VARIETIES OF THE METALLIC OXIDES.

347

anhydride Sne, and the titanic, niobic, and tantalic anhydrides, with which silica and zirconia ought also to be classed: these oxides do not unite with water when placed in contact with it, but yield hydrates possessed of feebly marked acid characters when precipitated with due precautions from their combinations. For each of these binoxides a corresponding chloride, containing 4 atoms of chlorine, may be obtained.

6. Teroxides, of the type Nig.

This class includes the metallic anhydrides in which the acid property is most strongly developed; such, for example, as chromic, vanadic, molybdic, tungstic, and ruthenic anhydrides. Although the ferric and manganic anhydrides have not been isolated, their place is obviously in this group. The molybdic, tungstic, and ruthenic anhydrides are insoluble in water.

7. Anhydrides, of the type R'.

5

5

Arsenic and antimonic anhydrides, As, and Sb2, are the representatives of this group of oxides: when combined with water they furnish well-marked metallic acids. Arsenic anhydride is deliquescent and freely soluble; the antimonic compound does not combine with water when mixed with it.

The properties of the metallic acids or hydrated compounds obtained by the action of these binoxides, teroxides, and peroxides upon water, directly or indirectly, will be referred to again hereafter, when the bodies themselves are described.

The compounds of the same metal with oxygen are often numerous; and the extremes, or the oxide with the maximum of oxygen, and the oxide with the minimum of oxygen, frequently present chemical qualities of opposite kinds, the former being electronegative, and possessing acid properties, whilst the lower oxides are electropositive, or basic in their nature.

An excellent instance of this kind is afforded in the various oxides of manganese: the protoxide (Mn) is a powerful base; the sesquioxide (Mn,,) is a much weaker base; the red oxide (MnO,Mn) is a saline or indifferent oxide, and shows little disposition to furnish corresponding salts by reaction either upon acids or alkalies, and the same may be said of the black oxide (Mn→2); while the two higher oxides, which, however, are only known in combination either with hydrogen or the metals, are soluble in water, when they constitute the manganic and permanganic acids (H,Mn, and HMO). As a general rule, the greater the number of atoms of oxygen which an oxide contains, the less is it disposed to form salts by reaction with the acids: on the

348

GENERAL PROPERTIES OF THE OXIDES.

contrary, its hydrate frequently possesses acid properties, and then it reacts upon bases to form salts.

The basic oxides in general are devoid of all metallic appearance, and present par excellence the characters of earthy matters. The protoxides, when solid, are all insulators of the voltaic current; but some of the higher oxides, such as the peroxides of silver, lead, and manganese, allow it to pass with facility. It is singular that all these conducting peroxides may be formed in solutions of salts of their respective metals by the action of the current itself.

These oxides, when found crystallized in the native state, are much harder than the metals that furnish them, and they generally have a specific gravity considerably less than that of the metals themselves. All the oxides are solid at ordinary temperatures; many of them are fusible at a red heat, such, for example, as the protoxides of potassium, sodium, and lead, and the sesquioxide of bismuth: but the oxide of copper, molybdic anhydride, sesquioxide of chromium, and black oxide of iron, require a much higher temperature to effect their fusion. Baryta, strontia, and alumina require the heat of the oxyhydrogen jet; while some oxides, such as lime and yttria, exhibit no appearance of fusion, even after the application of this intense heat.

As a general rule, the addition of oxygen to a metal renders it much less fusible and volatile. The protoxide of iron, the sesquioxide of chromium, and molybdic anhydride, are the only oxides which melt at a temperature below that of the metal from which they are produced. A few of the oxides are volatile at moderate temperatures; among these arsenious anhydride, sesquioxide of antimony, and tessaroxide of osmium. Nine only of the basic

oxides are soluble in water to any considerable extent-viz., the five alkalies, and baryta, strontia, lime, and oxide of thallium. The insolubility of the oxides, however, is far from being so complete in general as that of the corresponding sulphides, and consequently, except in particular cases, it is less advisable in analytical operations to separate the metals in the form of oxides than in that of sulphides; the oxides of lead, silver, and mercury in particular, are perceptibly soluble in pure water.

