Chemistry Reference
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cumvented because the insoluble product formed as a deposited layer in
the forward (oxidative) sweep direction is reduced in the backward-sweep
direction. This means that the formed layer is dissolved again in solution by
reducing the oxidised, initially deposited, layer of insoluble indigo to its
soluble form. Use will be made of this '
in situ
' cleaning property further in
this work (see section 6.7.4).
6.7
Simultaneous detection of sodium dithionite,
sulphite and indigo at a wall-jet electrode
6.7.1
Introduction
In this section, the aim is to explain how the previously discussed results
can be implemented in an industrially acceptable setup, taking into account
parameters such as long-term stability, selectivity, reproducibility, simplic-
ity and low cost. Therefore, it was the intention of the authors not to use
the rotating-disc electrode, but rather to implement a so-called wall-jet elec-
trode, which possesses similar characteristics but with a long-term stability,
simplicity and cost effectiveness that are much more favourable than for a
rotating-disc electrode. The setup of the wall-jet electrode is discussed in
Chapter 1, pages 19-21.
6.7.2
Characterisation of the wall-jet disc electrode
Prior to use for analytical purposes, the developed wall-jet electrode should
first be characterised and calibrated. This is described here, where the wall-
jet disc electrode is optimised by making use of a reversible, one-electron
exchanging, redox system ([Fe(CN)
6
]
4-
/[Fe(CN)
6
]
3-
) in order to obtain the
most favourable conditions for the determination of sodium dithionite, sul-
phite and indigo.
A first parameter that was investigated is the distance between nozzle
and working-electrode surface, further called NES gap. Current-potential
curves showed that, for different NES-gap values, the limiting current of
the [Fe(CN)
6
]
3-
reduction is almost independent of NES-gap value in a
range of 1-6 mm. In addition, this range can be selected as a good working
range because, for smaller NES-gap values, the risk of blocking of the cap-
illary (e.g. by textile fibres) becomes high. Larger NES gaps are, from an
economical point of view, not favourable. Therefore, a NES-gap value of
between 1 and 6 mm is used, with an electrode diameter of 3 mm.
A second parameter that was varied is the flow rate of the solution. This
was done in the same solution as was used in the previous section with the
same nozzle and electrode. The NES gap was fixed at 2.14 mm, the flow rate
