Chemistry Reference
In-Depth Information
6
Simultaneous detection of indigo and sodium
dithionite for control of dyeing processes
P. WESTBROEK AND P. KIEKENS
6.1
Introduction
Dyeing processes with indigo are carried out all over the world
1-6
. However,
dyeing of textiles with indigo in a reproducible way is complicated. Several
reasons for errors and irreproducible dyeing can be found, including a lack
of quality control during the dyeing itself and also in the preceding textile-
finishing processes. Ab- and adsorption of dyestuff by the textile is strongly
dependent on the nature, source and properties of the fibre and its surface
activity. These properties are determined by the preceding textile-finishing
processes such as bleaching and mercerisation. Optimisation of these
processes can result in a vast improvement of the dyeing process. In this
context, the authors of this topic developed a sensor for the continuous and
online detection and control of hydrogen peroxide concentrations in textile
bleaching baths, as described in references 5-9, in order to improve the
quality and reproducibility of the bleached textile fabric.
Another problem faced by the dyeing industry is the inability to main-
tain the concentration of leuco-indigo (reduced, water-soluble form of
indigo) at a constant level. Indigo is reduced to leuco-indigo by adding a
reducing agent (sodium dithionite)
1,10-12
,so, indirectly, the actual concen-
tration of leuco-indigo is dependent not only on the initial indigo concen-
tration but also on the concentration of reducing agent. Finally, quality
control of the dyed product is scarce, and in about 60% of the companies
in the world it is non-existent
13
. However, even if quality control is per-
formed, it is done at the dyed product, which means that if errors occurred
earlier they cannot be corrected
14-19
.
The aim of an indigo sensor is to keep the leuco-indigo concentration in
the solution at a constant value. In the past, different methods were devel-
oped for detection of the indigo and sodium dithionite concentration, but
up to now with limited success. The sodium dithionite concentration can be
determined by volumetric titration with iodine
20-22
or with K
3
[Fe(CN)
6
]
23
.
The endpoint detection of these titrations can be done visually
22,24-25
or
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