Chemistry Reference
In-Depth Information
E
(V) vs. Ag
|
AgCl
-3.5
-0.35
-0.3
-0.25
-0.2
-0.15
-0.1
-0.05
0
-3.7
-3.9
-4.1
-4.3
-4.5
8
-4.7
-4.9
-5.1
1
-5.3
-5.5
6.8
Relationship between potential and logarithm of the kinetic
current for the oxidation of sodium dithionite at the surface of a
platinum-disc electrode in alkaline solution (pH
=
12.5) at a rotation
rate of 1000 rpm. Sodium dithionite concentrations are (1)
6.0
¥
10
-
4
, (2) 1.2
¥
10
-
3
, (3) 1.8
¥
10
-
3
, (4) 2.4
¥
10
-
3
, (5) 3.6
¥
10
-
3
,
(6) 4.8
¥
10
-
3
, (7) 6.0
¥
10
-
3
, (8) 7.2
¥
10
-
3
mol l
-
1
. (Reprinted from
Journal of Electroanalytical Chemistry
, Vol 553, Gasana
et al
.,
'Kinetics and mechanism of . . .', pp 35-42, Copyright 2003, with
permission from Elsevier.)
with
n
a
the number of electrons exchanged in the rate-determining step. It
is assumed here that no more than one electron is transferred in each of
the elementary steps in the oxidation of sodium dithionite. The results
obtained are summarised in Table 6.1. A second method is based on the shift
of peak potential as a function of polarisation rate that obeys the follow-
ing equation
40,41
recorded at stationary electrodes:
-=
(
)
(
)
EE
30 a
n
log
vv
[6.6]
j
i
a
j
i
where
E
is the peak potential and
v
is the polarisation rate of curves i and
j. Numerical data are shown in Table 6.2. Tables 6.1 and 6.2 show the analy-
sis of the data, and values for the charge-transfer coefficient are obtained
with a=0.46 ± 0.04 if
n
a
is assumed to be 1. A final method, based on the
currents obtained in the second potential region of the voltammetric wave,
cannot be used at this stage of the analysis, because correction of the exper-
imental current for transport effects necessitates knowledge of the order of
the electron-transfer reaction in respect to sodium dithionite, which is not
yet the case. However, the data obtained in the first potential region allow
