Dyes and Pigments 73 (2007) 47e54

www.elsevier.com/locate/dyepig

Potentiostatic studies on indirect electrochemical reduction of vat dyes

M. Anbu Kulandainathan a,*, A. Muthukumaran a, Kiran Patil b, R.B. Chavan b
a
Electro Organic Division, Central Electrochemical Research Institute, Karaikudi-630 006, India
b
Department of Textile Technology, Indian Institute of Technology, Delhi, Hauz Khas, New Delhi-110 016, India
Received 10 May 2005; received in revised form 12 July 2005; accepted 10 October 2005
Available online 5 December 2005

Abstract

Dispersed vat dyestuffs can be electrochemically reduced by indirect electrolysis using ironetriethanolamine complex as a reducing agent.
The application and mechanism of indirect electrolysis as a reduction technique are described in detail in this paper. Electrochemically reduced
vat dye is tested on a laboratory scale in dyeing experiments, and the results of different reduction conditions are discussed. The influence of the
concentration of the complex-system on the build-up of colour depth, shade and fastness is discussed and compared with samples of the standard
dyeing procedure using sodium dithionite as the reducing agent. The new process offers environmental benefits as well as prospects for improved
process stability, because the state of reduction in the dye-bath can be readily monitored by measuring the reduction potential.
Ó 2005 Elsevier Ltd. All rights reserved.

Keywords: Electrochemical reduction; Vat dyes; Indirect electrolysis; Dyeing; Iron complex

1. Introduction a-hydroxyketone which meets requirements in terms of reduc-

tive efficiency and biodegradability had been tried. However,
Vat dye is most popular among dye classes used for colora- such compounds are expensive and their use is restricted to
tion of cotton, particularly, when high fastness standards are closed systems due to the formation of strong smelling con-
required to light, washing and chlorine bleaching [1]. Also, densation products in an alkaline solution [7,8]. Some other
from the commercial point of view, vat dyes (including indigo) sulphur containing compounds such as hydroxyalkyl sulphi-
have acquired a large share in the dyestuff market for the col- nate, thiourea, etc., have also been recommended [9,10]. These
oration of cellulosic fibres. The annual consumption of vat dyes compounds have relatively low amount of sulphur content and
including indigo is around 33,000 metric tons since 1992 and it also lower equivalent mass which leads to lower sulphur-based
holds 24% of cellulosic fibre dye market in value terms [2]. salt load in the wastewater. However, in these cases too, it is
Vat dyes being water insoluble have to be first converted not possible to dispense with sulphur-based problems totally.
into water soluble form (leuco dye) by reduction with a strong Fe(OH)2 being a strong reducing agent in alkaline medium,
reducing agent like sodium dithionate. In its reduced form, the the possibility of using it has also been explored for reducing
vat dye has substantivity towards cellulosic fibres and, after organic dyestuffs. The reducing effect of Fe(OH)2 increases
absorption, is reoxidised to the original water-insoluble form with increase in pH. However, Fe(OH)2 is poorly soluble in
in situ in the fibre [3,4]. The use of sodium dithionate is being an alkaline solution and gets precipitated. It has to be com-
criticized for the formation of non-environment friendly plexed in order to hold it in solution [11]. A stable complex
decomposition products such as sulphite, sulphate, thiosulphate with good reducing power is obtained with weaker ligand,
and toxic sulphur [5,6]. such as gluconic acid. Regarding eco-friendliness, gluconic
Attempts are being made to replace the sodium dithionite acid can be eliminated in the sewage tank through neutraliza-
by ecologically more attractive alternatives. In this connection tion with alkali; free Fe(OH)2 can be aerated and converted to
Fe(OH)3 which acts as a flocculent and brings down wastewa-
* Corresponding author. Tel.: þ91 4565 227772; fax: þ91 4565 227779. ter load. Tartaric acid has also been tried as a ligand by Chak-
E-mail address: manbu123@yahoo.com (M.A. Kulandainathan). raborty for complexing Fe(OH)2 in the presence of NaOH for

