Chemistry Reference
In-Depth Information
Gly MnO
: 2
−
+
3
NH CH COO
+
−
+
2
H
+
→
2
MnO
+
3
NH
+
3
HCHO
4
3
2
2
3
+
3
CO H O
+
2
2
(6.68)
Ala MnO
:
2
−
+
5
CH CH NH COOH
(
)
+
11
H
+
4
3
2
(6.69)
→
2
Mn
2
+
+
5
CH CHO NH
+
5
+
+
5
CO
+ H O
2
3
3
4
2
Ser MnO
:
2
−
+
3
OHCH CH NH COOH H
(
)
+
2
+
4
2
2
.
(6.70)
→
2
MnO
+
3
OHCH CHO NH
+
3
+
3
CO
+ H O
2
2
2
3
2
The kinetics results of the reactions between Mn(VII) and amino acids
showed an autocatalytic pattern. The formation of either the Mn(II) or Mn(III)
complex involving the amino acid as a ligand was proposed to understand the
autocatalytic behavior of the reactions in the acidic medium [239, 245].
However, in a neutral solution, a heterogeneous reaction pathway occurring
on the colloidal MnO
2
, the reduced product of Mn(VII), was proposed to
explain the kinetics of the reaction [240]. The soluble form of colloidal MnO
2
can either be recognized as the reaction product or as the long-lived interme-
diate, which may constitute molecular aggregates with varying nanoparticle
sizes [246].
Recently, a detailed role of MnO
2
colloidal particles in the autocatalytic
reaction pathway in the oxidation of Gly by permanganate in neutral aqueous
solution has been demonstrated [234]. Essentially, the kinetics of the reaction
in phosphate solutions under various conditions was monitored at 526 and
418 nm for
MnO
−
and colloidal MnO
2
species, respectively, to gain information
on different steps of the mechanisms. Significantly, long-lived intermediates
such as Mn(VI) and Mn(V) were not observed, although it is a possibility they
were present in insignificant steady-state concentrations. The rate law was
deduced from the plot of the belled-shaped rate (
v
) versus time (
t
) (Eq. 6.71):
v k c k c c
=
+
2
(
−
c
).
(6.71)
1
o
where
k
1
and
k
2
are pseudo-first-order and pseudo-second-order rate constants
of the noncatalytic and autocatalytic reaction pathways, respectively, in which
Gly was present in excess. The rate law assumed both the noncatalytic and the
autocatalytic reaction pathways. Reactions were first order with respect to
[
MnO
−
] and first order with respect to the colloidal MnO
2
surface for the
noncatalytic and the autocatalytic reaction pathways, respectively. Values of
k
1
and
k
2
were determined using integrated (Eq. 6.72) and differential (Eq. 6.73)
methods:
ln[(
k
+
k c
(
−
c
))
/
c
]
=
ln(
k c
/
)
+
(
k
+
k c t
)
(6.72)
1
2
o
1
o
1
2
o
v c
/
=
(
k
+
k c
)
−
k c
.
(6.73)
1
2
o
2
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