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