Chemistry Reference
In-Depth Information
5000
I
(
m
A)
7
4000
6
3000
5
2000
4
1000
3
2
1
0
1-4
-2.0
-1.5
-1.0
-0.5
0
0.5
1.0
1.5
5
6
-1000
E
(V) vs. SCE
7
-2000
4.3
Current-potential curves of hydrogen peroxide with a glassy-
carbon electrode at pH
=
11.45 and
T
=
298.0 K. The concentrations
equal (1) 0.012, (2) 0.025, (3) 0.05, (4) 0.1, (5) 0.2, (6) 0.4 and
(7) 0.8 mol l
-
1
.
current and hydrogen peroxide concentration) disappears with higher con-
centrations owing to the IR-drop effects. This effect remains absent in the
first wave, which occurs at ca. -0.2 V vs. SCE, and this wave is further called
the 'prewave'. A remarkable characteristic of the prewave is that it does not
represent a common limiting-current plateau, since the ideal transport-
limited current, in the presence of sufficient indifferent electrolyte to keep
transport by migration sufficiently low, shows small or no variations with
the potential. At the most positive potentials in the prewave, the inclina-
tion of the current-potential curve is considerably smaller than for more
negative potentials in the wave; however, the current still varies consider-
ably with applied potential. By means of variation of the cell geometry, it
could be determined that this was not due either to IR voltage drop or to
migration, and this was verified by variation of the electrolyte concentra-
tion. The potential range in the prewave, where the inclination of the
current-potential curve is the lowest, cannot be correlated with a classical
limiting current, since it still notably increases with the potential. Discus-
sion of this type of current will frequently return in the following section,
and to avoid mix up with other currents, it is called the pseudo-limiting
current, with symbol
I
Lp
.Figure 4.4 shows a calibration curve of the pseudo-
limiting current measured at a potential of 0.45 V vs. SCE as a function of
the hydrogen peroxide concentration.
In principle, the findings offer the possibility of determining hydrogen
peroxide at sufficiently high concentrations.
However,
the presented
