Chemistry Reference
In-Depth Information
Cr OH
III
(
)
−
+
O
−
Cr O OH
IV
(
)
2
−
(6.12)
4
4
[(
Cr
)
III III
,
]
+
O
−
[(
Cr
)
III IV
,
] .
(6.13)
z
z
2
2
The decay of Cr
IV
ultimately yielded Cr
VI
species (reactions 6.14 and 6.15).
Significantly, no evidence of Cr
V
as an intermediate was observed. This is in
contrast to the decay of Cr
IV
in acidic solution (see reactions 6.9 and 6.10):
2
Cr
IV
→ (
Cr
)
IV IV
,
(6.14)
2
(
Cr
)
IV IV
,
+
n
(
Cr
III
)
→
Cr
VI
(
-O-Cr
III
)
+
Cr
IV
+
2
Cr
III
.
(6.15)
2
n
6.1.1.5 Cr(V).
The pulse radiolysis of N
2
O-saturated solution containing
Cr
VI
and formate yields Cr
V
[41]. The formate radical produced in the system
reacts rapidly with Cr
VI
to form Cr
V
(Cr
VI
+
•
CO
2
→ Cr
V
;
k
≈ 10
8
/M/s). The
spectra obtained at different pH values are given in Figure 6.3. The spectrum
of Cr
V
did not vary much within the pH range of 1.75-4.75 [41]. However, in
the alkaline pH range, the spectrum had a higher molar absorptivity than in
the acidic pH range (Fig. 6.3) [41]. The spectrum of Cr
V
at pH 8 and 13.7 were
7
100
pH 4.75
pH 3.75
80
6
pH 2.75
60
5
40
pH 8
20
pH 1.75
4
0
300 320 340 360 380 400
λ
(nm)
3
2
pH 6.8
1
p
H 3.75
0
250
300
350
400
λ
(nm)
Figure 6.3.
Spectrum of Cr
V
at pH 3.75, 6.8, and 8.0. The inset shows a detail spectrum
at pH 1.75, 2.75, 3.75, and 4.75 (redrawn from Buxton and Djouider [41]).
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