MANGANESE AND CHROMIUM 529 



5. Permanganates, Mn 2 O 7 , HMn0 4 (hydrated), KMn0 4 . The 

 salts resemble the perchlorates, and are not hydrolyzed by water. 

 They are reddish-purple in color. 



It will be seen that the element manganese changes its char- 

 acter totally with change in valence, and in each form of combi- 

 nation resembles some set of elements of valence identical with 

 that which it has itself assumed. Since the valence represents 

 the number of electrons gained or lost by each atom (p. 217), it 

 is thus evident that the chemical properties of an element depend 

 more upon the electrical constitution of its atom than upon the 

 atomic weight (see p. 553). 



Compounds of Manganese. Manganese dioxide Mn02 

 is an oxidizing agent, and is mixed with black paints to make 

 them " dry " rapidly. For the same reason, and because it is 

 a conductor, it is used as a depolarizer in dry batteries and in the 

 Leclanche cell. It is added to molten glass to remove the green 

 color due to compounds of iron (p. 362). 



Potassium permanganate KMn0 4 crystallizes in needles which 

 have a deep purple color with a greenish luster by reflected light. 

 The purple color of the solution is the color of the ion Mn0 4 ~. 



The solution of potassium permanganate, especially when, by 

 addition of an acid, permanganic acid HMn0 4 has been formed, 

 is an active oxidizing agent. We employed it in oxidizing hydro- 

 chloric acid to make chlorine (p. 141). For the same reason a 

 dilute solution of potassium psrmanganate is used as an antiseptic 

 and disinfectant. 



CHROMIUM Cr 



Ore and Preparation. The chief ore of chromium is 

 chromite FeO,Cr 2 3 , which is mined chiefly in California, Rho- 

 desia and New Caledonia. The U. S. production in 1918, stim- 

 ulated by war requirements, exceeded 80,000 tons. In 1921 it 

 sank to less than 300 tons. 



