388 THE TRANSITION ELEMENTS
This ion is derived from manganese(IV) chloride, MnCl 4 , but the
latter has not been isolated. The MnCl^" ion is unstable, breaking
down to give chlorine thus :
[MnClJ^2 "^ Mn2+ + 4CP + C1 2 T
Hence, under ordinary conditions, manganese(IV) oxide oxidises
concentrated hydrochloric acid to chlorine, but the above shows
that the oxidation process is essentially :
Mnlv + 2e~ -> Mn°
An oxidation which can be used to estimate the amount of man-
ganese(IV) oxide in a sample of pyrolusite is that of ethanedioic acid :MnO 2 + (COOH) 2 + H 2 SO 4 -> MnSO 4 + 2CO 2 T + 2Hf OExcess standard acid is added, and the excess (after disappearance
of the solid oxide) is estimated by titration with standard potassium
manganate(VII).
Alternatively, a known weight of the pyrolusite may be heated
with concentrated hydrochloric acid and the chlorine evolved passed
into potassium iodide solution. The iodine liberated is titrated with
sodium thiosulphate :
MnO 2 =Cl 2 =I 2 =2S 2 Or
Manganese(IV) oxide is used as a depolariser in Leclanche cells
(the cells used in ordinary batteries), as a glaze for pottery and as a
decoloriser for glass. The decolorising action occurs because the
manganese(IV) oxide oxidises green iron(II) silicates to the less
evident iron(III) compounds; hence the one-time name of "glass-
maker's soap' and also "pyrolusite' (Greek pur and lusis, dissolution
by fire).
Although the complex ion [MnCl 6 ]^2 " is unstable, salts such as
K 2 [MnF 6 ] (containing the octahedral hexafluoromanganate(IV)
ion) are much more stable and can be crystallised from solution.
Oxidation state + 3
This state is unstable with respect to disproportionation in aqueous
solution :
2H 2 O -> Mn^2 + (aq) + MnO 2 + 4H +
However the Mn3+(aq) ion can be stabilised by using acid solutions
or by complex formation ; it can be prepared by electrolytic oxidation
of manganese(II) solutions. The alum CaMn(SO 4 ) 2. 12H 2 O contains