Chemistry Reference
In-Depth Information
TABLE 6.8. Dissociation Constants of Ferrate(VI), Ferrate(V), Gly, Glycylglycine,
and EDTA at 25°C
Ferrate(VI)
H
3
FeO
4
+
+ H
2
O
H
+
+ H
2
FeO
4
p
K
a1
= 1.6 [391]
p
K
a2
= 3.5 [392]
p
K
a3
= 7.3 [393]
H FeO H O
+
H HFeO
+
+
−
2
4
2
4
−
+
2
−
HFeO H O
+
H FeO
+
4
2
4
Ferrate(V) [394]
H FeO
H H FeO
+
+
−
5.5≤p
K
a1
≥6.5
p
K
a2
= 7.2
p
K
a3
= 10.1
3
4
2
4
H FeO
−
H HFeO
+
+
2
−
2
4
4
HFeO
2
−
H FeO
+
+
3
−
4
4
Glycine
+
H
3
N-CH
2
-COOH + H
2
O
+
H
3
N-CH
2
-COO
−
+ H
3
O
+
(H
2
Gly
+
)
p
K
1
= 2.3
(HGly)
+
H
3
N-CH
2
-COO
−
+ H
2
O
H
2
N-CH
2
-COO
−
+ H
3
O
+
(HGly)
p
K
2
= 9.6
(Gly
−
)
Glycylglycine
+
H
3
NCONHCH
2
COOH + H
2
O
+
H
3
NCONHCH
2
COO
−
+ H
3
O
+
p
K
1
= 3.1
+
H
3
NCONHCH
2
COO
−
+ H
2
O
H
2
NCONHCH
2
COO
−
+ H
3
O
+
p
K
2
= 8.2
EDTA
H
4
Y + H
2
O
H
3
O
+
+ H
3
Y
−
p
K
3
= 2.0
H
3
Y
−
+ H
2
O
H
3
O
+
+ H
2
Y
2−
p
K
4
= 2.7
H
2
Y
2−
+ H
2
O
H
3
O
+
+ HY
3−
p
K
5
= 6.2
HY
3−
+ H
2
O
H
3
O
+
+ Y
4−
p
K
6
= 10.3
HFeO H N-CH -COO
−
+
−
→
Fe OH
(
)
+
product s
( )
4
2
2
3
(6.135)
k
=
3 0 0 1
. (
±
. )
×
10
4
/M
s
145
FeO
2
−
+
+
H N-CH -COO
−
→
Fe OH
(
)
+
product s
( )
4
3
2
3
(6.136)
k
=
1 5 0 1
. (
±
. )
×
10
2
/
M/s.
146
Reactions (6.135) and (6.136) introduce the proton ambiguity in the reactiv-
ity of ferrate(VI) with Gly. Thus, the experimental values of
k
fit reasonably
well (a solid line in Fig. 6.35) by considering the set of reactions in model I or
model II.
The reactivity of ferrate(VI) with the dipeptide, glycylglycine (Gly-Gly),
has also been studied [378]. The rate law for the oxidation of Gly-Gly was first
order with respect to each reactant in the acidic to alkaline pH range. The rate
of the oxidation of Gly-Gly was slower than that of Gly at pH > 9.0 (Table
6.9), while the trend is reversed at pH < 9.0. The differences observed in the
p
K
a
values of the two substrates may explain the differences in reactivity (see
Table 6.8). The amine moiety of Gly-Gly has a p
K
a
of 8.2, 1.4 units lower than
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