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.
It decomposes when heated to
200°-240° C. : 2KMnO4 = K2MnO4 + MnO2 + O2;
and when warmed with hydrochloric acid it yields chlorine:
2KMnO4 + 16HCl = 2KCl + 2MnCl2 + 8H2O + 5Cl2.
Sodium Permanganate, NaMnO4.3H2O (?), may be prepared in a similar manner, or by precipitating the silver salt with sodium chloride. It crystallizes with great difficulty. A solution of the crude salt is used as a disinfectant under the name of “Condy’s fluid.”