Chemistry Reference
In-Depth Information
oughly investigated and incorporated in the relation for the pseudo-
limiting-current of the prewave (Equation 5.2).
Organic additives
Organic additives, as described in Chapter 4, section 4.2, are divided into
classes according to their role in the bleaching process. Several additives
per class were investigated, and the first type of organic additive to be
studied was the class of complexing agents. Complexing agents cause the
inactivation of metal ions and prevent the associated damage of the cata-
lyst. Although these additives did not appear to have electroactive proper-
ties, they do interfere in the potential area of the prewave with the hydrogen
peroxide signal by drastically lowering the pseudo-limiting-current. This
presumably means that adsorption occurs of one or more components of
the additives at the glassy-carbon surface, leading to a decrease of the free
surface and a lowering of the current. To verify whether this effect is con-
nected with the nature of the electroactive component, it was examined
whether the additives also influence the reaction of components other than
hydrogen peroxide. Experiments based on the hexacyano ferrate system
(K
4
[Fe(CN)
6
]/K
3
[Fe(CN
6
)]), displayed an identical signal-lowering effect
both for the oxidation reaction and the reduction reaction. This leads us to
conclude that the current-lowering effect of this class of additives is very
likely to be ascribed to a decrease of the active free surface of the working
electrode.
Characteristic for the commercial complexing agents investigated in this
work is the fact that the maximum current-lowering effect is already
attained in the presence of additive concentrations that are much lower
than those used in practice, and this is illustrated in Fig. 5.2. The manufac-
turers of the additive mixtures all refused to communicate the identity and
concentration of the complexing agent in the mixtures. In the case of what
is here called in short 'adsorption of an additive', it most likely concerns
complex adsorption equilibria, where synergist effects between the differ-
ent components can play a part.
The fact that the electrode surface is not completely blocked by adsorp-
tion of complexing agent can be explained by means of the hypothesis of
sterical hindrance: the complexing agents are relatively large organic mol-
ecules, and substances that are already adsorbed can prevent further
adsorption. Smaller components, e.g. hydrogen peroxide, can still penetrate
through the adsorbed layer towards the surface of the electrode. To verify
this hypothesis, it was checked how the oxidation of a larger molecule,
notably an indanthrene dye, is influenced by the presence of complexing
agents. The relatively low complexing agent concentration, which is enough
to obtain the maximum current-lowering effect on the hydrogen peroxide
