642

PERMANGANIC ACID-PERMANGANATES.

(801) Permanganic Acid (HMnO4 or HO,MnO

= 120).If a solution of manganate of potassium be largely diluted with water, the colour changes from green to violet; the manganic acid passes to a higher state of oxidation, and permanganate of potassium is formed.

This salt may be prepared by mixing intimately 4 parts of finely powdered peroxide of manganese with 34 parts of chlorate of potassium; 5 parts of hydrate of potash are dissolved in a small quantity of water, and added to the mixture, which is dried and reduced to powder, and then heated to dull redness for an hour in an earthen crucible (Gregory). When cold, the mass is treated with water, and filtered through a funnel plugged with asbestos; the solution, after being neutralized with sulphuric acid, on evaporation yields beautiful red acicular crystals of permanganate of potassium (KMnO4, or KO,Mn2O,=158). If a solution of manganate, prepared by Béchamp's process, be decomposed by transmitting a current of carbonic acid until the green colour has been converted into red, very fine crystals of the permanganate are obtained on evaporating the clear liquid after decantation from the precipitated oxide of manganese. The crystals of the permanganate are isomorphous with those of perchlorate of potassium; they require about 16 parts of cold water for solution.

Permanganate of potassium is in certain cases a useful oxidizing agent it may be employed to detect the occurrence of sulphurous acid in solution in sulphuric or hydrochloric acid; sulphurous acid quickly deoxidizes it, and destroys its colour if present. Neutral solutions of the sulphides and the pentathionates quickly discharge the colour of a solution of permanganate of potassium, and a similar effect is produced by acid solutions of the sulphites, hyposulphites, tetrathionates, sulphocyanides, and nitrites. The trithionates produce the same effect, but more slowly. Acidulated solutions of the mercurous, ferrous, stannous, and antimonious salts, and acid solutions of arsenious acid, likewise decolorize a solution of permanganate rapidly. A solution of this salt constitutes a test-liquid which may be very usefully employed in many cases of volumetric analysis, as already exemplified in the instance of the ores of iron (778).

The permanganates are much more stable than the manganates; their solutions may be boiled without undergoing decomposition. Organic matter, however, combines with part of the oxygen contained in the acid, and reduces it first to manganic acid and then to the binoxide of the metal, which is precipitated

SULPHIDE AND CHLORIDES OF MANGANESE.

643

in flocculi as a hydrate: their solutions, therefore, must not be filtered through paper, but through a funnel loosely plugged with asbestos. When ignited, oxygen is given off, and a manganate is reproduced, which, if the heat be too great, is in turn decomposed with a further extrication of oxygen. Most of the permanganates are freely soluble in water; the permanganate of silver is the least soluble of these salts. If concentrated solutions of permanganate of potassium and nitrate of silver be mixed together, a red crystalline permanganate of silver is deposited. It may be employed for the preparation of the other permanganates; if it be levigated with water, and mixed with a solution of the chloride of the metal of which the permanganate is required, double decomposition occurs, and chloride of silver is formed, whilst the desired permanganate is obtained in solution. In this way the permanganate of barium may be procured, and from it the permanganic acid may be obtained in solution, by the cautious addition of diluted sulphuric acid, so long as any precipitate is produced on evaporation it may be obtained as a brown partially crystalline mass, which is very soluble in water: its solution is decomposed by mere elevation of temperature; at a little beyond 100° F. hydrated peroxide of manganese is deposited, and oxygen gas escapes.

Condy has employed solutions of manganate, and of permanganate of potassium, as disinfecting agents, for which purpose they are admirably adapted: the organic matter is rapidly and completely oxidized by their means, and as the solutions have no corrosive action, solutions of these salts form valuable local applications to fœtid sores.

(802) Protosulphide of manganese (MnS,H,O) is obtained as a yellowish-red hydrate, by precipitating a manganous salt by sulphide of ammonium. The presence of traces of iron, cobalt, or nickel renders it black: it speedily becomes oxidized by exposure to the air. A crystalline sulphide may be obtained in black rhombic prisms, by heating the hydrated sesquioxide in the vapour of bisulphide of carbon. A native sulphide of manganese is occasionally met with, as manganese blende, of a brownish-black or steel-grey colour, and feeble metallic lustre. The other sulphides of manganese have not been accurately examined.

