Chemistry Reference
In-Depth Information
0.0025
2
0.0020
0.0015
0.0010
0.0005
1
0
0
200
400
600
800
1000
Time (min)
6.14
Variation of the concentrations of dithionite (curve 1) and
sulphite (curve 2) during dithionite decomposition obtained by
measuring limiting currents at 0.45 and 0.8 V vs. Ag|AgCl at a
platinum wall-jet electrode (data of Fig. 6.13). A flow rate of
1.5 l min
-
1
, a diameter of the electrode of 3.0 mm, a diameter of
the nozzle of 2.0 mm and a NES gap of 2.5 mm were used. The
crosses indicate concentrations determined by titration.
(Reprinted from
Analytica Chimica Acta
, Vol 486, No 1, Gasana
et
al
., 'A wall-jet disc . . .' pp 73-83, Copyright 2003, with permission
from Elsevier.)
formed (Equation 6.1). Detection limits for dithionite and sulphite were
found to be 5 ¥ 10
-5
mol l
-1
, while the upper limit of linearity of the calibra-
tion line was located around 1 ¥ 10
-2
mol l
-1
.
6.7.4
Detection of sodium dithionite, sulphite and indigo
Previous investigations (discussed in section 6.6) demonstrated that indigo
is soluble in water in the reduced form only. Oxidation of the reduced form
at the surface of an electrode inevitably causes deposition of the sparingly
soluble reaction product, even in the presence of sodium dithionite con-
centrations that are common in the textile dyeing industry. The deposition
of indigo at the electrode surface during oxidation cannot be prevented
completely by reduction with dissolved sodium dithionite because of limi-
tations in transport of dithionite towards the electrode surface. Therefore,
the electrode surface should be cleaned in an alternative way after each
measurement of a limiting current by applying a potential where indigo is
reduced electrochemically. Preliminary experiments proved that suitable
potentials for oxidation of leuco-indigo or reduction of indigo in alkaline
solutions are located between -0.7 and -0.3 V and -0.5 and -0.8 V vs.
Ag|AgCl, respectively. However, a fraction of the usable potential range for