0143-7208/$ - see front matter Ó 2005 Elsevier Ltd. All rights reserved.
doi:10.1016/j.dyepig.2005.10.007
48 M.A. Kulandainathan et al. / Dyes and Pigments 73 (2007) 47e54

reduction and dyeing of cotton with indigo and other vat dyes chemicals. The mediator system necessary for indirect electro-
at room temperature [12]. Efforts have been made to complex chemical reduction of vat dyes was prepared in an alkaline
Fe(OH)2 with single and double ligand systems using tartaric medium from triethanolamine and ferric sulphate, which
acid, citric acid and gluconic acid which have shown encour- were analytical grade chemicals. Potentiometric titrations
aging results [13]. were carried out to measure the dye reduction potential using
Electrochemical reduction of vat, indigo and sulphur dyes is K4Fe(CN)6 as an oxidizing agent which was also an analytical
also suggested as an alternative route for ecological and eco- grade chemical. Dye systems investigated were commercial
nomic reasons. Electrochemical reduction can be achieved by products from Atul Ltd.: Novatic Yellow 5G (CI Vat Yellow
direct and indirect electrochemical reduction. In direct electro- 2), Novinone Brown RRD (CI Vat Brown 5), Novinone Green
chemical reduction chemical reducing agents are replaced by FFB (CI Vat Green 1), Novinone Blue RSN (CI Vat Blue 4),
electrons from electric current, and effluent contaminating sub- Novinone Blue BO (CI Vat Blue 20), Novinone Brown BR
stances can be dispensed with altogether [14,15]. Although this (CI Vat Brown 1), Novinone Black BB (CI Vat Black 36),
technique is ideal, the stability of reduced dye species formed Novinone Brill. Violet RR (CI Vat Violet 1) and Novinone
is poor thereby affecting the colour yield and the rate limiting Black CH.
step of electrochemical reduction is electron transfer from the
cathode surface to the surface of the microcrystal of the dis- 2.2. Dyeing procedure and process control
persed dye pigment [16e20]. In indirect electrochemical re-
duction technique the reduction of dye is achieved through 2.2.1. Conventional dyeing
a redox mediator system [21,22]. This process is mediated by Conventional vat dyed samples were prepared in the labo-
an electron carrier whereby reduction takes place between sur- ratory, with a standard procedure using sodium dithionite
faces of the electrode and the dye pigment instead of direct and sodium hydroxide as described elsewhere [13].
contact between both surfaces. Among the various mediator
systems suggested in the literature, ironetriethanolamine com- 2.3. Electrochemical dyeing
plex (ironeTEA) seems to be promising [23e29]. In order to
meet all requirements in an economical way, a multi-cathode 2.3.1. Design of the electrochemical cell
cell with a large number of cathodes, electrically connected The two-compartment electrochemical cell consists of
with one or two anodes, has been suggested [30,31]. This con- a rectangular polyvinyl chloride vessel containing the capacity
figuration allows the operation of the cell with a maximum area of 1 l catholyte and 0.1 l anolyte and the separator used was
cathode and minimum area anode [32e34]. Very recently, 423 Nafion membrane. The anode used was a thin stainless
Bechtold et al. have demonstrated that mediator system obtain- steel rod with the surface area of 6 cm2. A three-dimensional
able by mixing one or more salts of a metal capable of forming copper wire electrode with a surface area of 500 cm2 served as
a plurality of valence states with at least one amino-containing the cathode. The cathode compartment was thus providing
complexing agent and at least one hydroxyl-containing but porous flow through electrode filling the cathode compartment
amino-devoid complexing agent in an alkaline medium has as coil. The catholyte was agitated by an analytical rotator at
the improved capacity to reduce the vat dyes [35,36]. a constant speed to create homogeneous and stable conditions
Both the electrochemical reduction techniques are not yet in the cathode chamber. The dye-bath of the dyeing apparatus
commercialized and research and developmental efforts are was circulated through the electrochemical reactor for contin-
in progress in this direction. Here, the most challenging engi- uous renewal of the reducing capacity of the dye-bath.
neering task is to achieve a dye reduction rate and a current A schematic drawing of the dye-bath circulation through
efficiency, which are high enough to make electrochemical the laboratory dyeing unit and the catholyte circulation
reaction industrially feasible. through the electrochemical cell is given in Fig. 1.
In the present study attempts are being made to understand
the fundamentals of indirect electrochemical reduction of 2.3.2. Determination of reduction potential of vat dyes
selected vat dyes using ironeTEA complex as a mediator. Here, the reduction potential of the dye is referred as the
Essential requirements for the design of electrochemical cell potential developed on the bright platinum electrode vs the ref-
are suggested. IroneTEAeNaOH molar ratio has been stan- erence electrode when dipped in the dye-bath in which the dye
dardized to get the dyeing of cotton by indirect electrochemi- gets just solubilised. In order to measure the reduction poten-
cal reduction technique. Colour yields are compared with tial of vat dyes, a titration was carried out using bright plati-
those of conventional sodium dithionate method. Repeated num electrode with Hg/HgO/OH
as a reference electrode.
use of dye-bath after dye separation is also explored. A solution of 100 mg vat dye was prepared which was then re-
duced with 25 ml of 0.1 M NaOH and 100 mg Na2S2O4. This
2. Experimental solution was then diluted to 50 ml with distilled water and then
titrated against 0.05 M K4[Fe(CN)6]. The K4[Fe(CN)6] solu-
2.1. Materials tion was added in the steps of 0.5 ml and the dye solution po-
tential was measured at every step. The volume of 0.05 M
The sodium dithionite and sodium hydroxide used for vat K4[Fe(CN)6] solution is then plotted against the dye solution
dyeing by conventional method were laboratory grade potential in order to find out dye reduction potential.
 M.A. Kulandainathan et al. / Dyes and Pigments 73 (2007) 47e54 49

