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taneously operational with all the hydrogen peroxide concentrations inves-
tigated, that these mechanisms are competing with each other, and that the
relative fraction of each mechanism in the global reaction depends on the
hydrogen peroxide concentration. If one supposes for the time being that
the number of operational mechanisms equals two, this hypothesis implies
that the variation of the hydrogen peroxide concentration involves a tran-
sition in the relative predominance of one mechanism in relation to the
other. With sufficiently high or low hydrogen peroxide concentrations,
respectively, this should, in principle, lead to a situation where one or the
other mechanism, respectively, has only a negligible influence.
When examining Fig. 4.9 and Fig. 4.10, it seems that these extreme situa-
tions have not yet been reached, neither with the highest nor with the lowest
hydrogen peroxide concentrations. Hence, the search for a global reaction
mechanism becomes rather complicated and potentially time-consuming: it
is not only necessary to combine two mechanisms, but, moreover, little
support is found in the reaction orders of hydrogen peroxide and OH
-
in
both submechanisms. For hydrogen peroxide, the experimentally obtained
orders vary between 0.49 (highest pH) and 1.22 (lowest pH) with the lowest
hydrogen peroxide concentrations applied, and between 1.05 (highest pH)
and 1.28 (lowest pH) with the highest hydrogen peroxide concentrations
applied. For the OH
-
ions, reaction orders of ca. 0.54 and 0.30, respectively,
are obtained. For the sake of clarity, it should be emphasised that what we
call 'experimentally obtained orders' are apparent, and there are no real
reaction orders when, as is mentioned above, two reaction mechanisms are
operational. Hence mechanisms belonging to mechanism pairs postulated
in the framework of the above hypothesis need to imply reaction orders
that are, respectively, higher or lower than the experimental limit values
mentioned earlier in this paragraph. In addition, both of them should lead
to a transfer coefficient of 0.5 within the potential range of ca.
E
=-0.10 V
to
E
= 0.20 V vs. SCE.
There is no routine procedure for determining hypothetical reaction
mechanisms. Normally, one starts with the obvious, simple sub-stage reac-
tion sequences which should contain the components that are known to
interfere in the overall reaction. In this research, it appeared totally impos-
sible to explain the experimentally observed relations if only the compo-
nents already mentioned (H
2
O, H
2
O
2
or HO
2
-
,O
2
and OH
-
) were to occur
in the reactions of the postulated mechanism pairs. In such a case, possible
intermediates need to be inserted. Hydrogen peroxide is an intermediate
product itself in the oxygen-water system in an alkaline environment.
Together with the hydrogen system, these are largely the most investigated
redox systems. Hence, an extensive literature is available in order to verify
which of the possible intermediates show sufficient evidence. Table 4.1 pre-
sents a number of particles which display a considerable evidence for their
