Chemistry Reference
In-Depth Information
10
5
EDTA
10
4
10
3
Gly
10
2
10
1
10
0
10
-1
10
-2
4.0
6.0
8.0
10.0
12.0
pH
Figure 6.35.
Second-order rate constants (
k
, /M/s) as a function of pH at 25°C for the
oxidation of Gly and EDTA by Fe(VI). Solid lines represent best-fitted curves (adapted
from Noorhasan et al. [378] with the permission of Elsvier, Inc.).
The reaction of ferrate(VI) with Gly in the acidic to basic pH range has
been recently performed [378]. Generally, the rates decreased with a decrease
in pH, particularly in alkaline media (Fig. 6.35). However, the reactivity with
Gly showed only a small variation in the rates at pH values between 6.0 and
8.0. Also, an increase was observed with an increase in pH from 4.0 to 6.0. This
dependence is unusual because ferrate(VI) is stronger upon protonation [370,
380]. For example, the oxidation of EDTA had a decrease in rate with an
increase in pH (Fig. 6.35). Significantly, the oxidation rate of EDTA (3
o
amine,
Fig. 6.36) was less than that of Gly in a strong alkaline medium, but the trend
reversed in an acidic medium (Fig. 6.35). The pH dependence is usually related
to the protonation of ferrate(VI) and substrates [381]. The equilibrium reac-
tions of ferrate(VI) and Gly are given in Table 6.8. Based on the dissociation
constants (Table 6.8), the calculated fractions of various ferrate(VI) and Gly
as a function of pH are shown in Figures 6.37 and 6.38.
The pH dependence of
k
for the reaction of Fe(VI) with substrates can be
quantitatively modeled by Equation (6.132):
k
[
Fe VI
(
)]
[ ]
S
= Σ α β
k
[
Fe VI
(
)]
[ ]
S
,
(6.132)
tot
tot
ij
ij
ij
tot
tot
i
= 1 2 3 4
,
,
,
j
= 1 2 3 4 5
,
,
,
,
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