Chemistry Reference
In-Depth Information
I
Lp
(mA)
8000
7000
6000
5000
S6
4000
S5
3000
S4
2000
c
additives
S3
1000
S2
0
S1
1
2
3
4
5
6
7
c
peroxide
5.2
Influence of increasing concentration of complexing agent
Cottoclarin
®
SV (Henkel) on the current signal of the glassy-
carbon electrode onto which hydrogen peroxide is oxidised, with
E
=
0.45 V vs. Ag|AgCl. Hydrogen peroxide concentration increased
from 0 mol l
-
1
(series 1) to 0.8 mol l
-
1
(series 7); additive concentra-
tion in series S6 is 100 times lower than the lowest concentrations
used in bleaching processes; additive concentration in series S2
corresponds to the minimally used concentration in bleaching
processes.
prewave, causes complete disappearance of the oxidation/reduction peak
of the indanthrene dye. This leads to the conclusion that sterical hindrance
of adsorbed complexing agent additives is responsible for the fact that the
working electrode surface is still accessible, in a limited way, by relatively
smaller molecules, such as hydrogen peroxide.
The adsorption of the additive at the electrode surface is achieved
quickly, since after adding an additive to a solution that contains hydrogen
peroxide, the signal almost immediately decreases and a new stable condi-
tion occurs after a few seconds (Fig. 5.3). The adsorption leading to maximal
shielding of the electrode surface is reached in a few seconds with an addi-
tive concentration that is 100 times lower than the minimal concentrations
used in bleaching processes. An important consequence is that the signal-
lowering effect is constant during the bleaching process, even if the addi-
tive concentrations vary.
A second and third class of additive are the de-foamers and de-aeraters,
respectively, which are always integrated together in one commercial
product. They also display a current-lowering effect, which strongly resem-
bles that of the complex formers. Here as well, the effect occurs quickly and
already reaches a maximum with extremely low additive concentrations.
Everything indicates that here steric hindrance is also responsible for not
