CHAPTER XLV 



MANGANESE AND CHROMIUM 



THE first metallic elements we considered form simple positive 

 ions (e.g. Na+, C&++) only. The last two, on the other hand, 

 appear almost exclusively in complex, negative ions, just as do 

 the non-metallic elements (AuCl 4 ~, Au(CN) 2 ~, PtCl 6 = ). Some of 

 the intermediate metals can give negative ions (e.g. Zn0 2 = , AlO^), 

 although in most of their compounds they are positive (Zn++, 

 A1+++). The two elements taken up in the present chapter are 

 equally familiar in both roles. 



MANGANESE Mn 



Ore and Preparation. The commonest ore of manganese is 

 pyrolusite Mn0 2 , a soft, black mineral. The metal is obtained in 

 pure form by mixing the pulverized dioxide with aluminium 

 (Goldschmidt's process, p, 468) in a clay crucible and starting the 

 reaction with magnesium: 



3MnO 2 + 4A1 - 3Mn + 2A1 2 O 3 . 



At the high temperature (over 3000) the molten manganese 

 sinks to the bottom, and the alumina floats above it. 



Properties and Uses. Manganese is hard and crystalline, 

 with a greyish-white luster. It is tarnished superficially by moist 

 air. In fine powder it slowly displaces hydrogen from boiling 

 water. It acts vigorously on dilute hydrochloric and sulphuric 

 acids, giving manganous salts : 



Mn + 2H+ - Mn++ + H 2 1 



The preparation and uses of spiegeleisen and ferro-manganese 

 (p. 488), and of manganese steel (p. 493) have already been de- 



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