Manganese forms several oxides, the most important of which are
manganous oxide, MnO, trimanganese tetroxide, Mn3O4, manganese
sesquioxide, Mn2O3, manganese dioxide, MnO2, manganese trioxide, MnO3,
and manganese heptoxide, Mn3O7.
_Manganous oxide_, MnO, is obtained by heating a mixture of anhydrous
manganese chloride and sodium carbonate with a small quantity of
ammonium chloride (J. v. Liebig and F. Wöhler, _Pogg. Ann._, 1830, 21,
p. 584); or by reducing the higher oxides with hydrogen or carbon
monoxide. It is a dark coloured powder of specific gravity 5.09.
_Manganous hydroxide_, Mn(OH)2, is obtained as a white precipitate on
adding a solution of a caustic alkali to a manganous salt. For the
preparation of the crystalline variety identical with the mineral
pyrochroite (see A. de Schulten, _Comptes rendus_, 1887, 105, p.
1265). It rapidly oxidizes on exposure to air and turns brown, going
ultimately to the sesquioxide. _Trimanganese tetroxide_, Mn3O4, is
produced more or less pure when the other oxides are heated. It may be
obtained crystalline by heating manganese sulphate and potassium
sulphate to a bright red heat (H. Debray, _Comptes rendus_, 1861, 52,
p. 985). It is a reddish-brown powder, which when heated with
hydrochloric acid yields chlorine. _Manganese sesquioxide_, Mn2O3,
found native as the mineral braunite, may be obtained by igniting the
other oxides in a mixture of nitrogen and oxygen, containing not more
than 26% of the latter gas (W. Dittmar, _Jour. Chem. Soc._, 1864, 17,
p. 294). The hydrated form, found native as the mineral manganite, is
produced by the spontaneous oxidation of manganous hydroxide. In the
hydrated condition it is a dark brown powder which readily loses water
at above 100° C., it dissolves in hot nitric acid, giving manganous
nitrate and manganese dioxide: 2MnO(OH) + 2HNO3 = Mn(NO3)2 + MnO2 +
2H2O. _Manganese dioxide_, or pyrolusite (q.v.), MnO2, the most
important oxide, may be prepared by heating crystallized manganous
nitrate until red fumes are given off, decanting the clear liquid, and
heating to 150° to 160° C. for 40 to 60 hours (A. Gorgen, _Bull.
Soc._, 1890 [3], 4, p. 16), or by heating manganese carbonate to 260°
C. in the presence of air and washing the residue with very dilute
cold hydrochloric acid. It is a hard black solid which readily loses
oxygen when strongly heated, leaving a residue of Mn3O4. When heated
with concentrated hydrochloric acid it yields chlorine, and with
concentrated sulphuric acid it yields oxygen. It is reduced to the
monoxide when heated in a current of hydrogen. It is a strong
oxidizing agent. It dissolves in cold concentrated hydrochloric acid,
forming a dark brown solution which probably contains manganic
chloride (see R. J. Meyer, _Zeit. anorg. Chem._, 1899, 22, p. 169; G.
Neumann, _Monats._, 1894, 15, p. 489). It is almost impossible to
prepare a pure hydrated manganese dioxide owing to the readiness with
which it loses oxygen, leaving residues of the type _x_MnO·_y_MnO2.
Such mixtures are obtained by the action of alkaline hypochlorites on
manganous salts, or by suspending manganous carbonate in water and
passing chlorine through the mixture. The solid matter is filtered
off, washed with water, and warmed with 10% nitric acid (A. Gorgen).
It is a dark brown powder, which reddens litmus. Manganese dioxide
combines with other basic oxides to form _manganites_, and on this
property is based the Weldon process for the recovery of manganese
from the waste liquors of the chlorine stills (see CHLORINE). The
manganites are amorphous brown solids, insoluble in water, and
decomposed by hydrochloric acid with the evolution of chlorine.
