Chemistry Reference
In-Depth Information
0.6
I
(mA)
0.5
0.4
2-5
2-5
1
0.3
1
0.2
1
0.1
0
-0.9
-0.4
0.1
0.6
-0.1
E
(V) vs. Ag
|
AgCl
6.2
Five successively recorded current-potential curves at a platinum
rotating-disc electrode in a solution containing 7.94
¥
10
-
3
mol l
-
1
sulphite at 298.0 K,
N
=
16.67 Hz and pH
=
12.43 after initial
polarisation at
-
0.5 V vs. Ag|AgCl for 5 min. (Reprinted from
Electrochemistry Communications
, Vol 2, No 10, Gasana
et al
.,
'Influence of changes of . . .' pp 727-732, Copyright 2000, with
permission from Elsevier.)
Other particles like S
2
O
6
2-
and S
4
O
6
2-
are not considered, because in the lit-
erature no evidence can be found of them being stable in alkaline solution
31
.
In the second oxidation wave, sulphite is further oxidised according to:
¨
3
2
-
-
4
2
-
-
SO
+
2
OH
S O
+ H O
+
2
e
[6.3]
2
The ratio of the number of electrons totally exchanged in the first and the
second oxidation wave of dithionite is 3.0 for the combination of reactions
1 (2e
-
) and 3 (6e
-
), and 1.5 for reactions 2 (4e
-
) and 3 (2e
-
). Note that for
these ratios, the electrons exchanged in the first oxidation wave are also
taken into account in the second wave. A choice between both hypotheses
can be made because in principle the number of electrons
n
1
and
n
2
trans-
ferred in both waves can be obtained theoretically from the postulated reac-
tion scheme and experimentally from the ratio of the limiting-currents
(
I
L,2
/
I
L,1
) of both waves. An important condition is that the currents
I
L,1
and
I
L,2
are true limiting-currents, which means that the corresponding reaction
rate should be controlled by transport of electroactive species only. This can
be verified by plotting the logarithm of the plateau currents vs. log w. If the
reaction rate is controlled by transport only, a slope of 0.5 must be obtained
according to the Levich equation (Chapter 1, Equation 1.15):
23 16 12
FD nw
/
-
/
/
I
=
062
.
n
A
c
[1.15]
lim
where
I
lim
is the limiting-current (mA),
n
is the number of electrons, F is the
Faraday constant (C mol
-1
),
A
is the surface of the electrode (cm
2
), D is the
diffusion coefficient (cm
2
s
-1
), n is the kinematic viscosity (cm
2
s
-1
), w=2p
N
