Chemistry Reference
In-Depth Information
implies that styrene is a poor stabiliser of dissolved ions by solvation. As a
consequence, clusters of electrolyte are formed explaining the much lower
conductivity in styrene than in water.
The results of Fig. 12.1 explain the reason for using ultramicro electrodes
instead of commonly used millimeter-sized electrodes. A solution resistance
that is about 200 times larger than the one observed in water means that
ohmic-drop effects will influence the recorded voltammetric waves at
current signals about 200 times smaller than is the case if the experiment
were to be performed in aqueous solution. However, using an ultramicro
electrode with a diameter of 100 mm reduces the current signal significantly,
allowing voltammetric measurements in the styrene/THAP system. There-
fore, the investigation was continued with ultramicro electrodes, which
explains the presence of limiting-current plateaux instead of peak-shaped
voltammetric waves (except for the anodic dissolution of Cu(0),
as
explained later).
Preliminary experiments were performed to study the (electroactive)
behaviour of styrene containing THAP at a platinum ultramicro disc elec-
trode, which was obtained as described in Chapter 1, pages 21-24. How-
ever, in the range where copper activity is observed (see later) no additional
reactions of the solvent or the electrolyte were observed. Outside this
potential window, oxidation of styrene and reduction of the electrolyte were
observed. As these potential ranges are not interesting for the purpose of
detection of Cu(II) and Cu(I), they are not further described in this chapter.
In Fig. 12.2, current-potential curves are shown which were obtained by
performing cyclic voltammetry at a platinum ultramicro disc electrode in
styrene solutions containing 0.11 mol l
-1
THAP and different concentrations
of CuBr
2
(Fig. 12.2a) and CuBr (Fig. 12.2b). The potential was swept from
0.5 to -1.1 V vs. reference electrode (RE) (forward-sweep direction) and
back to 0.5 V vs. RE (backward-sweep direction) with a sweep rate of
50 mV s
-1
. In both figures, a reduction wave resulting in a limiting-current
plateau is observed in the forward-sweep direction. The starting potential
of this wave is located at -0.35 V vs. RE in solutions containing CuBr
2
and
-0.2 V vs. RE for CuBr-containing solutions. From the formation of a lim-
iting current, it can be concluded that transport of Cu(II) and Cu(I) towards
the electrode surface is rate determining, and it was found that the product
of this reaction is a metallic copper layer deposited at the electrode surface.
In the backward-sweep direction, two waves are observed, for solutions
containing CuBr
2
as well as for those of CuBr. A first wave, resulting in a
limiting-current plateau, is observed in the potential region of -0.4 to -0.15
V vs. RE, while a second, peak-shaped, wave starts at potentials around -
0.15 V vs. RE. The second wave can be attributed to the dissolution of the
copper layer by oxidation because of its peak shape (limited amount of
Cu(0)). The first oxidation wave is determined by steady-state phenomena,
