Chemistry Reference
In-Depth Information
Determination of the optimal values for the parameters investigated in
the first part of the research, and which will be used as fixed values in the
following sections, resulted in a flow rate of 1.5 l min
-1
, an electrode diame-
ter of 3.0 mm and a nozzle diameter of 2.0 mm. The relatively large nozzle
diameter is used to avoid blocking by fibres. For the same reason, a NES
gap of 2.5 mm is used. A second reason is that in the region of 1-3 mm, the
limiting current is almost independent of the NES-gap distance, therefore
some deviation is allowed without distortion of the electrode signal. An
electrode diameter of 3.0 mm is used to obtain a large signal without the
risk of IR-drop effects at large currents associated with large sodium
dithionite concentrations. For the same reason, a flow rate of 1.5 l min
-1
is
selected. The obtained optimal parameter values resulted in satisfactory
signal-to-noise ratios.
6.7.3 Detection of sodium dithionite and
sulphite concentrations
Linear-sweep experiments recorded in solutions containing sodium dithion-
ite or sulphite were discussed in sections 6.2 and 6.3 using rotating-disc elec-
trodes. With these electrodes, well-shaped limiting-current plateaux were
obtained for the oxidation of dithionite to sulphite, while the second plateau
shows some kinetic influence. This could be explained by changes in elec-
trode surface conditions, which inevitably have an effect on the kinetics of
sulphite oxidation. Despite this disadvantage, it is still possible to measure
sulphite concentrations based on the plateau currents of the second oxida-
tion wave. Therefore, the investigation in this section is started immediately
with a double potential step chronoamperometric experiment in order to
measure the limiting current
I
0.45 V
of sodium dithionite oxidation at a first
applied potential
E
dit
=+0.45 V vs. Ag|AgCl. It will also measure the step
related to the sum of sodium dithionite and sulphite oxidation (
I
0.8 V
) at a
second applied potential
E
sul
=+0.8 V vs. Ag|AgCl using the wall-jet elec-
trode setup. Each potential is alternately applied for 5 s, and the resulting
current signal is measured at the end of each potential step.
In Fig. 6.13, the variation of the limiting currents of the oxidation of
sodium dithionite and sulphite is shown as a function of time during decom-
position of sodium dithionite in an alkaline solution. In the oxidation of
dithionite corresponding to the first potential, two electrons are exchanged,
while at the second potential, six electrons are released. This means that the
current contribution of sulphite (
I
L,sul
) present in solution, also shown in Fig.
6.13, can be calculated as:
I
=
I
-
3
I
[6.26]
sul
08
.
V
045
.
V