Analytical rotator

+ -

Anode: SS Cathode: Cu Peristaltic

Anolyte: Catholyte: Pump 1 Dye bath Peristaltic
Mediator (Mediator system Pump 2
175 ml/min
100 ml + dye) 1200ml 175 ml/min

Electrochemical
cell

Cu wire to measure solution potential w.r.t. Reference electrode

Reference electrode (Hg / HgO/OH-)

Fig. 1. Schematic representation of the electrochemical flow cell.

2.3.3. Dyeing procedure mediator system should be made as low as possible. High con-
Selected vat dyes were dyed under the standardized condi- centrations of chemicals in the dye-bath enable more stable re-
tions of the mediator system as described in Section 3. While duction conditions and prevent the oxidation of the reduced
following the electrochemical dyeing technique, the dye-bath dyestuff. However, if we use the mediator solution towards
was circulated through the cathodic compartment of the elec- its higher concentration end then there will be an eventual in-
trolytic cell for continuous renewal of the reducing capacity. crease in the loss of chemicals in the recycling loop and also
All the experiments were performed under potentiostatic con- carried along the fabric at the end of dyeing operation. This
ditions (
1050 mV vs Hg/HgO/OH
) at room temperature. makes the process uneconomical.
The catholyte was called as a mediator solution which com-
prised triethanolamine, ferric sulphate and sodium hydroxide. Table 1
The potential prevailing in the catholyte, during electrolysis, Different composition of the mediator solutions used for dyeing experiments
was measured with a copper wire vs a reference electrode Mediator solution pH Mediator solution composition
(Hg/HgO/OH
). The catholyte was agitated in the catholyte Fe2(SO4)3 (M) TEA (M) NaOH (M)
compartment itself and was also circulated through the dye-
Group I
bath continuously. When the potential achieved in the catho- 1.1 14 0.025 0.30 0.357
lyte was equivalent to the reduction potential of the dyestuff, 1.2 14 0.03 0.35 0.434
the dyestuff was introduced into the catholyte compartment. 1.3 14 0.035 0.40 0.500
In all the experiments 2% of weight of fabric dye was used 1.4 14 0.04 0.45 0.570
1.5 14 0.045 0.50 0.643
in the dyeing recipe. The fabric sample was introduced into
the dye-bath after 10 min of introduction of dyestuff in order Group II
to allow reduction of the dyestuff. Experiments were carried 2.1 10.5 0.03 0.35 0.185
2.2 12 0.03 0.35 0.225
out at a relatively large liquor ratio i.e. 240:1. The dyeing 2.3 13 0.03 0.35 0.275
was continued in the dye-bath for 1 h with constant agitation 2.4 14 0.03 0.35 0.315
of the fabric sample with glass rods, while electrolysis and cir- 2.5 14 0.03 0.35 0.375
culation of the catholyte were in progress. After dyeing the 2.6 14 0.03 0.35 0.429
sample was withdrawn from the dye-bath, air oxidized, cold 2.7 14 0.03 0.35 0.480
rinsed, soaped at boil, cold rinsed and air dried. Group III
3.1 14 0.025 0.30 0.43
3.2 14 0.025 0.315 0.43
2.3.4. Optimization of mediator solution 3.3 14 0.025 0.33 0.43
In order to make the indirect electrochemical dyeing tech- 3.4 14 0.025 0.345 0.43
3.5 14 0.025 0.360 0.43
nique eco-friendly and economical, the concentration of the
50 M.A. Kulandainathan et al. / Dyes and Pigments 73 (2007) 47e54

