Chemistry Reference
In-Depth Information
probably transport of Cu(II) or Cu(I). It should be noted that the electro-
chemical behaviour of Cu(II) and Cu(I) in styrene is similar to that in
aqueous solutions
11
.
The voltammetric waves observed in Fig. 12.2 can be attributed to four
reactions, partly based on literature results. It was found experimentally
that the reaction product of the reduction waves observed for Cu(II) and
Cu(I) reduction is metallic copper. Therefore the following reactions are
suggested:
Æ
()
+
()
Cu II
e
-
Cu I
[12.2]
Æ
()
+
()
Cu I
e
-
Cu 0
[12.3]
Equation (12.2) starts at a potential around -0.35 V, while Equation 12.3
starts at -0.2 V vs. RE. Indeed in styrene solutions containing CuBr
2
(Fig.
12.2a), the wave starts at -0.35 V vs. RE according to Equation 12.2.
However, in that reaction Cu(I) is formed, which is reduced easily at poten-
tials more negative than -0.2 V vs. RE. Therefore one voltammetric wave
is observed corresponding to the two-electron reduction of Cu(II) to metal-
lic copper. This wave is not initiated at -0.2 V vs. RE, because at that poten-
tial Cu(I) is not yet formed by Equation 12.2. For styrene solutions
containing CuBr (Fig. 12.2b), it is now clear that the wave observed at
potentials starting from -0.2 V vs. RE corresponds to Equation 12.3.
However, at that potential Cu(II) is not yet reduced. In Cu(I) solutions,
some Cu(II) will be present owing to homogeneous oxidation of Cu(I),
which is a relatively unstable compound, and this can indeed be seen in the
voltammetric curves. At potentials more negative than -0.35 V vs. RE, a
slight increase of the limiting-current is observed, which can be attributed
to reduction of Cu(II), present in solution in small quantities.
12.2.3 Analytical considerations
The transport-controlled limiting-current of the reduction waves can be
used for electroanalytical purposes. Figure 12.3 shows a linear relationship
between limiting-current at -0.80 V vs. RE and the Cu(II) (o) and Cu(I) (¥)
concentration. The slope for Cu(II) is twice as high as the one for Cu(I),
which can be explained by the fact that Cu(II) reduction involves the
exchange of two electrons (Equations 12.2 and 12.3), while the Cu(I) reduc-
tion involves only one electron (Equation 12.3). In both cases, a stable back-
ground current of 1.40 ± 0.16 nA was found, which resulted in a detection
limit of 1.4 ¥ 10
-4
and 2.1 ¥ 10
-4
mol l
-1
for Cu(II) and Cu(I), respectively,
taking into account the rule of twice the deviation of the background
current (equal to 0.32 nA).
The first oxidation wave is attributed to the following reaction:
