362 PREPARATION AND DECOMPOSITIONS OF METALLIC CHLORIDES.

metals. Perchloride of tin may thus be obtained by heating metallic tin with an excess of corrosive sublimate; and terchlorides of antimony and bismuth may be obtained in a similar manner. Sometimes this process is employed for the purpose of isolating those metals the oxides of which resist decomposition by the usual means. In this way sodium is employed to decompose the chloride of aluminum or of magnesium for the purpose of procuring the aluminum or magnesium in an uncombined form; and in a similar manner potassium is employed to obtain uranium from uranous chloride. 9.-The insoluble chlorides, such as those of silver and lead, and mercurous chloride, may be formed by the addition of hydrochloric acid, or of a soluble chloride, to a solution of the corresponding salts of these metals.

Decompositions.-All the metallic chlorides, excepting those of the metals of the alkalies and earths, are reduced when sufficiently heated in a brisk current of hydrogen. In many cases the reduction is easily effected, and this process is occasionally resorted to as a means of procuring certain metals in a state of purity. Iron, for example, may be obtained in fine cubic crystals by reducing ferrous chloride in this manner. It is necessary, however, to maintain a current of hydrogen of sufficient rapidity to carry away the hydrochloric acid from the reduced metal, as otherwise, in most cases, the chloride would be reproduced by the decomposition of the acid. All the chlorides, except those of the alkaline metals, and of barium and mercury, are decomposed when heated in a current of steam, generally leaving corresponding oxides, but chloride of bismuth leaves an oxychloride (Kunheim). All the soluble chlorides, when heated with sulphuric acid and black oxide of manganese, evolve chlorine gas. Other particulars relating to the chlorides have been already mentioned (373, 376).

(539) Estimation of Chlorine in Metallic Chlorides.-Chlorine is almost always estimated in the form of chloride of silver, 100 parts of which represent 24'74 of chlorine. The solution should be acidulated with nitric acid, and gently warmed, and then the nitrate of silver should be added. If iodine or bromine be present, it will be precipitated with the chlorine, and must be determined separately, and the corresponding weight of iodide or bromide of silver deducted.

The composition of an insoluble chloride or of a basic chloride may be ascertained, except in the case of chloride of silver, by boiling a given weight of the compound with a pure solution of potash, and then determining the quantity of chlorine in the

METALLIC BROMIDES-IODIDES.

363

alkaline solution by means of nitrate of silver: before adding the solution of silver, the alkaline liquid must be filtered from the undissolved metallic oxide, and acidulated with nitric acid.

(540) The BROMIDES (389, 391) closely resemble the chlorides in chemical characters, and may be arranged in corresponding groups: the bromides of the metals of the alkalies and alkaline earths may be prepared by digesting a solution of the alkali or of the earth with bromine in slight excess; a bromide and a bromate of the metal are thus formed, and by gentle ignition the bromate is decomposed, leaving a pure bromide: a small quantity of charcoal may be added previously to the ignition, by which the decomposition of the bromate is more easily effected. The bromide is removed from the excess of charcoal by solution in water. The other bromides may be procured by acting upon the metals by bromine, either in a dry state or in the presence of water. They are also easily formed by dissolving the oxides or the carbonates in hydrobromic acid.

Bromine may be precipitated from its solutions, and, in the absence of chlorine, its quantity may be estimated, by means of nitrate of silver, which occasions a white precipitate of bromide of silver, 100 parts of which indicate 41'47 of bromine. If chlorine be present the precipitate will consist of a mixture of the bromide and chloride of silver: it must be collected and weighed, then digested with metallic zinc and a drop or two of sulphuric acid; in a day or two the zinc will have reduced the bromide and. chloride to metallic silver; this must be well washed, dried, and weighed. It should be wholly soluble in nitric acid, since if it be not, a portion of the precipitate has escaped decomposition.

From the above data the relative proportions of the bromide and chloride of silver may be calculated :—let m be the weight of the mixed bromide and chloride, and let s be the weight of the reduced silver; then if a represent the proportion of bromide, and y that of chloride of silver, it will be found that

=

consequently, or the bromide of silver in the mixture, my; and y, or the chloride of silver in the mixture, 3*2247 m.

=

5'6134 $

(541) The IODIDES (396) may be formed by processes analogous to those employed for the bromides: the insoluble iodides, such as those of mercury, silver, and lead, may be obtained from

364

METALLIC IODIDES AND FLUORIDES.

a solution of iodide of potassium, by mixing it with a solution of the metallic salt.

The iodides exhibit a strong tendency to form double salts, the iodides of the strongly basylous metals combining readily to form crystallizable double iodides with those of the electronegative metals, such as those of silver, mercury, and lead. The iodides also form double compounds with the oxides and chlorides: for example, there are several compounds of the iodide with the oxide of lead; and a combination of perchloride of tin with the stannous iodide (SnI,,SnCl) may be obtained in orange-coloured crystals (Kane).

2

The quantity of iodine in a solution which contains iodides, if chlorides be absent, may be estimated by the addition of nitrate of silver slightly acidulated with nitric acid: the resulting buffcoloured iodide of silver, when collected and dried, contains 54'0 per cent. of iodine. If chlorine or bromine be present, the iodine must be precipitated by means of nitrate of palladium; the precipitate must be allowed to subside during ten or twelve hours, and it may then be collected on a filter and dried at a temperature not exceeding 160°: this precipitate is insoluble in cold diluted nitric or hydrochloric acid, but soluble in ammonia. contains 700 per cent. of iodine. Iodine may also be separated from bromine and chlorine, but less perfectly, by a mixture of green sulphate of iron and sulphate of copper (883).