(803) CHLORIDES OF MANGANESE.-Two chlorides of this metal may be obtained: manganous chloride, MnCl,, and manganic chloride, Mn,Cl..

Manganous Chloride (MnCl2,4 H2O=126+72), or Protochloride of Manganese (MnCl,4 Aq=63+36); Sp. Gr. 2.01.-This sub

644

CHLORIDE OF MANGANESE.

stance is obtained abundantly as a waste product in the preparation of chlorine, by acting on the black oxide of the metal: the chlorine escapes, and the chloride of manganese is dissolved.* If this solution be evaporated to dryness, redissolved in water, and about one-fourth of its bulk be precipitated by means of carbonate of sodium, an impure carbonate of manganese is obtained. If this precipitate, after it has been well washed, is boiled with the remainder of the solution, the whole of the iron will be precipitated in the form of peroxide, while oxide of manganese takes its place, and carbonic anhydride is expelled, leaving a solution of chloride of manganese freed from all metallic impurities except cobalt and nickel. A still better process consists in concentrating a solution of the crude chloride by evaporation, to expel the excess of acid, and afterwards diluting with water. A current of sulphuretted hydrogen is transmitted, by which the iron is reduced to the state of ferrous salt; the manganese may then be obtained free from iron, nickel, and cobalt by suspending freshly precipitated sulphide of manganese in water, and adding it to the liquid as long as the fresh portions of sulphide become blackened; the manganese displaces the other metals from their solution, and they are precipitated as black hydrated sulphides: for example, FeCl + MnS‚æĦ ̧Ð =MnCl2+ FeS,H,. On evaporation the chloride of manganese crystallizes in a tabular form with 4 H2O. It is of a delicate pink colour and slightly deliquescent; by heat an anhydrous chloride may be procured, which is soluble in alcohol; from this solution it crystallizes with 4 atoms of alcohol (MnCl2,4,H ̧Ð).

2

Various attempts have been made to economize the vast quantities of chloride of manganese formed during the manufacture of chloride of lime. The most successful is one employed by Mr. Dunlop. It consists in precipitating the manganese as carbonate, and roasting the carbonate at a temperature of about 600°. The crude acid solution of the chloride is treated with milk of lime in quantity sufficient not only to neutralize the excess of acid, but to precipitate the iron as sesquioxide. The clear liquid is then pumped up into an iron boiler and mixed with a proportion of chalk just sufficient to precipitate the whole of the manganese, an excess being carefully avoided. The mixture is heated by the injection of steam, which is maintained for about 24 hours under a pressure of from 2 to 2 atmospheres; under these circumstances carbonate of manganese is precipitated, and chloride of calcium formed in the liquid. The solution is run off from the precipitate, and this is well washed, pressed, and partially dried. In this condition it is introduced in shallow sheet-iron cases on wheels into large vaulted galleries of brickwork heated to about 600°: through these a current of air is maintained. The carbonate is gradually made to pass from the cooler to the hotter portions of these galleries, being from time to time sprinkled with water. The carbonic anhydride is gradually expelled, and oxygen absorbed; so that at the end of 48 hours about 72 per cent. of the carbonate has been converted into black oxide. The process, however, is expensive. (Hofmann's Int. Exhib. Jury Report, 1862, p. 36.)

OTHER SALTS OF MANGANESE.

645

Manganic chloride (Mn,Cl) may be obtained in solution by acting on the sesquioxide of manganese with cold hydrochloric acid it is of a dark brown colour: it must be concentrated by evaporation in vacuo. It is converted by heat into 2 MnCl, +Cl2.

An oxychloride (the perchloride, Mn,Cl,? of Dumas) is obtained by dissolving permanganate of potassium in oil of vitriol and adding fused chloride of sodium, in small portions at a time it is a greenish-yellow gas, which condenses at o° F. to a greenishbrown liquid. The fumes in a moist air assume a purple colour from the formation of permanganic acid: water decomposes it instantly, forming a red solution of permanganic and hydrochloric acids. It is probable that this compound is an oxychloride of the metal, somewhat analogous to chlorochromic acid (789).

