Chemistry Reference
In-Depth Information
attention is given to the nature of this kinetic influence. The contribution
of kinetic parameters can also be observed from the curves in Fig. 6.1. For
the second oxidation wave, a weak inclination of the limiting-current in the
limiting-current plateau is observed. For a purely transport-controlled reac-
tion, a potential-independent limiting-current should be obtained as is the
case for the limiting-current of the first oxidation wave.
In a general form, the voltammetric current measured for a system that
is not reversible, which is the case for the dithionite and sulphite reactions
concerned (no return peak is observed), can be expressed by
32-33
:
11 1
II I
=+
KL
[6.4]
where
I
K
and
I
L
are the kinetic current and the limiting-current, respectively.
If the rate of the electrochemical reaction is controlled only by the rate of
electron transfer, which means that
I
L
>>
I
K
, then Equation 6.4 simplifies into
I
=
I
K
. In Fig. 6.1, this situation approximately corresponds with the poten-
tial region from
E
=-0.45 to -0.15 V vs. SCE at the onset of the first oxida-
tion wave of sodium dithionite. However, if the rate of the electrochemical
reaction is controlled only by transport phenomena, then
I
=
I
L
, and this
current is independent of potential.
The relationships between log
I
L
and log w for the first (Fig. 6.3)
and second (Fig. 6.4) dithionite oxidation wave (Fig. 6.1) do not allow
the derivation of the number of electrons exchanged in the electrochemi-
cal reaction from the ratio of the limiting-currents, because the second one,
I
L,2
, is not purely controlled by transport of electroactive species. However,
an indirect method can be used. Extrapolation of the relationship between
I
L,2
/
I
L,1
and the rotation rate of the electrode (w) to very low rotation
rates allows the number of electrons to be obtained. As outlined earlier, it
is presuming that, at these slow rotation rates, the velocity of the second
oxidation reaction is controlled by transport only. The driving force in this
case is that at very slow rotation rates of the electrode, the transport
rate is much slower than the one for charge transfer and becomes rate
determining.
It should be mentioned that this situation is experimentally not accessi-
ble and is therefore performed by an extrapolation method. The value of
I
L,2
/
I
L,1
at these slow rotation rates is equal to the value of
n
2
/
n
1
.The rela-
tionship between the logarithm of log w and log[log(
I
L,2
/
I
L,1
)] was plotted
for various concentrations of sodium dithionite and revealed that all the
curves tend towards a value of -0.33 for log[log(
I
L,2
/
I
L,1
)] at slow rotation
rates (e.g. w=10). This corresponds to a
I
L,2
/
I
L,1
ratio of approximately 3. In
the Levich equation (Chapter 1, Equation 1.15),
I
L
is proportional to
n
;thus
it can be concluded that the combination of Equations (6.1) and (6.3) can
explain the experimentally obtained results.
