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-H
+
Cr
III
aq
OO
2+
Cr
III
aq
OO
2+
(OH)Cr
III
aq
OO
+
Cr-products + O
2
+H
+
H
2
O
(OH)Cr
III
aq
2+
+ O
2
•
-
Cr
III
aq
2+
+ O
2
Cr
III
aq
OO
2+
Cr
III
aq
OO
2+
, H
+
Cr
III
aq
3+
, HCrO
4
-
Cr
III
aq
OOH
2+
+ O
2
Cr
aq
(III) + H
2
O
2
Figure 6.2.
Decomposition of Cr
aq
OO
2+
in acidic solutions (adapted from Bakac [35]
with permission from Elsevier Inc.).
The rate constants of the electron transfer steps are of the order of 10
6
and
10
4
/s for
•
OH and
SO
•−
, respectively. The spectrum of Cr
IV
showed a weak
rising band between 420 and 250 nm, which had an increase in the molar
absorption coefficient from 4.3 to 48.0/M/cm [39]. The final product was Cr
VI
.
A mechanism was proposed in which the first step was the formation of Cr
III
and Cr
V
(reaction 6.9). This step was followed by the rearrangement of the
coordination shell of Cr
V
from octahedral to tetrahedral (reaction 6.10). The
reaction between Cr
V
and Cr
IV
yielded Cr
VI
(reaction 6.11):
IV
IV
III
oc
V
Cr
+
Cr
→
Cr
+
Cr
slow
(6.9)
oct
oct
oct
Cr
V
Cr
V
fast
(6.10)
oct
tet
Cr
V
+
Cr
IV
→
Cr
VI
+
Cr
III
fast
.
(6.11)
tet
oct
tet
oct
The aquachromyl(IV) ion, Cr
aq
O
2+
, had also been produced from the reac-
tion of
Cr
a
2+
with oxygen in dilute perchloric acid [40]. This study suggested a
formation of a precursor before the formation of Cr
III
and Cr
V
. The cleavage
of the O-H bond to yield the product was confirmed by a large solvent kinetic
effect (
k
H
/
k
D
= 6.9). The additional steps were the same as proposed earlier
(reactions 6.10 and 6.11) [39].
Alkaline Medium.
Cr
IV
was obtained in the pulse radiolysis of solution of Cr
III
in 0.5 M NaOH [30]. Aging of Cr
III
solution formed both monomeric and
dimeric species (reactions 6.1-6.4), and the pulse radiolysis of these species
results in Cr
IV
species (reactions 6.12 and 6.13):
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