Modern inorganic chemistry

386 THE TRANSITION ELEMENTS

oxidation of manganese(IV), by fusion of MnO 2 with potassium

hydroxide, the usual method. This fusion, in air or in the presence

of a solid oxidising agent (KNO 3 ) produces manganate(VI) ( + 4 to

+ 6):

2MnO 2 + 4KOH + O 2 -» 2K 2 MnO 4 + 2H 2 O

The green manganate(YI) is extracted with water, then oxidised to

manganate(VII). This is usually carried out electrolytically, at an

anode, but in the laboratory chlorine may be used :

C1 -* 2MnO + 2C1

(Note that here "chlorine' is oxidising the manganate(VI) to man-
ganate(VII) ; under more acid conditions, the latter oxidises chloride
to chlorine, p, 103).
Potassium manganate(VII) disproportionates on heating :
2KMnO 4 -» K 2 MnO 4 + MnO 2 + O 2

• 7 +6 +4

The manganate(VII) ion slowly oxidises water, the essential reaction
being

4MnO4 + 4H+ -> 4MnO 2 + 2H 2 O + 3O 2

This reaction proceeds very slowly in absence of light, and aqueous
solutions of potassium manganate(VII) are effectively stable for
long periods if kept in dark bottles.
The manganate(VII) ion is one of the more useful oxidising agents ;
in acid solution we have

MnO^aq) + 8H 3 O+ + 5e~ -» Mn2+(aq) + 12H 2 O : E^= + 1.52 V
Hence manganate(VII) is used in acid solution to oxidise, for
example,

Fe(III), NO 2 " -* NOj, H 2 O 2 ^ O 2 ,C 2 Ol" -. 2CO 2

quantitatively; the equivalence point is recognised by persistence
of the purple colour. (Sulphuric acid is used to acidify, since hydro-
chloric acid is oxidised to chlorine, and nitric acid is an oxidising
agent.) Manganate(VII) is also used extensively in organic chemistry.
lor example to oxidise alcohols to aldehydes ; here it may be used
in acid or (more commonly) in alkaline solution, when manganesed V)
oxide is the product :

(aq) 4- 2H 2 O + 3^~ -> MnO 2 (s) + 4OH~ (aq) : E^ = -h 0.59 V