Chemistry Reference
In-Depth Information
4
7
6
5
4
3.5
3
2
1
3
2.5
2
1.5
1
0.5
10.0
10.5
11.0
11.5
12.0
12.5
13.0
13.5
14.0
pH
4.9
Logarithmic relationship between pseudo-limiting-current of the
prewave at
E
=
0.45 V vs. SCE and the hydroxide concentration
(pH) for various hydrogen peroxide concentrations, recorded at a
glassy-carbon electrode rotating at 16.67 Hz. The numbered curves
correspond to hydrogen peroxide concentrations of (1) 0.02, (2)
0.05, (3) 0.1, (4) 0.2, (5) 0.3, (6) 0.4 and (7) 0.5 mol l
-
1
.
ide concentration; with the highest pH values (ca. 12.6-14), the slope of the
calibration curves initially decreases with an increasing hydrogen peroxide
concentration, and the concentration range where this applies increases
when the pH is higher. After this initial decrease, the slope of the calibra-
tion curve increases again with higher hydrogen peroxide concentrations.
A third group of curves, with intermediate pH values, can be considered as
a transition between both curves mentioned above.
When the data of Fig. 4.8 (
I
lp
vs.
c
OH
-
) are set out logarithmically, Fig. 4.9
is obtained. Although the relationships are linear, their slope, which one
expects to reflect the reaction order of the OH
-
ions, varies with the hydro-
gen peroxide concentration. The inclination varies from 0.5365 to 0.3035
with hydrogen peroxide concentrations of 0.02 and 0.5 mol l
-1
, respectively.
When the data of Fig. 4.7 are also displayed logarithmically, which results
in Fig. 4.10, no linear relations are obtained.
All the relationships mentioned above were obtained from measurement
of the current in the pseudo-limiting-current range at
E
= 0.45 V vs. SCE
(Figs. 4.7, 4.8, 4.9 and 4.10). They were also found when the current was
measured in the ascending part of the prewave at
E
= 0.15 V vs. SCE, with
the difference that the absolute value of the currents is smaller. An impor-
tant implication here is that the reaction orders, of hydrogen peroxide as
well as hydroxyl ions, are the same in the ascending part of the prewave
and in the pseudo-limiting-current range. Since the potential of
E
= 0.45 V