_Manganese trioxide_, MnO3, is obtained in small quantity as an
unstable deliquescent red solid by dropping a solution of potassium
permanganate in sulphuric acid on to dry sodium carbonate (B. Franke,
_Jour. prak. Chem._, 1887 [2], 36, p. 31). Above 50° C. it decomposes
into the dioxide and oxygen. It dissolves in water forming manganic
acid, H2MnO4. _Manganese heptoxide_, Mn2O7, prepared by adding pure
potassium permanganate to well cooled, concentrated sulphuric acid,
when the oxide separates as a dark oil (H. Aschoff, _Pogg. Ann._,
1860, 111, p. 217), is very unstable, continually giving off oxygen.
It decomposes violently on heating, and explodes in contact with
hydrogen, sulphur, phosphorus, &c. It dissolves in water to form a
deep red solution which contains _permanganic acid_, HMnO4. This acid
is also formed by decomposing barium or lead permanganate with dilute
sulphuric acid. It is only known in aqueous solution. This solution is
of a deep violet-red colour, and is somewhat fluorescent; it
decomposes on exposure to light, or when heated. It is a monobasic
acid, and a very powerful oxidizing agent (M. M. P. Muir, _Jour. Chem.
Soc._, 1907, 91, p. 1485).
_Manganous Salts._--The anhydrous _chloride_, MnCl2, is obtained as a
rose-red crystalline solid by passing hydrochloric acid gas over
manganese carbonate, first in the cold and afterwards at a moderate
red heat. The hydrated chloride, MnCl2·4H2O, is obtained in rose-red
crystals by dissolving the metal or its carbonate in aqueous
hydrochloric acid and concentrating the solution. It may be obtained
in at least two different forms, one isomorphous with NaCl·2H2O, by
concentrating the solution between 15° C. and 20°C.; the other,
isomorphous with FeCl2·4H2O, by slow evaporation of the mother liquors
from the former. It forms double salts with the chlorides of the
alkali metals. The _bromide_ MnBr2·4H2O, _iodide_, MnI2, and
_fluoride_, MnF2, are known.
_Manganous Sulphate_, MnSO4, is prepared by strongly heating a paste
of pyrolusite and concentrated sulphuric acid until acid fumes cease
to be evolved. The ferric and aluminium sulphates present are thus
converted into insoluble basic salts, and the residue yields manganous
sulphate when extracted with water. The salt crystallizes with varying
quantities of water, according to the temperature at which
crystallization is effected: between -4° C. and +6° C. with 7H2O,
between 15° C. and 20° C. with 5H2O, and between 25° C. and 31° C.
with 4H2O. It crystallizes in large pink crystals, the colour of which
is probably due to the presence of a small quantity of manganic
sulphate or of a cobalt sulphate. It combines with the sulphates of
the alkali metals to form double salts.
_Manganous Nitrate_, Mn(NO3)2·6H2O, obtained by dissolving the
carbonate in nitric acid and concentrating the solution, crystallizes
from nitric acid solutions in long colourless needles, which melt at
25.8° C. and boil at 129.5° C. with some decomposition.
_Manganous Carbonate_, MnCO3, found native as manganese spar, may be
prepared as an amorphous powder by heating manganese chloride with
sodium carbonate in a sealed tube to 150° C., or in the hydrated form
as a white flocculent precipitate by adding sodium carbonate to a
manganous salt. In the moist condition it rapidly turns brown on
exposure to air.
_Manganous Sulphide_, MnS, found native as manganese glance, may be
obtained by heating the monoxide or carbonate in a porcelain tube in a
current of carbon bisulphide vapour. R. Schneider (_Pogg. Ann._, 1874,
151, 449) obtained a crystalline variety by melting sulphur with
anhydrous manganous sulphate and dry potassium carbonate, extracting
the residue and drying it in a current of hydrogen. Four sulphides are
known; the red and green are anhydrous, a grey variety contains much
water, whilst the pink is a mixture of the grey and red (J. C. Olsen
and W. S. Rapalje, _Jour. Amer. Chem. Soc._, 1904, 26, p. 1615).
