Chemistry Reference
In-Depth Information
Originally, attempts were made to replace the manual titration
20
by an
automatic titration unit where the equivalence point was no longer deter-
mined by visual colour changes but via a potential change at redox elec-
trodes. The MnO
4
-
/Mn
2+
system, as well as the Ce
4+
/Ce
3+
system
21-23
, was
investigated as a titration reagent. The most serious drawbacks, which
explain why this method is seldom applied in industry, are the relatively
high investment costs for such an automatic system and the excessively long
response times (of the order of minutes). Moreover, cerium (IV) is a rather
expensive reagent. The arsenite system (AsO
2
-
) was also investigated.
However, this method was more used for bleaching processes on the basis
of chlorite and hypochlorite.
Calorimetric, potentiometric
24
and conductometric
25
methods of deter-
mination appeared to be insufficiently precise and/or accurate. However,
the sensitive spectrophotometric determination was discontinuous with
relatively long response times (a few minutes). A continuously measuring
variant of this method, based on the flow injection analysis (FIA) principle,
did have a short response time (a few seconds, depending on the length of
the delivery pipe), but a lower precision and stability, due to the frequent
congestion of the small pipes through which the bath solution is pumped.
Furthermore, the purchase of a spectrophotometric apparatus appeared
to be relatively expensive. Research groups (in Germany
26-27
and the
Netherlands
25
) have recently worked on methods to measure hydrogen
peroxide in a continuous way.
4.4
Voltammetric behaviour
In the development of an amperometric sensor, voltammetric research is
of utmost importance. In literature, much information can be found on the
voltammetric behaviour of hydrogen peroxide with numerous electrode
materials. This behaviour can be divided into two large groups, namely oxi-
dation and reduction of hydrogen peroxide. The largest group of publica-
tions deals with the reduction on platinum
28-33
, palladium
34
, gold
35-37
,
silver
38-43
, several carbon forms
35,39,44
, iron
45-47
, and others
48-51
, and this in an
acid as well as an alkaline medium. The fact that more attention is paid to
the reduction reaction is partly due to the fact that hydrogen peroxide is
formed as an intermediate product in the oxygen reduction
52-56
.The latter
is one of the most studied electrode reactions, amongst others, because of
the importance in corrosion and in research regarding batteries
57
. In this
research, no attention was paid to the oxygen reduction itself. The oxida-
tion reaction of hydrogen peroxide was studied to a more limited extent
58-60
.
Since hydrogen peroxide can decompose faster under the influence of
catalysts, this so-called catalyst decomposition was also studied by means
of voltammetric methods applied to the oxidation and reduction reactions
