The document discusses using iron(II) salt complexes as reducing agents for dyeing cotton with vat dyes as an alternative to sodium hydrosulfite. Single ligand complexes of iron(II) were ineffective for reducing most vat dyes except with gluconic acid. A stable two ligand complex of iron(II) with tartaric acid/triethanolamine or citric acid/triethanolamine effectively reduced vat dyes and dyed cotton comparably to sodium hydrosulfite at room temperature. Optimization of the two ligand system concentrations improved stability and dye uptake. Dyeing with various vat dyes showed the two ligand systems were generally suitable but resulted in over-reduction of some blues and an olive

1. REVISED PAPER
Dyeing of Cotton with Vat Dyes using Iron(II) Salt Complexes
J N Chakraborty, Department of Textile Technology, National Institute of Technology,
Jalandhar-144011
R B Chavan, Department of Textile Technology, Indian Institute of Technology,
New Delhi -110016, India
ABSTRACT
Sodium hydrosulfite is universally used reducing agent for dyeing of cotton with vat dyes.
However it forms various decomposition products containing sulfur, which go into wastewater
creating environmental problems. Search is therefore on for alternative reducing systems for vat
dyeing. In the present work, the use of the co-ordination complexes of Fe(II) salts with suitable
ligands is reported.
Key Terms
Hydrosulfite, Dye Strength, Fe(II) salt, Ligand,
Vat colors possess pairs of carbonyl (C=O) groups in their structure and are water insoluble.
These are converted to water soluble form in presence of strong reducing agent and alkali which
only then exhibit affinity for cellulosics1,2. Sodium hydrosulfite is universally used reducing
agent for dyeing of cotton with vat dyes. However, there are certain drawbacks associated with
use of sodium hydrosulfite viz. reduction and dyeing both are performed at different temperature
for different classes of vat dyes2 ; wastage of sodium hydrosulfite due to its thermal and
oxidative decomposition in bath, which is compensated by means of adding it in excess3,4 (a cost
factor), as well as formation of various decomposition products containing sulfur, which go into
waste-water creating environmental problems5,6 .

2. Various alternative eco-friendly reducing systems viz. hydroxy acetone7,8, glucose-NaOH9,
electrochemical reduction10-11 are reported in the literature. Fe(OH)2, though a strong reducing
agent, its reducing capacity is not revealed due to its poor water solubility. In order to use
Fe(OH)2 as reducing agent it is necessary to keep it in solution. This is possible by the formation
of co-ordination complexes of Fe(II) salts with suitable ligands in presence of alkali like NaOH.
It was reported that gluconic acid co-ordinates with Fe(II), improves its water solubility to
generate reduction potential for vat dye reduction and subsequent dyeing of cotton at 60oC5. In
our previous work, we had shown that Fe(II) salts can be successfully complexed with tartaric
acid, citric acid or triethanolamine in presence of NaOH12. These co-ordination complexes were
termed as single ligand complexes or single ligand systems. These Fe(II)–single ligand
complexed reduction baths were turbid due to incomplete solubilisation of Fe(OH)2 ; though
effective for reduction of indigo were ineffective for other vat dyes12 .
The present paper aims at the use of co-ordination complexes of Fe(II) salts with two ligands
(viz. citric acid and triethanolamine or tartaric acid and triethanol amine, termed as two ligand
systems) along with NaOH for reduction and application of vat dyes other than indigo on cotton
at room temperature. Fe(II) complexed with gluconic acid was also used for dyeing at 60oC as
reported in literature5 to compare with the dyeing efficiency of our system.
EXPERIMENTAL
Materials (with specifications), determination of alkali equivalence of ligands, calculation of
total NaOH requirement, measurement of color strength of samples, measurement of reduction
potential, estimation of iron in dyed samples, estimation of ferric iron, estimation of soluble iron
in dyebath were reported in earlier paper12. Commercial vat dyes were used. C.I.Generic names
are reported in this paper13.
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3. Preparation of Reduction Bath
Single Ligand System
2.16g of tartaric acid or 3.0g of citric acid or 5.6ml of gluconic acid (50%) or 1.92ml of
triethanolamine was dissolved in 100 ml water in a glass beaker in open air ; 2 g of FeSO4 was
dissolved in this bath ( FeSO4 : ligand = 1 : 2 molar ) followed by addition of NaOH ( 1.47g,
2.62g or 1.21g for tartaric, citric or gluconic acid system respectively ). All the baths were turbid.
Quantity of dye required to get 1% shade was then added. Vatting and dyeing were carried out
at room temperature, except gluconic acid bath which was heated up at 60oC as reported in
literature12.
Double Ligand System
3.78g of tartaric or 6g of citric acid was dissolved in 50 ml water in a glass beaker in open air
followed by addition of 2g of FeSO4 with constant stirring till the latter gets completely
dissolved. Triethanolamine was added in this solution (1ml in tartaric and 1.5ml in citric acid
system) and stirred well to ensure thorough mixing. In another beaker, desired weight of NaOH
( 3.09g for tartaric and 4.3g for citric acid system ) was dissolved in 50 ml water followed by
addition of this NaOH solution to the previously prepared FeSO4, tartaric (or citric) acid and
triethanolamine mixture. A clear solution was obtained. Quantity of dye required to get 1%
shade was added to this bath. Instantaneous reduction of dye occurs (though 10 minutes were
allowed to ensure complete reduction). The vatting and dyeing were carried out at room
temperature at material to liquor ratio of 1:20
Sodium hydrosulfite system
Concentrations of hydrosulfite, NaOH and vatting and dyeing conditions were as follows1,2 :
3

