Chemistry Reference
In-Depth Information
4.0
3.5
3.0
5
2.5
4
2.0
3
1.5
2
1.0
1
0.5
-1.8
-1.6
-1.4
-1.2
-1.0
-0.8
-0.6
-0.4
-0.2
Log
c
(mol l
-1
)
4.10
Logarithmic relationship between pseudo-limiting-current of the
prewave at
E
=
0.45 V vs. SCE and the hydrogen peroxide con-
centration for various pH values, recorded at a glassy-carbon
electrode rotating at 16.67 Hz. The numbered curves correspond
to pH values of (1) 10, (2) 11, (3) 12, (4) 13 and (5) 14; other
curves correspond to each increment of 0.2 pH units.
vs. SCE is important for the possible analytical application of the prewave,
we will work with the results obtained with this potential.
A rough qualitative analysis tells us that, for the highest hydrogen per-
oxide concentrations, the slope of all log
I
LP
vs.
c
-curves, obtained for the
different pH values, appears to strive towards the same value of ca. 1.28
with the highest hydrogen peroxide concentrations. With lower hydrogen
peroxide concentrations, the slope varies from 1.22, with the lowest pH
value, to 0.49 at pH = 14.
If the experimental observations are summarised, a constant finding is
the value of 0.5 for the transfer coefficient of the oxidation reaction, with
all the combinations of hydrogen peroxide concentration and pH. Obvi-
ously, this is valid only in the potential range in which this transfer coeffi-
cient was experimentally determined. With potentials outside this range, the
transfer coefficient cannot be used as a criterion. This value of the transfer
coefficient is a primary requirement which every postulated reaction
mechanism should meet theoretically. Furthermore, it is certain that
hydroxide ions interfere in the oxidation reaction.
What is confusing at first sight is the conclusion that the reaction orders
of hydrogen peroxide and of OH
-
have a different value for each hydrogen
peroxide concentration. A possible explanation could be found in the
hypothesis that in the prewave at least two reaction mechanisms are simul-