Table 2
Reduction potential of selected vat dyes
Dye Reduction potential (V) Dye Reduction potential (V) Dye Reduction potential (V)
Novatic Yellow 5G
0.818 Novinone Blue RSN
0.880 Novinone Black BB
0.952
Novinone Brown RRD
0.841 Novinone Blue BO
0.903 Novinone Brill. Violet RR
0.953
Novinone Green FFB
0.850 Novinone Brown RR
0.935 Novinone Black CH
0.964
Reduction potential (V vs Hg/HgO/OH
).

Indirect electrochemical dyeing was carried out with the se- 3. Results and discussion
lected dyestuffs at different mediator solution concentrations
in three sets of mediator systems as shown in Table 1. In group 3.1. Reduction potential of vat dyes
I, the ferric sulphate to TEA and ferric sulphate to sodium hy-
droxide molar ratio was kept constant as 1:11.48 and 1:14.3, The reduction potential of vat dyes (Table 2) gives an idea
respectively as described elsewhere [13]. In group II, the ferric about the level of difficulty involved in their reduction. Higher
sulphate to TEA molar ratio was kept constant at 1:11.48 the reduction potential of the dye, higher is the reducing power
while the ferric sulphate to sodium hydroxide molar ratio required in the dye-bath for its reduction. Thus, from the
was varied. Whereas in group III, the ferric sulphate to sodium reduction potential values, we can group the dyes under con-
hydroxide molar ratio was kept constant at 1:14.3 and the fer- sideration in three groups viz. easy to reduce, normal to reduce
ric sulphate to TEA molar ratio was varied. and difficult to reduce.
The mediator solution was prepared according to the proce- While following the indirect electrochemical dyeing tech-
dure described elsewhere [14]. Caustic soda was dissolved in nique, three dyestuffs such as, Green FFB (an easy to reduce
a small amount of water in which TEA was added. Ferric sul- dye), Blue RSN (a normal to reduce dye) and Violet RR (a
phate was separately dissolved in a small amount of water and hard to reduce dye) were selected.
then added to the TEA/caustic soda mixture with continuous
stirring with the magnetic stirrer. After the complete dissolu-
tion of the precipitated iron oxide, the solution was diluted 3.2. Dyeing results
to the full volume. The solution was subjected to constant stir-
ring for 90 min. The rates of potential development while carrying out po-
tentiostatic electrolysis in the group I, II and III dyeing experi-
ments with Violet RR dye only are shown in Figs. 2, 3 and
2.3.5. Material to liquor ratio experiments
4, respectively. In each dyeing experiment, the dye was
As the electrolysis cell was not optimized with regard to
introduced in the cathodic chamber as soon as the reduction
short liquor ratios, experiments were carried out at a relatively
potential of Violet RR had reached i.e.
953 mV. The final
large material to liquor ratio i.e. 1:240. To understand the ef-
dye-bath potential attained at the end of dyeing in few
fect of the various materials to liquor ratios on the colour
cases was lower than that attained during the course of dyeing,
depth developed on the fabric samples by indirect electro-
especially while dyeing with Green FFB and Blue RSN.
chemical dyeing, experiments were also carried out with
1:120, 1:80 and 1:60 material to liquor ratios by making
appropriate modifications in the system.
1000
Solution Potential (-mV vs Hg/HgO/OH-)

