MANGANESE
Occurrence. Manganese is found in nature chiefly as the dioxide MnO2, called pyrolusite. In smaller amounts it occurs as the oxides Mn2O3 and Mn3O4, and as the carbonate MnCO3. Some iron ores also contain manganese.
Preparation and properties. The element is difficult to prepare in pure condition and has no commercial applications. It can be prepared, however, by reducing the oxide with aluminium powder or by the use of the electric furnace, with carbon as the reducing agent. The metal somewhat resembles iron in appearance, but is harder, less fusible, and more readily acted upon by air and moisture. Acids readily dissolve it, forming manganous salts.
Oxides of manganese. The following oxides of manganese are known: MnO, Mn2O3, Mn3O4, MnO2, and Mn2O7. Only one of these, the dioxide, needs special mention.
Manganese dioxide (pyrolusite) (MnO2). This substance is the most abundant manganese compound found in nature, and is the ore from which all other compounds of manganese are made. It is a hard, brittle, black substance which is valuable as an oxidizing agent. It will be recalled that it is used in the preparation of chlorine and oxygen, in decolorizing glass which contains iron, and in the manufacture of ferromanganese.
Compounds containing manganese as a base-forming element. As has been stated previously, manganese forms two series of salts. The most important of these salts, all of which belong to the manganous series, are the following:
| Manganous chloride | MnCl2·4H2O. |
| Manganous sulphide | MnS. |
| Manganous sulphate | MnSO4·4H2O. |
| Manganous carbonate | MnCO3. |
| Manganous hydroxide | Mn(OH)2. |
The chloride and sulphate may be prepared by heating the dioxide with hydrochloric and sulphuric acids respectively:
MnO2 + 4HCl = MnCl2 + 2H2O + 2Cl,
MnO2 + H2SO4 = MnSO4 + H2O + O.
The sulphide, carbonate, and hydroxide, being insoluble, may be prepared from a solution of the chloride or sulphate by precipitation with the appropriate reagents. Most of the manganous salts are rose colored. They not only have formulas similar to the ferrous salts, but resemble them in many of their chemical properties.
Compounds containing manganese as an acid-forming element. Manganese forms two unstable acids, namely, manganic acid and permanganic acid. While these acids are of little interest, some of their salts, especially the permanganates, are important compounds.
Manganic acid and manganates. When manganese dioxide is fused with an alkali and an oxidizing agent a green compound is formed. The equation, when caustic potash is used, is as follows:
MnO2 + 2KOH + O = K2MnO4 + H2O.
The green compound (K2MnO4) is called potassium manganate, and is a salt of the unstable manganic acid (H2MnO4). The manganates are all very unstable.
Permanganic acid and the permanganates. When carbon dioxide is passed through a solution of a manganate a part of the manganese is changed into manganese dioxide, while the remainder forms a salt of the unstable acid HMnO4, called permanganic acid. The equation is
3K2MnO4 + 2CO2 = MnO2 + 2KMnO4 + 2K2CO3.
Potassium permanganate (KMnO4) crystallizes in purple-black needles and is very soluble in water, forming an intensely purple solution. All other permanganates, as well as permanganic acid itself, give solutions of the same color.
Oxidizing properties of the permanganates. The permanganates are remarkable for their strong oxidizing properties. When used as an oxidizing agent the permanganate is itself reduced, the exact character of the products formed from it depending upon whether the oxidation takes place (1) in an alkaline or neutral solution, or (2) in an acid solution.
1. Oxidation in alkaline or neutral solution. When the solution is either alkaline or neutral the potassium and the manganese of the permanganate are both converted into hydroxides, as shown in the equation
2KMnO4 + 5H2O = 2Mn(OH)4 + 2KOH + 3O.
2. Oxidation in acid solution. When free acid such as sulphuric is present, the potassium and the manganese are both changed into salts of the acid:
2KMnO4 + 3H2SO4 = K2SO4 + 2MnSO4 + 3H2O + 5O.
Under ordinary conditions, however, neither one of these reactions takes place except in the presence of a third substance which is capable of oxidation. The oxygen is not given off in the free state, as the equations show, but is used up in effecting oxidation.
Potassium permanganate is particularly valuable as an oxidizing agent not only because it acts readily either in acid or in alkaline solution, but also because the reaction takes place so easily that often it is not even necessary to heat the solution to secure action. The substance finds many uses in the laboratory, especially in analytical work. It is also used as an antiseptic as well as a disinfectant.