Chemistry Reference
In-Depth Information
H CrO
H HCrO p
+
+
−
K
=
0 74
.
(
ionic strength
=
0 16 25
.
,
°
C
)
(6.5)
2
4
4
a
1
HCrO
−
H CrO
+
+
2
−
p
K
=
5 84
.
(
ionic strength
=
0 5 25
. ,
°
C
)
(6.6)
4
4
a
2
2
HCrO
−
Cr O
2
−
+
H O p
K
= −
1 87
.
(
ionic strength
=
0 5 25
. ,
°
C
).
(6.7)
4
2
7
2
a
3
Overall, the acidity and total concentration of Cr(VI) determine the par-
ticipation of the species. The equilibria and the rate constants of reaction
(6.7) depend on the type of buffer and the concentrations of H
+
and OH
−
ions
[32]. In dilute solution (<10
−3
M), the monomeric form predominates and
HCrO
−
is the principal species in the pH range of 1-5. The dimeric form is
not present in any significant concentration in biological and environmental
conditions. At pH > 8, the yellow ion,
CrO
2−
, is the major species. The relative
proportion of the species depends on both the pH and the total Cr(VI) con-
centration [33].
6.1.1.3 Cr(III) Superoxo and Hydroperoxo Complexes.
The chemistry of
chromium(III) superoxo (Cr
aq
OO
2+
) and hydroperoxo (Cr
aq
OOH
2+
) complexes
have been studied in detail due to their importance in oxygen activation in
industrial and biological processes [34-36]. These complexes were synthesized
from the reaction of Cr(II) with molecular oxygen [37]. The decomposition of
Cr
aq
OO
2+
in acidic solution depends on the pH, and either hemolytic or het-
erolytic steps are responsible for the decomposition of Cr
aq
OO
2+
. This is given
as a scheme in Figure 6.2. A homolysis Cr-O
2
bond is the major step in acidic
solution, which is followed by the reaction between
Cr
a
2+
and Cr
aq
OO
2+
to ulti-
mately produce Cr
3+
and Cr
6+
. under the conditions that the pH is changed,
the conjugate base, (OH)Cr
aq
OO
2+
, dissociates to
Cr
a
3+
, and the free superoxide
disproportionates to O
2
and H
2
O
2
(Fig. 6.2). The products O
2
,
Cr
a
3+
, and Cr(VI)
are also generated by the parallel reactions between Cr
aq
OO
2+
and (OH)
Cr
aq
OO
2+
(Fig. 6.2).
The decay kinetics of Cr
aq
OOH
2+
depends on the concentrations of H
+
and
the hydroperoxo complex. Similar to superoxo complexes, the decay reaction
generates mixtures of O
2
, H
2
O
2
,
Cr
a
3+
, and Cr
6+
[35]. The multiple steps with the
involvement of Cr(IV) and Cr(V) as intermediates of the reactions were sug-
gested. In recent years, substitution of the hydroperoxo group in Cr
aq
OOH
2+
with nitrate was carried out to form a
Cr NO
aq
2+
complex, which may be impor-
tant transients in the biological system and in aqueous atmospheric photo-
chemistry [38].
2
6.1.1.4 Aqueous Cr(IV) Ion.
Acid Medium.
In aqueous solution, Cr
IV
was formed from the oxidation of
Cr
III
by
•
OH and
SO
•−
, produced by pulse radiolysis at pH 3.0 and 3.7 [39].
Initially, a precursor complex is formed, followed by an electron transfer from
Cr
III
to the radical to generate Cr
IV
(reaction 6.8):
Cr
III
+
•
OH/SO
•−
(
Cr OH /SO
III
•
)
•−
→
Cr
IV
.
(6.8)
4
4
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