2.3.6. Recycling of mediator solution experiments

After completing the dyeing experiment, the remaining dye
and reduced mediator were oxidized by bubbling air though 900
the solution to form an insoluble dye.
The oxidized dye particles can be segregated out through
800
the vacuum filtration using G3 type of porcelain membrane.
The clear filtered out mediator solution was recycled further
to carry out dyeing experiments. 700 Mediator index: 1.1
Mediator index: 1.2
Mediator index: 1.3
2.3.7. Dyed samples evaluation 600 Mediator index: 1.4
Results of the dyeing experiments were characterized by Mediator index: 1.5
colour measurement in the form of K/S and CIELab coordi-
nates with the help of spectrophotometer X4000 (Jaypak) us- 500
-50 0 50 100 150 200 250 300 350
ing D65 light source, 10  viewing angle.
Time (min)
Dyed samples were also evaluated for wash fastness
(SOURCES: IS 764; 1979), light fastness (SOURCE: IS Fig. 2. Dye-bath potential development with varying ferric sulphate concentration
2454; 1985) and rubbing fastness (SOURCE: IS 766; 1988). (group I) electrolysis with Violet RR dye.
 M.A. Kulandainathan et al. / Dyes and Pigments 73 (2007) 47e54 51

Table 3
Mediator index:2.1 Comparison of colour yield and colour coordinates of conventional and
1000
Solution Potential (-mV vs Hg/HgO/OH-)

Mediator index:2.2 electrochemical dyeing with Green FFB

Mediator index:2.3
Mediator solution Final dye-bath potential K/S L* a* b*
900 Mediator index:2.4 index (V vs Hg/HgO/OH
)
Mediator index:2.5
With dithionite* 15.4626 35.44
35.20
0.98
Mediator index:2.6
800 1.1*
0.860 5.1460 55.37
39.92
2.67
Mediator index:2.7 1.2*
0.880 4.7258 53.39
32.58
6.24
1.3*
0.870 5.6865 52.95
38.25
3.72
700 1.4*
0.874 3.3465 58.97
33.46
4.71
1.5*
0.996 4.1874 57.42
36.84
3.27

600 Initial dye-bath potential before starting the electrolysis in each case was

0.350 V vs Hg/HgO/OH
.

500

cell due to the deposition of a block material as a hard layer on

0 50 100 150 200 250 300 350
Time (min)
the cathode. As can be seen from the corresponding K/S values
in Table 4, the colour depth appears to be increasing with an in-
Fig. 3. Dye-bath potential development with varying sodium hydroxide crease in NaOH concentration above pH 13.
concentration (group II) during electrolysis with Violet RR dye. There is no effect of the TEA concentration variation in the
mediator solution in the given range of group III experiments on
the colour depth and shade which is shown in Table 5. Also the
From the variation of the CIELab coordinates from the solution potential development rate is found to be unaffected
group I experiments as shown in Table 3, it becomes clear (Fig. 4). However, at 44.75 g/l TEA, the iron complex becomes
that the colour depth and shade did not show strong depen- unstable and ferric hydroxide is found to be precipitating.
dence on the iron salt concentration in the mediator solution.
However, the depth appears to be good around 12e14 g/l
ferric sulphate concentration. The potential development rate 3.2.1. Influence of MLR on colour yield
appears to be increasing with increase in ferric sulphate All the dyeing experiments for mediator solution optimiza-
concentration as can be seen in Fig. 2. tion were carried out at 1:240 material to liquor ratio. The
In case of group II experiments, the solution potential could depth of the dyed samples was very low as compared to the
not be developed above
953 mV vs Hg/HgO/OH
while conventionally dyed samples which were dyed at 1:30 material
working up to pH 13. Therefore dyeing was not possible during to liquor ratio. In order to investigate the possibilities of im-
working with NaOH concentration below 11 g/l in the mediator proving the colour depth, dyeing experiments were carried
solution. During electrolysis of the solution with pH below 13, out with Blue RSN dye at 1:240, 1:120, 1:80 and 1:60 material
the complex formed was found to be unstable and caused addi- to liquor ratios using 1.2 mediator index solution.
tional problems in the proper functioning of the electrochemical Values of K/S and CIELab coordinates of dyeing experi-
ments are given in Table 6 which show marginal improvement
in colour depth of the dyed samples with shorter material to
liquor ratios.
1000
Solution Potential (-mV vs Hg/HgO/OH-)

3.2.2. Recycling of mediator solution

Dyeing without effluent i.e. dye-bath recycling seems to
900
be very attractive in the present environment conscious era.

