen through neutral or acid solutions of their salts, when they are precipitated in the insoluble form. 7. The hydrated sulphides of zinc, iron, manganese, cobalt, and nickel, which are not thrown down by sulphuretted hydrogen, may be prepared by double decomposition, by mixing a solution of the salts of any of these metals with that of a sulphide of one of the alkalifiable metals: thus sulphate of manganese if mixed with sulphide of potassium yields sulphate of potassium and sulphide of manganese; MnᎦᎾ, + @H Ꮎ, Ꮶ Ꭶ=K ᎦᎾ + #H, Ꮎ, MnᎦ. In many cases the colours of these hydrated sulphides are characteristic of the metal-for example, the hydrated sulphide of zinc is white; that of manganese flesh-red; those of cadmium, arsenic, and persulphide of tin are yellow; and that of the hydrated protosulphide of tin is chocolate-brown. The sulphides of molybdenum, rhodium, iridium, and osmium are brown, each with its peculiar shade; whilst in a large number of instances-including the sulphides of iron, cobalt, nickel, uranium, vanadium, bismuth, copper, lead, silver, mercury, gold, platinum, and palladium-the precipitated sulphides are of a black, more or less pure.

(536) Estimation of Sulphur in Metallic Sulphides.—Sulphur is always estimated in the form either of sulphuric acid or of free sulphur. The sulphur in a sulphide is easily converted into sulphuric acid by the agency either of gaseous chlorine or of aqua regia; and the soluble sulphates, when mixed in slightly acid solution with a salt of barium, yield an insoluble sulphate of barium, the amount of which, after it has been well washed with boiling water and ignited, furnishes data for the calculation of the sulphur; 100 parts of sulphate of barium representing 34'34 of sulphuric anhydride, or 13.74 of sulphur. If a salt of silver be present, nitrate of barium must be employed to precipitate the sulphuric acid.

If during the solution of a sulphide in aqua regia, the sulphur have collected into clear yellow balls, and the action upon the