CHKOMIUM, MOLYBDENUM, TUNGSTEN, UKANIUM, ETC. 313 



instance, in the presence of an excess of sulphuric acid, the decom- 

 position is accomplished in the following manner : 2KMn0 4 + 3H 2 SO 4 

 ==K 2 S0 4 + 2MnSO 4 + 3H 2 O + 5O. This decomposition, however, does 

 not proceed directly on mixing a solution of the salt with sulphuric 

 acid, and crystals of the salt even dissolve in oil of vitriol without the 

 evolution of oxygen, and this solution only decomposes by degrees after 

 a certain time. This is due to the fact that sulphuric acid liberates 

 free permanganic acid from the permanganate, 24 which acid is stable 

 in solution. But if, in the presence of acids and a permanganate, there 



24 If sulphuric acid is allowed to act on potassium permanganate without any special 

 precautions, a large amount of oxygen is evolved (it may even explode and inflame), and 

 a violet spray of the decomposing permanganic acid is given off. But if the pure salt 

 (i.e. free from chlorine) be dissolved in pure well-cooled sulphuric acid, without 

 any rise in temperature, a green-coloured liquid settles at the bottom of the vessel. 

 This liquid does not contain any sulphuric acid, and consists of permanganic anhydride, 

 Mn a O 7 (Aschoff, Terreil). It is impossible to prepare. any considerable quantity of the 

 anhydride by this method, as it decomposes with an explosion as it collects, evolving 

 oxygen and leaving red oxide of manganese. Permanganic anhydride, Mn.^O;, in 

 dissolving in sulphuric acid, gives a green solution, which (according to Franke, 1887) con- 

 tains a compound Mn 2 S0 10 = (MnO 3 ) i SO 4 that is, sulphuric acid in which both hydro- 

 gens are replaced by the group MnO 3 , which is combined with OK in permanganate of 

 potassium. This mixture with a small quantity of water gives Mn.^,O 7 , according to the 

 equation: (MnO 3 ) 2 SO 4 -t- H^O = IL^O.^ + Mn 2 7 , and when heated to 80 it gives man- 

 ganese- trioxide, (MnO 5 J2SO 4 + H.jO = 2MnO a + H 2 SO 4 + O. Pure manganese trioxide is 

 obtained if the solution of (Mn0 3 ) 2 SO 4 be poured in drops on to sodium carbonate. Then, 

 together with carbonic anhydride, a spray of manganese trioxide passes over, which 

 may be collected in a well-cooled receiver, and this shows that the reaction proceeds 

 according to the equation . (MnO 3 ) 2 SO 4 + Na^COj - Na.;S0 4 + 2MnO 3 + C0 2 + O (Thorpe). 

 The trioxide is decomposed by water, forming manganese dioxide and a solution of 

 permanganic acid: 8MnO 3 + H ; O = MnO i + 2HMn0 4 . The same acid is obtained by 

 dissolving permanganic anhydride in water. 



Barium permanganate when treated with sulphuric acid gives the same acid. This 

 barium salt may be prepared by the action of barium chloride on the difficultly soluble 

 silver permanganate, AgMnO 4 , which is precipitated on mixing a strong solution of the 

 potassium salt with silver nitrate. The solution of permanganic acid forms a bright red 

 liquid which reflects a dark violet tint. A dilute solution has exactly the same colour 

 as that of the potassium salt. It deposits manganese dioxide when exposed to the action 

 of light, and also when heated above 60, and this proceeds the more rapidly the more 

 dilute the solution. It shows its oxidising properties in many cases, as already 

 mentioned. Even hydrogen gas is absorbed by a solution of permanganic acid ; and 

 charcoal and sulphur are also oxidised by it, as they are by potassium permanganate. 

 This may be taken advantage of in analysing gunpowder, because when it is treated 

 with a solution of potassium permanganate, all the sulphur is converted into sulphuric 

 acid and all the charcoal into carbonic anhydride. Finely-divided platinum immediately 

 decomposes permanganic acid. With potassium iodide it liberates iodine (which may 

 afterwards be oxidised into iodic acid) (Mitscherlich, Fromherz, Aschoff, and others). 

 Ammonia does not form a corresponding salt with free permanganic acid, because it is 

 oxidised with evolution of nitrogen. The oxidising action of permanganic acid in a 

 strong solution may be accompanied by flame and the formation of violet fumes of 

 permanganic acid ; thus a strong solution of it takes fire when brought into contact with 

 paper, alcohol, alkaline sulphides, fats, &c. 



We may add that, according to Franke, 1 part of potassium permanganate with 18 



