Chemistry Reference
In-Depth Information
of hydrogen peroxide with the catalyst as electrode material, to clarify the
mechanism of this decomposition
61-68
.Finally, it can be mentioned that
the reduction of hydrogen peroxide was also investigated with semi-
conductors
69-70
. In none of the above-mentioned publications was a study
performed into the relatively high hydrogen peroxide concentrations that
are used in bleaching baths.
4.5
Detection of high hydrogen peroxide
concentrations with amperometric method
4.5.1
Preliminary research
In this part of the work, electrode configurations were searched that qualify
to be submitted to a thorough further investigation with regard to the pos-
sibility to measure high concentrations of hydrogen peroxide in a continu-
ous way. A necessary condition is that the electrode signal is independent
of time. In this respect, the rotating-disc electrode is a suitable configura-
tion because the limiting current is independent of time and well described
by the equation of Levich (Chapter 1, Equation 1.15). This limiting current
is proportional to the bulk concentration of the reacting component.
With numerous electrode materials (Pt, Pd, Ni, Sn, Rh, Ir, Cr, Cu, Au, Ag,
Fe, vitreous carbon, graphite, Re, Ti and Zr), current-potential curves were
registered for 8-10 different hydrogen peroxide concentrations in the full
potential range, limited by the oxygen and hydrogen evolution reactions.
With none of the examined materials did the transport-controlled oxida-
tion or reduction current of hydrogen peroxide appear to qualify for further
investigation. Certain materials have specific disadvantages, such as an
unfavourable position of the wave, interference of reactions from the elec-
trode material itself, an excessively high or excessively low limiting current
plateau and strongly irreversible behaviour of hydrogen peroxide. Never-
theless, besides these disadvantages, which they all demonstrate, for the
oxidation as well as the reduction wave of hydrogen peroxide, it was also
found that the linearity between the limiting current and the hydrogen
peroxide concentration was limited to approximately 15 ¥ 10
-3
mol l
-1
(Fig. 4.2), whereas the concentrations used in bleaching processes can
increase to 2 mol l
-1
.
This effect apparently could be ascribed to the occurrence of the so-called
ohmic potential drop (IR-drop)
71
with concentrations higher than ca.
15 ¥ 10
-3
mol l
-1
.This ohmic potential drop is the product of the electrical
resistance of the cell solution between working electrode and reference
electrode and the electrical current, resulting in a high value of the
last-mentioned parameter in a potential loss that cannot be neglected.
Consequently, the effective potential of the working electrode compared
