Chemistry Reference
In-Depth Information
similar though [41]. using the disproportionation rate of Cr
V
as a function of
pH as well as the conductivity measurements, the following sequence of reac-
tions were proposed for the decay of Cr
V
(reactions 6.16-6.20):
HCr O
VI
−
+
CO
•−
→
HCr O
V
2
−
+
CO
(6.16)
4
(
tet
)
2
2
4
(
tet
)
HCr O
V
2
−
+
H
+
H Cr O
V
−
K
=
6 0 10
.
×
3
(6.17)
2
4
17
4
(
tet
)
V
−
+
V
2
(6.18)
H Cr O H
+
H Cr O
K
=
5 6 10
.
×
2
4
3
4
(
oct
)
18
HCr O
V
2
−
+
H Cr O
V
→
HCr O
VI
−
+
H Cr O
V
−
(6.19)
3
4
(
oct
)
4
(
tet
)
3
4
(
oct
)
4
(
tet
)
HCr O
V
2
−
H CrO
+
+
3
−
K
=
1 0 10
.
×
−
7
.
(6.20)
4
20
4
(
tet
)
The speciation of Cr
V
as a function of pH in dilute aqueous solution using
the constants of protonic equilibria of reactions (6.17), (6.18), and (6.20) is
shown in Figure 6.4. At a neutral pH, both monoprotonated and deprotonated
species of Cr
V
coexist.
6.1.2 Chromium(VI, V, and IV) Complexes
In addition to the oxo complexes of Cr(VI), there are other simple complexes
such as CrO
3
X (
X ClO
4
,
HSO
−
, and HaI
−
[halogenation]) [25, 42]. The reac-
tions between Cr(VI) and glutathione (GSH) and smaller thiolate reductants
form complex species [43]. For example, the reaction between K
2
CrO
7
and
=
−
1.0
H
3
CrO
4
HCrO
4
2-
CrO
4
3-
0.8
H
2
CrO
4
-
0.6
0.4
0.2
0.0
0
1
2
3
4
5
6
7
8
9
pH
Figure 6.4.
Speciation of Cr
V
as a function of pH in dilute aqueous solution (redrawn
from Buxton and Djouider [41]).
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