800
Mediator index:3.1
Table 4
Mediator index:3.2 Comparison of colour yield and colour coordinates of conventional and
700
Mediator index:3.3 electrochemical dyeing Blue RSN
Mediator index:3.4
Mediator solution Final dye-bath potential K/S L* a* b*
600 Mediator index:3.5
index (V vs Hg/HgO/OH
)
With dithionite 9.1666 35.94 6.62
38.03
500 1.1
0.875 2.6641 52.92
3.29
27.93
1.2
0.900 3.6045 48.44 0.58
32.87
-20 0 20 40 60 80 100 120 140 160
1.3
0.890 2.2319 55.17
2.25
28.24
1.4
0.905 2.5897 52.71 0.23
31.25
Time (min)
1.5
0.983 1.9702 56.10
1.10
27.58
Fig. 4. Dye-bath potential development with varying TEA concentration Initial dye-bath potential before starting the electrolysis in each case was
(group III) during electrolysis with Violet RR dye. around
0.350 V vs Hg/HgO/OH
.
52 M.A. Kulandainathan et al. / Dyes and Pigments 73 (2007) 47e54

Table 5 Table 7
Comparison of colour yield and colour coordinates of conventional and Regeneration of mediator solution
electrochemical dyeing with Violet RR Recycling step Fresh solution Absorbance
Mediator solution Final dye-bath potential K/S L* a* b* addeda (ml) Before oxidation After filtration
index (V vs Hg/HgO/OH
) and filtration
With dithionite 15.73 23.10 16.93
23.13 Original 0 1.384 0.072
1.1
0.922 0.2810 77.92 10.16
9.76
1 30 1.533 0.027
1.2
0.996 3.0499 47.82 22.82
28.25 2 41 1.403 0.050
1.3
0.994 3.2752 47.87 26.70
30.07 3 28 1.417 0.079
1.4
0.979 2.4236 52.75 26.42
27.63 4 42 1.420 0.070
1.5
0.1001 2.9228 49.78 26.78
29.23
a
2.1
0.539 Dyeing not possible Fresh solution was added to make the total volume to 1 l before starting
2.2
0.545 Dyeing not possible each experiment.
2.3
0.584 Dyeing not possible
2.4
0.987 2.0706 54.97 25.45
26.87
2.5
0.993 2.2632 53.53 25.15
26.66
2.6
0.1001 3.1007 48.72 26.54
30.03
Table 8
2.7
0.1001 4.4808 43.71 28.11
30.84
Colour yield and CIELab coordinates for repeated dyeing with regenerated
3.1
0.1002 2.8823 49.71 26.59
30.50
mediator solution
3.2
0.992 3.5124 47.04 27.51
30.79
3.3
0.997 3.9246 45.32 27.49
31.02 Recycling step Final dye-bath potential K/S L* a* b*
3.4
0.1001 3.0499 47.82 22.68
28.25 (V vs Hg/HgO/OH
)
3.5
0.995 3.1660 48.79 26.99
28.83 Original
0.995 2.4954 52.95 0.62
31.06
Initial dye-bath potential before starting the electrolysis in each case was 1
0.965 2.3803 53.82
0.97
29.35
around
0.350 V vs Hg/HgO/OH
. 2
0.950 2.8766 51.25
0.55
30.31
3
0.958 2.6414 52.39
0.39
29.67
4
0.953 3.0783 50.65
1.09
30.50

