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(a)
Ferrate(VI)
1.0
0.8
FeO
4
2-
HFeO
4
-
0.6
H
3
FeO
4
+
p
K
a2
=
3.5
p
K
a3
=
7.23
p
K
a1
=
1.6
0.4
0.2
H
2
FeO
4
0.0
0.0
2.0
4.0
6.0
8.0
10.0
pH
(b)
Ferrate(V)
1.0
H
2
FeO
4
-
0.8
FeO
4
2-
HFeO
4
2-
H
3
FeO
4
0.6
p
K
a1
=
6.0
p
K
a2
=
7.2
p
K
a3
=
10.1
0.4
0.2
0.0
4.0
6.0
8.0
10.0
12.0
14.0
pH
Figure 6.37.
Speciation of ferrate(VI) (a) and ferrate(V) (b).
that of Gly. If the deprotonated amine moieties of Gly-Gly and Gly determine
the reactivity with Fe(VI), the observed difference in reactivity would be
expected. The maximum reactivity of Gly-Gly is higher than that of Gly with
Fe(VI). The N-terminal of Gly-Gly is less electrophilic than the amino nitro-
gen of Gly, suggesting cause of the observed differences in reactivity.
The pH dependence of the reaction of ferrate(V) with amino acids has also
been studied [382]. The rate-determining step was the two-electron reduction
of ferrate(V) to Fe(III). The subsequent reaction yielded deamination prod-
ucts. The reaction rates as a function of pH showed a bell shape, which resulted
in a maximum between pH 9 and 10 (Fig. 6.39). The maxima are in the pro-
tonation equilibria of both ferrate(V) and amino acids (Tables 6.8 and 6.10).
The speciation of ferrate(V) is shown in Figure 6.37. Based on the species of
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