CHROMIUM, MOLYBDENUM, TUNGSTEN, URANIUM, ETC. 811 



is expressed in the following manner : 3K 2 Mn0 4 -f 2H 2 0=2KMn0 4 

 -f MnO 2 + 4KHO. If there is a large proportion of acid and the de- 

 composition is aided by heat, the manganese dioxide and potassium 

 permanganate are also decomposed, with formation of raanganous salt. 

 Exactly the same decomposition as takes place under the action of acids 

 is also accomplished by magnesium sulphate, which reacts in many cases 

 like an acid. When water holding atmospheric oxygen in solution acts 

 on a solution of potassium manganate, the oxygen combines directly 

 with the manganate and forms potassium permanganate, without 

 precipitating manganese dioxide, 2K 2 MnO 4 + O + H 2 O = 2KMnO 4 

 + 2KHO. Thus a solution of potassium manganate undergoes a very 

 characteristic change in colour and passes from green to red ; hence this 

 salt received the name of chameleon mineral. 2 ' 2 



Potassium permanganate, KMnO 4 , crystallises in well-formed, long 

 red prisms with a bright green metallic lustre. In the arts the potash 

 is frequently replaced by soda, and by other alkaline bases, but no salt 

 of permanganic acid crystallises so well as the potassium salt, and 

 therefore this salt is exclusively used in chemical laboratories. One 

 part of the crystalline salt dissolves in 15 parts of water at the ordinary 

 temperature, The solution is of a very deep red colour, which is so 

 intense that it is still clearly observable after being highly diluted with 

 wa'ter. In a solid state it is decomposed by heat, with evolution of- 



23 It was known -to the alchemists by this name, but the true explanation of the> 

 change in colour is due to the researches of Cbevillot, Edwards, Mitscherlich, and 

 Forchhammer. The change in colour of potassium manganate is due to its insta- 

 bility and to its splitting up into two other manganese compounds, a higher and & 

 lower 8MnO 3 = Mn . O 7 + MnO.^. Manganese trioxide is really decomposed in this manner 

 by the action of water (see later) : 8MnO 3 + H 2 O = 2MnHO 4 + MnO 2 (Franke, Thorpe, 

 and Humbly). The instability of the salt is proved by the fact of its being deoxidised by 

 organic matter, with the formation of manganese dioxide .and alkali, so that, for instance, 

 a solution of this salt cannot be filtered through paper. The presence of an excess of 

 alkali increases the stability of the salt ; when heated it breaks up in the presence of 

 water, wi f ,h the evolution of oxygen. 



The method of preparing potassium permanganate will be understood from the above. 

 There are many recipes for preparing this substance, as it is now used in considerable 

 quantities both for technical and laboratory purposes. But in all cases the essence of 

 the methods is one and the same : a mixture of alkali with any oxide of manganese 

 (even manganous hydroxide, which may be obtained from manganous chloride) is first 

 heated in the presence of air or of an oxidising substance (for the sake of rapidity, with 

 potassium chlorate) , the resultant mass is then treated with water and heated, when 

 manganese dioxide is precipitated and potassium permanganate remains in solution. 

 This solution may be boiled, as the liquid will contain free alkali ; but the solution 

 cannot be evaporated to dryness, because a strong solution, as well as the solid salt, ia 

 decomposed by heat. 



By adding a dilute solution of manganous sulphate to a boiling mixture of lead 

 dioxide and dilute nitric acid, the whole of the manganese may be converted into per 

 manganic acid (Crtun) 



