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E
o
(acidic)
E
o
(basic)
MnO
4
-
0.56
0.56
MnO
4
2-
0.27
0.27
MnO
4
3-
4.27
0.96
MnO
2
0.95
0.15
Mn
3+
1.5
-0.25
Mn
2+
-1.18
-1.56
Mn
Figure 6.15.
Potential diagram of different oxidation states of oxo compounds of Mn.
6.2.1 Aqueous Chemistry of Oxo-Mn Compounds
6.2.1.1 Mn(III).
The oxidation of Mn
2+
by O
3
in acidic solution resulted in
Mn(III) (Fig. 6.16) [178]. The absorption maximum is at 220 nm with a molar
absorptivity of ∼5000/M/cm. A small difference in HClO
4
and H
2
SO
4
solutions
was not significant. A formation of Mn(II)/Mn(III)-
SO
2−
complexes in H
2
SO
4
solutions was suggested. The proposed mechanism is presented in Equations
(6.46) and (6.47). The formation of
•
OH in the reaction steps was ruled out
experimentally. Initially, the manganyl ion (MnO
2+
) was formed, which rapidly
reacted with excess Mn
2+
to result in only Mn(III) as the final product:
Mn
2
+
+ →
O
MnO
2
+
+
O
(6.46)
3
2
2
+
2
+
+
MnO Mn
+
+
2
H
→
Mn III H O
(
)
+
.
(6.47)
2
In a recent detailed mechanistic study of the O
3
-treated Mn
2+
-containing
waters also yielded MnO
2
and
MnO
−
ions [179]. In neutral solution, aqua-Mn
2+
was mainly oxidized to colloidal MnO
2
by O
3
. Comparatively, Mn(III) was
formed in acidic solution (pH 0) when an excess [Mn
2+
] over [O
3
] was present.
However, with low concentrations of Mn
2+
and a large excess of [O
3
] under
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