Ammonium sulphide alone gives incomplete precipitation of the
sulphide. In the presence of ammonium salts the precipitate is dirty
white in colour, whilst in the presence of free ammonia it is a buff
colour. This form of the sulphide is readily oxidized when exposed in
the moist condition, and is easily decomposed by dilute mineral acids.
_Manganese Disulphide_, MnS2, found native as hauerite, is formed as a
red coloured powder by heating manganous sulphate with potassium
polysulphide in a sealed tube at 160°-170° C. (H. v. Senarmont, _Jour.
prak. Chem._, 1850, 51, p. 385).
_Manganic Salts._--The sulphate, Mn2(SO4)3, is prepared by gradually
heating at 138° C. a mixture of concentrated sulphuric and manganese
dioxide until the whole becomes of a dark green colour. The excess of
acid is removed by spreading the mass on a porous plate, the residue
stirred for some hours with nitric acid, again spread on a porous
plate, and finally dried quickly at about 130° C. It is a dark green
deliquescent powder which decomposes on heating or on exposure to
moist air. It is readily decomposed by dilute acids. With potassium
sulphate in the presence of sulphuric acid it forms potassium
manganese alum, K2SO4·Mn2(SO4)2·24H2O. A. Piccini (_Zeit. anorg.
Chem._ 1898, 17, p. 355) has also obtained a manganese caesium alum.
_Manganic Fluoride_, MnF3, a solid obtained by the action of fluorine
on manganous chloride, is decomposed by heat into manganous fluoride
and fluorine. By suspending the dioxide in carbon tetrachloride and
passing in hydrochloric acid gas, W. B. Holmes (_Abst. J.C.S._, 1907,
ii., p. 873) obtained a black trichloride and a reddish-brown
tetrachloride.
_Manganese Carbide_, Mn3C, is prepared by heating manganous oxide with
sugar charcoal in an electric furnace, or by fusing manganese chloride
and calcium carbide. Water decomposes it, giving methane and hydrogen
(H. Moissan); Mn3C + 6H2O = 3Mn(OH)2 + CH4 + H2.
_Manganates._--These salts are derived from manganic acid H2MnO4.
Those of the alkali metals are prepared by fusing manganese dioxide
with sodium or potassium hydroxide in the presence of air or of some
oxidizing agent (nitre, potassium chlorate, &c.); MnO2 + 2KHO + O =
K2MnO4 + H2O. In the absence of air the reaction proceeds slightly
differently, some manganese sesquioxide being formed; 3MnO2 + 2KHO =
K2MnO4 + Mn2O3 + H2O. The fused mass has a dark olive-green colour,
and dissolves in a small quantity of cold water to a green solution,
which is, however, only stable in the presence of an excess of alkali.
The green solution is readily converted into a pink one of
permanganate by a large dilution with water, or by passing carbon
dioxide through it: 3K2MnO4 + 2CO2 = 2K2CO3 + 2KMnO4 + MnO2.
_Permanganates_ are the salts of permanganic acid, HMnO4. The
_potassium_ salt, KMnO4, may be prepared by passing chlorine or carbon
dioxide through an aqueous solution of potassium manganate, or by the
electrolytic oxidation of the manganate at the anode [German patent
101710 (1898)]. It crystallizes in dark purple-red prisms, isomorphous
with potassium perchlorate. It acts as a powerful oxidizing agent,
both in acid and alkaline solution; in the first case two molecules
yield five atoms of available oxygen and in the second, three atoms:
2KMnO4 + 3H2SO4 = K2SO4 + 2MnSO4 + 3H2O + 5O;
2KMnO4 + 3H2O = 2MnO2·H2O + 2KHO + 3O.
It completely decomposes hydrogen peroxide in sulphuric acid
solution--
2KMnO4 + 5H2O2 + 3H2SO4 = K2SO4 + 2MnSO4 + 8H2O + 5O2.