It

(542) FLUORIDES.-The general properties of these compounds have been already stated (403). The fluorides are usually prepared by the direct action of hydrofluoric acid either upon the metal or more usually upon the oxide of the metal. Those which are insoluble may be procured by mixing a solution of the metallic salt with one of fluoride of potassium or of sodium.

Estimation of Fluorine.-A simple method of detecting and of approximatively estimating fluorine, when present, even in very small quantities, has been proposed by Dr. G. Wilson. The following is the process, slightly modified:-the substance, if it does not already contain silica, is mixed with pounded glass, placed in a retort, and made into a thin cream with oil of vitriol; the mixture is next heated, and distilled into a flask containing a solution of ammonia; the fluoride of silicon comes over, and is immediately decomposed on evaporating the liquid in the flask to dryness on a water-bath, the silica is rendered insoluble, and can be collected and weighed, whilst the fluoride of ammonium may be dissolved out with a little water, and the presence of fluorine shown by mixing it with oil of vitriol; the vapour which

METALLIC NITRIDES-PHOSPHIDES-CARBIDES.

365

is evolved produces the usual corrosive action of hydrofluoric acid on glass (403): the proportion of silica in the insoluble residue to the fluorine, however, is not very uniform.

(543) NITRIDES.-It is not improbable that the fulminating compounds, obtained by digesting the hydrated oxides of gold, of silver, and of platinum, in a solution of ammonia, may owe their explosive character to the formation of a nitride: the composition of these bodies has, however, been but imperfectly investigated, on account of the ease with which they explode. So weak is the chemical attraction of nitrogen for most metallic bodies, that a slight alteration of circumstances often suffices to restore it suddenly to the gaseous state. Nitride of copper is formed by passing dry ammonia over oxide of copper, at a temperature not exceeding 480°, in which case water is formed at the expense of the hydrogen of the ammonia and the oxygen of the oxide, and part of the nitrogen escapes; thus, 6 Єu✪ + 4 H ̧N = 2 €u2N + 6 H2+ N2. Nitrides of mercury and iron may be prepared by passing ammonia over oxide of mercury and oxide of iron in a similar manner. Titanium, molybdenum, and vanadium absorb nitrogen rapidly at a red heat; and crystalline nitrides of chromium and magnesium have also been obtained.

(544) The PHOSPHIDES of the metals are of comparatively small importance: they are never met with in the native state. The phosphides of the metals of the alkalies and alkaline earths decompose water when thrown into it; self-lighting phosphuretted hydrogen is disengaged, and a hypophosphite of the metal is retained in solution. In some cases, as for example in that of phosphide of calcium, the phosphide is formed by heating the oxide strongly, and driving the vapour of phosphorus over it; in this case it is mixed with a large proportion of phosphate of calcium. The insoluble phosphides may often be obtained by transmitting a current of phosphuretted hydrogen through a solution of the salt of the metal in water: phosphides of copper and silver may be thus obtained.

When heated in air phosphides are converted into phosphates, or into phosphoric anhydride, while the metal is liberated.

(545) CARBIDES.-The only carbides of importance are those of iron, which will be considered in detail when treating of that metal. Manganese, palladium, iridium, and a few other metals, also combine with carbon; generally speaking, these carbides are more fusible than the metals which enter into their formation.

SILICON and BORON form with the metals analogous compounds of small importance. Amongst the most interesting of

366

HYDRIDES-THEORY OF SALTS.

the silicides are those of calcium, aluminum, magnesium, iron,

and copper.

(546) HYDRIDES.-Hydrogen is not known to combine with more than five metals :-viz., arsenic, antimony, copper, iron, and potassium. The first two of these compounds are gaseous, and are decomposed by a red heat into metal and hydrogen gas. A solid hydride of arsenic is also said to exist. A few metals, such as zinc and potassium, appear under peculiar circumstances to undergo partial volatilization along with the hydrogen at the moment that this gas is evolved.

§ III. HYPOTHESES ON THE CONSTITUTION OF SALTS.

(547) Acids and Bases.-It has already been stated (6) that any substance which is produced by the action of an acid upon a base is termed a salt. It is, however, necessary to examine more minutely into the nature both of bases and acids, and into that of the compounds formed by their combination with each other.

By the word base, is meant a body always of a compound nature, very frequently an oxide of a metal, which is capable of effecting a double decomposition with an acid, whilst a salt and water are formed, and the distinctive characters of the acid are more or less completely neutralized. A base, however, is not necessarily a metallic oxide; the hydrates of ammonia, quinia, and morphia, for example, are powerful bases, but they contain no metallic substance.

(548) Oxyacids and Hydracids.—When Lavoisier imposed the name of oxygen upon one of the constituents of the atmosphere, he supposed that the presence of that energetic body was essential to the existence of an acid; and this view was supported by the known composition of the principal acids, such as the sulphuric, the sulphurous, the nitric, the carbonic, the phosphoric, and the boracic acids. The term acid was indifferently applied to the anhydrides and to the compounds produced by the action of anhydrides upon water; to which latter class of compounds the term is more properly restricted by many later writers, and to which it is limited in this work. Lavoisier considered an acid to be an oxidized body more or less soluble in water, with a sour taste, capable of reddening vegetable blues, and entering into combination with the alkalies, the distinctive properties of which it neutralized.

By degrees, however, acids were discovered in which no oxygen could be detected: such, for example, as the hydrochloric,