Fluorides of manganese, corresponding to each of these chlorides, have been formed.

(804) SULPHATE OF MANGANESE (MnSO,5 H2O=151+90, or MnO,SO,, 5 Aq=75'5+45) is obtained for the use of the calico-printer, by digesting the binoxide in diluted sulphuric acid, in order to remove the carbonates, then heating the oxide with oil of vitriol, evaporating to dryness, and gently igniting the residue to decompose the sulphate of iron, which does not resist so high a temperature as the sulphate of manganese. On digesting the mass, after it has become cool, in water, the sulphate of manganese is dissolved, and may be obtained in crystals by evaporation: it crystallizes below 42° with 7 H, in efflorescent prisms; between 45° and 68° with 5 H,; and between 68° and 86° with 4 H2O (Brandes). It forms a double salt with sulphate of potassium (MnSOK2SO4,6 н,O), which is isomorphous with the corresponding double sulphate of magnesium.

2

The manganic sulphate cannot be obtained in crystals, but it was obtained by Mitscherlich combined with sulphate of potassium crystallized in octohedra (KMn"" 2 SO. 12 H2O), and corresponding in form and composition with common alum.

(805) CARBONATE OF MANGANESE (MnC0, 115, or MnO, CO2=575).—The anhydrous carbonate forms the native manganese spar, and frequently accompanies spathic iron: the artificial carbonate may be obtained as a white hydrate (2 Mn€Ð ̧,H ̧¤) on precipitating the chlorine by a carbonate of one of the alkalies : it becomes brownish by drying.

(8c6) CHARACTERS OF THE SALTS OF MANGANESE.-The salts formed from the protoxide are the only salts of manganese of importance: they are of a delicate rose colour, and have an astringent

646

TESTS FOR MANGANESE.

taste. With the hydrates of potash and soda their solutions yield a white precipitate of hydrated protoxide, which absorbs oxygen rapidly, and becomes brown by exposure to the air. Ammonia gives a similar precipitate, which is soluble in excess of the ammoniacal liquid, especially when it contains chloride of ammonium ; the solution absorbs oxygen quickly, and deposits a brown hydrated protosesquioxide of manganese. The carbonates of the alkali-metals give a white precipitate of carbonate of manganese, soluble in chloride of ammonium. With sulphide of ammonium a characteristic flesh-coloured hydrated sulphide of manganese is formed, which is readily dissolved by hydrochloric and by nitric acid; it becomes brown by exposure to air. Sulphuretted hydrogen gives no precipitate in the solutions of manganese; but a neutral solution of acetate of manganese is partially precipitated by this gas. Ferrocyanide of potassium gives in neutral solutions a white precipitate soluble in acids; in neutral solutions ferricyanide of potassium produces a brown precipitate. Mr. Crum has pointed out an extremely delicate test of the presence of a salt of manganese, provided that the solution is free from chlorides; the liquid must be mixed with diluted nitric acid, and a little peroxide of lead added : on boiling the mixture, the red colour of permanganic acid is produced by a trace of manganese which is too small to be otherwise recognized. Before the blowpipe, when fused on platinum wire or foil, with a little carbonate of sodium, the compounds of manganese give a very characteristic bluish-green opaque bead: a bead of borax or of microcosmic salt becomes violet in the oxidizing flame, if manganese be present; the colour disappears in the reducing flame.

(807) Estimation of Manganese, and Separation from the Alkalies.-Manganese is generally estimated in analysis in the form of the red oxide of manganese, which contains 72.7 per cent. of the metal. For this purpose it is precipitated from a boiling solution of its salts by carbonate of potassium or of sodium; the precipitated carbonate is well washed and then heated to redness, by which carbonic anhydride is expelled, and the red oxide is produced by absorption of oxygen from the air.

Separation of Manganese from the Alkaline Earths.-The solution must be rendered nearly neutral, and sulphide of ammonium added, which precipitates the manganese as sulphide: the sulphide must then be redissolved in hydrochloric acid, precipitated by carbonate of potassium, and the manganese estimated, after ignition, as red oxide. It is apt however to retain some portions of the earths when thus separated. The oxide must therefore be