Those hydrated compounds of oxygen with the metals which possess acid characters-such as the chromic, manganic, and arsenic acids are often freely soluble in water; but even those acids, which, like the tantalic, molybdic, and tungstic, are nearly insoluble, usually redden litmus-paper, though their anhydrides have no such effect.

Preparation. Most of the oxides may be procured in com

PREPARATION OF THE METALLIC OXIDES.

349

bination with water; generally speaking, these hydrated oxides are obtained by double decomposition, on the addition of the solution of an alkali to one of their soluble salts: in this manner sulphate of zinc yields hydrated oxide of the metal on adding hydrate of potash to its solution; ZnSO4+2 KHO=K2SO ̧+ ZnH2. The metals which form powerful bases, like those of the alkalies and alkaline earths, retain the water with great obstinacy; while others, which are less powerful bases, such as the hydrated oxide of copper, are decomposed at a temperature below that of boiling water.

2

The anhydrous oxides may be obtained in several ways:1.—They may often be formed directly, by burning the metal in air, or in oxygen gas. This process is best adapted to metals which, like zinc or arsenic, are volatile, or which produce fusible oxides, like iron or lead; in such cases the oxide is removed as fast as it is formed, and fresh surfaces of the metal are continually exposed to the action of the gas. Anhydrous potash and soda are obtained by this method; and it is resorted to on the large scale in the preparation of arsenious anhydride, and of the oxides of zinc and lead. Several of the metallic protoxides, if roasted at a low red heat in a current of air or of oxygen, absorb an additional quantity of oxygen. Litharge, or protoxide of lead, is thus converted into red lead, 2 Pb✪,Pb,; and peroxide of barium, Bae, may in this way be obtained from baryta. 2.-Another method consists in the formation of a nitrate of the metal by means of nitric acid; the nitrate is then decomposed by heat, which expels the elements of the anhydride and leaves the oxide: in this way the oxides of mercury, bismuth, antimony, copper, barium, and strontium are prepared. 3.-In some cases it is found advantageous to prepare the oxide by the decomposition of the carbonate of the metal by heat. All the carbonates, with the exception of those of cœsium, rubidium, sodium, potassium, and barium, are decomposed at a red heat. Lime is thus commonly obtained from limestone, which is an impure carbonate. 4.-Sometimes the hydrated oxide is first precipitated, as already mentioned, and then rendered anhydrous by heat; in this manner the sesquioxides of iron and uranium are often prepared. 5.-Occasionally the ignition of a sulphate is resorted to, as in preparing alumina and sesquioxide of iron. 6.-All the acid oxides may be obtained by deflagrating the metal or its sulphide with nitre; the tendency of the metallic acid to unite with the alkali favours the oxidation of the metal: the higher oxides of osmium, titanium, manganese, and chromium, as well as of some other metals, may be obtained in this way.

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DECOMPOSITIONS OF THE METALLIC OXIDES.

Decompositions.-1. By the action of a red heat many of the oxides lose their oxygen, either partially or entirely. The oxides of gold, silver, mercury, platinum, and palladium may thus be completely reduced; the peroxides of lead, cobalt, nickel, and barium, return to the state of protoxide; and the metallic anhydrides lose a portion of their oxygen; for example, arsenic and chromic anhydrides are thus converted respectively into arsenious anhydride and sesquioxide of chromium. The higher oxides of iron and manganese furnish the magnetic oxide, Fe, and the red oxide Mng. It may be stated as a general rule, liable however to exception, especially in the case of the acidifiable metals, that the attraction of a metal for oxygen increases in the inverse proportion of its specific gravity; the lightest metals, such as potassium and sodium, being the most readily oxidized, while platinum, iridium, and gold, which are densest metals, are also those which show the smallest tendency to combine with oxygen.

