Chemistry Reference
In-Depth Information
mechanism, where
x
= 1, can correspond exclusively to the situation of an
extremely low hydrogen peroxide concentration, which is in practice also
unreal, because the electrode signals for concentrations smaller than ca.
1 ¥ 10
-4
mol l
-1
become very small and unmeasurable owing to disturbing
factors such as capacitive current, presence of impurities or noise.
Plotting the data presented in Fig. 4.11 in a logarithmic way results in a
linear relation; from this, it is possible to deduce the following relation
between
x
and the hydrogen peroxide concentration:
-
0 0791
.
x
=
0 5071
.
c
[4.57]
-
HO
2
As appears from Equation 4.55, in the factor
G
of Equation 4.56, the
product of a number of equilibrium constants and reaction-rate constants
is elevated to a power in which
x
occurs. The values of these constants are
not known, so that
G
can be determined only experimentally. Figure 4.12
represents the relation between
G
¢ and the hydrogen peroxide concentra-
tion with a working electrode potential of 0.45 V vs. SCE, where
G
¢ was
obtained by using Equation 4.56 from current data for a number of selected
hydrogen peroxide concentrations where pH and
x
are known. From a com-
bination of the data of Fig. 4.11 and Fig. 4.12, the relation between
G
¢ and
x
follows, with
E
= 0.45 V vs. SCE, represented in Fig. 4.13. In general, this
relation can be written as follows:
G
¢=
93
.
¥
10
56607
x
.
[4.58]
Equations 4.55 and 4.56, combined with the knowledge of the relations
between
G
¢ and
x
, respectively, and the hydrogen peroxide concentration,
90
000
80
000
70
000
60
000
50
000
40
000
30
000
20
000
10
000
0
0
0.1
0.2
0.3
0.4
0.5
Concentration (mol l
-1
)
4.12
Relation between the factor
G
and the hydrogen peroxide con-
centration;
E
=
0.45 V vs. SCE.