Table 6 Table 9
Effect of ML ratios on colour yield and CIELab coordinates Comparison of fastness properties of electrochemical and conventional dyeing
MLR Final dye-bath potential K/S L* a* b* Reduction/mediator Washing Light Rubbing fastness
(V vs Hg/HgO/OH
) system fastness fastness
Wet Dry
1:240
0.900 3.6045 48.44 0.58
32.87
With dithionitea 4e5 5 4 5
1:120
0.958 2.9914 50.85
0.72
30.43
With dithioniteb 4e5 5 4 5
1:80
0.950 3.0879 50.49
1.65
29.25
With dithionitec 4e5 5 4 4e5
1:60
0.965 4.0401 46.55
1.01
29.91
0.025 M Fe2(SO4)3 þ 4e5 4e5 4e5 5
0.345 M TEA þ 0.45 M
NaOH (MLR 1:240)a
0.025 M Fe2(SO4)3 þ 4 4e5 4 4e5
0.345 M TEA þ 0.45 M
NaOH (MLR 1:240)b
0.025 M Fe2(SO4)3 þ 4e5 5 4 4e5
0.345 M TEA þ 0.45 M
1000
NaOH (MLR 1:240)c
Solution Potential (-mV vs Hg/HgO/OH-)

0.025 M Fe2(SO4)3 þ 4 4e5 4e5 5

900
0.345 M TEA þ 0.45 M
NaOH (MLR 1:60)a
0.025 M Fe2(SO4)3 þ 4 4e5 4e5 5
800 0.345 M TEA þ 0.45 M
NaOH (MLR 1:60)b
0.025 M Fe2(SO4)3 þ 4 4e5 4e5 5
Mediator Index:1.2
700 0.345 M TEA þ 0.45 M
Original
NaOH (MLR 1:60)c
First Recycle
a
Second Recycle
With Green FFB.
600 b
With Blue RSN.
Third Recycle c
With Violet RR.
Fourth Recycle
500

In order to reuse the mediator solution with different dyes, the

0 20 40 60 80 100 120
residual dye has to be removed from the dye liquor. However,
Time (min)
this is very straightforward especially in case of vat dyes
Fig. 5. Effect of mediator recycling on the dye-bath potential development. which become insoluble in the dye-bath on oxidation. In the
 M.A. Kulandainathan et al. / Dyes and Pigments 73 (2007) 47e54 53

laboratory dyeing experiments, this can be achieved by development of the solution potential as shown by curves in
bubbling air through the dye-bath. Insoluble oxidized dye par- Fig. 5. Thus, reproducibility of the dyeing results was con-
ticles were removed by vacuum filtration with G3 porcelain firmed along with the electrochemical regeneration of the re-
filter. ducing agent.
The mediator solution used for the recycling experiments
was composed of 0.025 M Fe2(SO4)3 þ 0.345 M
TEA þ 0.45 M NaOH (1.2 index mediator solution). Table 7
3.2.3. Fastness results
shows the absorbance of the dye-bath before oxidation and
A few selected dyed samples with indirect electrochemical
the filtered mediator solution after oxidized dye-bath at
dyeing technique were tested for washing fastness, light fast-
620 nm. The difference in the absorbance shows a high degree
ness, wet and dry rubbing fastness. The results are shown in
of dye removal. The filtered mediator solution was again used
Table 9. All the fastness properties appear to be equivalent
for the next dyeing experiment. However, there was a loss of
with the conventionally dyed samples obtained using sodium
4e5% of the mediator solution throughout the process which
dithionite as a reducing agent.
had to be replenished with fresh mediator solution which is
also given in Table 7. Thus the dye-bath was recycled four
times. Blue RSN dye was used during each dyeing experiment.
The CIElab coordinates of the dyed samples (Table 8) for 3.2.4. Indirect electrochemical reduction mechanism
four recycling dyeing experiments, carried out under similar A general reaction for indirect electrochemical dyeing pro-
experimental conditions show no effect on the colour depth cess using Fe(II)eTEA complex is established [26,37] as
and shade. There was also no effect on the rate of follows.

Fe3+L + e- Fe2+L ---(1)

O O

Fe2+L + ---(2)

O O
Anthraquinone Dye Radical Anion

O O O

+ ---(3)

O O O
Dye Radical Anion
Dye Dianion Oxidised dye

O O

+ Fe2+L + Fe3+L ---(4)

O O
Dye Radical Anion Dye Dianion

O O
2e- ---(5)
+
O O
Dye Dianion Oxidised dye

General Reaction Scheme of Dye Reduction by Fe(II) complex

system `L` denotes the associated TEA ligand
54 M.A. Kulandainathan et al. / Dyes and Pigments 73 (2007) 47e54

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