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2. The oxides are not affected by hydrogen gas at the ordinary temperature of the air. All the higher oxides of the metals are readily reduced to protoxides by hydrogen at a low red heat, whilst water is formed; and at a full red heat a large number of them are reduced to the metallic state. This reduction goes on most readily when the current of hydrogen is brisk, so that the vapour of water is carried away as fast as it is formed. The oxides of many of the metals which decompose water at a red heat may nevertheless be deprived of their oxygen in a brisk current of hydrogen; this is the case, for example, with the oxides of iron, zinc, and cadmium; but not with chromium or manganese. The metals of the alkalies and of the earths are not reducible by hydrogen.

3.-The reducing action of carbon at a high temperature is still more important; all the metals which yield their oxygen to hydrogen do so to carbon, and potassium and sodium are obtainable from their compounds by its agency. This arises in part from the volatility of these two metals, which is sufficient to enable them to be distilled from the carbonaceous mixture. Lithium and the metals of the earths are not sufficiently volatile to pass over in vapour, and though their attraction for oxygen is less intense than that of potassium or of sodium, they cannot be obtained from their oxides by the action of carbon. It depends upon the nature of the metal, and upon the temperature employed, whether the gas that is formed during the reduction be carbonic oxide or carbonic anhydride. The more readily oxidizable metals, such as

DECOMPOSITIONS OF THE METALLIC OXIDES.

351

potassium, zinc, and iron, at a high temperature decompose carbonic anhydride, so that carbonic oxide only is formed when they are reduced; while if the reduction takes place readily, as is the case with copper and lead, carbonic anhydride is obtained.

4.—Dry chlorine sometimes, even without the application of heat, decomposes the basic metallic oxides, such as oxide of silver and the red oxide of mercury, expelling their oxygen, and converting them into chlorides. At an elevated temperature few of them, excepting those of magnesium and of the earths in the third group, resist its action: the oxides of gold and platinum are simply reduced to the metallic state, but chlorides of the metals are formed in other cases.

If the oxides be hydrated and suspended in water, the action of chlorine is quite different; the metals of the first two groups yield bleaching compounds, and by heat are converted into chlorates and chlorides, in the manner already explained (379). The oxides of the third group, the earths proper, experience no particular change, but those in the ferric group are converted into a mixture of chloride and hydrated sesquioxide. The sesquioxides of cobalt and of nickel are usually prepared in this manner: for example, 3 ЄoH‚Ð ̧+Cl2=ЄoCl2+¤01⁄2Ð3, 3 H2Ð.

2

2

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If the liquid be strongly alkaline, the whole of the metal may be converted into sesquioxide; 2ЄoH,2+2KHO+Cl2=Єo23,3 H2O +2 KCl. The potash in this case parts with its oxygen, which is transferred to the cobalt, whilst the chlorine combines with the potassium. The protoxide of manganese, under these circumstances, yields the hydrated peroxide, Mu✪„,2 H2O.

If the metal be capable of forming an acid with 3 atoms of oxygen, the process of oxidation may even proceed further, and the sesquioxide may, in the presence of a large quantity of potash, become converted into the metallic acid, which reacts upon a portion of the excess of alkali and forms a salt, as in the case of sesquioxide of iron, when ferrate of potassium is produced; Fe,,, 3 H2+10 KHO+3 Cl2=2 K2Fe1 + 6 KCl+8 H2→.

5.-Most of the oxides are decomposed more or less completely when heated with sulphur; the alkalies and alkaline earths are converted into a mixture of sulphate and sulphide, but magnesia, oxide of chromium (?), stannic and titanic anhydrides, as well as the metals of the earths, or those of the third group, are unaltered. Most of the other oxides are converted into sulphides, with escape of sulphurous anhydride. The oxides are more readily decomposed by sulphur if they be previously mixed with carbon.

(534) Estimation of Oxygen in Metallic Oxides.-The compo