Chemistry Reference
In-Depth Information
be positive or negative as the effect of the decrease in electrical current or
the increase in resistance predominates.
A third possibility is based on the presence of small current densities
due to high coverage of the electrode surface. Frequently, current-
potential curves show multiple waves or peaks which cannot immediately
be explained. These are due to more complicated reaction mechanisms
where phenomena such as adsorption/desorption, homogeneous chemical
reactions in solution and slow balances play a role. These phenomena can
strongly depend on the environment and the electrode material. The liter-
ature, in this respect, often refers to pre (after)-waves or pre (after)-peaks,
which can have a considerably lower current density than the normal trans-
port-limited reaction. On condition of proportionality with the bulk con-
centration, these currents are useful for analytical purposes and give
occasion to a lower ohmic voltage drop. With a glassy-carbon electrode,
hydrogen peroxide shows two oxidation waves. The second wave is
transport-controlled and, because of IR-drop or ohmic-drop effects, it is not
suitable for the determination of the high hydrogen-peroxide concentra-
tions in this research. In the literature, nothing was found on the prewave,
probably because it is easily overlooked due to the relatively low current
density. For this reason, the wave is noticed only with relatively high con-
centrations, whereas the majority of published papers deal with the lower-
ing of detection limits and the measuring of very small concentrations.
On the basis of the results obtained in the preliminary research, three
potentially useful electrode configurations were investigated further:
•
ultramicro-electrodes having a diameter of the order of mm;
•
electrodes modified with membranes;
•
the rotating glassy-carbon electrode in consideration of a more pro-
found research of the oxidation prewave.
Various experiments and procedures were performed to investigate the
possibilities using ultramicro-electrodes and membranes to lower the IR-
drop effects and to increase the linearity of the relationship between elec-
trode signal and concentration to higher hydrogen peroxide concentrations.
This led to limited success, inhibited by low accuracy and precision and poor
long-term stability. The third option, based on glassy carbon, showed much
better results and is described more in detail below and in the following
sections of this chapter. Figure 4.3 shows the current-potential curves of
hydrogen peroxide with a glassy-carbon electrode for different hydrogen
peroxide concentrations.
It is possible to distinguish clearly two waves in the oxidative area. The
second wave, occurring at a potential more positive than ca. 0.6 V vs. satu-
rated calomel electrode (SCE), is of no use in attaining this aim, since the
S-shaped character (and the related linear connection between the limiting