4. IK IW IN IN special
Temperature of vatting 35-40oC 45-50oC 55-60oC ≥ 60oC
Temperature of dyeing 35oC 45oC 50-55oC ≥60oC
Hydrosulfite (g/l) 8 10 12 15
NaOH (g/l) 8 10 12 15
Dyeing of Samples
Cotton fabric samples were dyed in open air in glass beakers for 1 hr. at room temperature in
single and double ligand reducing systems separately, at 60oC in gluconic acid system and at
specific temperature range in hydrosulfite system. Dyed samples were air oxidized, rinsed with
water, soaped at boil with anionic detergent (5g/l) for 15 minutes followed by thorough washing.
Wash and light fastness were determined as per standard procedure reported in Bureau of Indian
Standards( Test methods IS: 764 : 1979 and IS 2454 : 1985 respectively)14. Color strength was
measured in Datacolor Color Matching Instrument.
RESULTS AND DISCUSSION
For the simplicity of understanding the co-ordination complex system essentially consisting of
Fe(II) salt like FeSO4, NaOH and ligands like citric acid, tartaric acid or triethanolamine either
alone (single ligands) or tartaric acid and citric acid separately in combination with
triethanolamine (two ligands), it is envisaged that FeSO4 reacts with NaOH with formation of
Fe(OH)2 of poor water solubility. Its water solubility is improved when Fe(II) forms co-
4

5. ordination complex with single ligand or two ligands as mentioned above. In the following
paragraphs such co-ordination complexes are referred as single ligand or two ligand systems.
Dyeing of Cotton with Anthraquinoid Vat Dye
Single Ligand System
Reducing systems based on Fe(OH)2 complexed with single ligand though worked successfully
for dyeing of cotton with indigo12, showed their inability to reduce anthraquinoid vat dyes except
gluconic acid system. Table 1 shows values of pH, reduction potential, complexed iron and dye
strength of samples. It was observed that no dye reduction took place from FeSO 4 + NaOH as
well as FeSO4 + NaOH + triethanolamine systems, hence no dye yield. In case of tartaric and
citric acid systems, partial reduction of dye was visually observed, however there was no dyeing,
whereas in case of gluconic acid system dye reduction and dyeing took place, though the
reduction potential at all stages of dyeing was highest in triethanolamine system.
Different ligands showed varying amounts of complexed Fe(II) in bath before dye addition, as
shown in Table I. Complexed Fe(II) was least in FeSO4 + NaOH system and highest in gluconic
acid system. Thus it appears that the amount of complexed Fe(II) in triethanolamine system was
not adequate for vat dye reduction and therefore no dyeing took place. Complexed Fe(II)was
higher in case of tartaric and citric acid systems ; thus when dye was added to these reducing
systems, partial reduction of vat dye was observed but no dyeing took place. In contrast, in
gluconic acid system complexed Fe(II) was highest causing complete reduction of vat dye and
good dyeing. Failure of anthraquinoid vat dye reduction and its subsequent dyeing in spite of
high reduction potential and pH of baths at different stages of dyeing clearly indicated that it was
not only the reduction potential but also amount of complexed Fe(II) in bath was equally
important for dye reduction and to keep the dye in reduced form during dyeing.
5

6. Chemically vat dyes can be classified in two types - indigoid and anthraquinoid. Indigoid vat dye
can be reduced and maintained in reduced condition at low reduction potential (-700 mV)
compared to anthraquinoid vat dyes -(850-900)mV. Therefore a single ligand system was
capable of reducing indigo and keeping it in reduced condition giving good dyeing of cotton 12
but the same systems failed to reduce anthraquinoid vat dyes. This analysis made it clear that the
dyeing of cotton with vat dye did not occur from co-ordination complexes of Fe(II) with single
ligand systems (except gluconic acid) due to inadequate complexion of Fe(II). It was therefore
thought to use two ligand systems for vat dyeing.
Chakraborty replace the words highlighted with unstable co-ordination complex of Fe(II)
Two Ligand Systems
Limited complexion of Fe(II) in presence of single ligand like tartaric or citric acid might be due
to formation of weak co-ordination linkages with limited stability. It was thought that a second
ligand like triethanolamine may enter along with tartaric and citric acid in co-ordination linkage
with Fe(II), resulting in more stable complex. Based on this assumption, reduction baths
consisting of FeSO4-tartaric acid-triethanolamine-NaOH or FeSO4 -citric acid - triethanolamine -
NaOH ( FeSO4 -25 g/l, Tartaric acid or Citric acid-15 g/l, Triethanolamine - 40 ml/l and
NaOH-50 g/l) were prepared so as to get colorless, clear reduction baths indicating complete
complexion of Fe(II). In these formulations, molar ratio of FeSO4 with triethanolamine was 1: 3,
that with tartaric acid 1: 1.5 and with citric acid 1 : 1. Dyeing with few randomly selected vat
dyes was carried out in 1% shade at room temperature and were compared with those obtained
from hydrosulfite system. Results are shown in Table II. Except C.I. Vat Blue 7 and C.I. Vat
Violet 1, the dyeings were comparable to sodium hydrosulfite system.
6

7. This observation gave support to our hypothesis that for satisfactory dyeing with vat dyes, it was
not only the reduction potential at different stages of dyeing but also maximum complexion of
Fe(II) are very important.
Chakraborty Replace the highlighted portion by: the formation of stable co-ordination complex
of Fe(II) is important
Optimisation of Dyebath Recipe for Double Ligand Systems Replace double by two
The above experiment gave the clue for successful dyeing of cotton with vat dyes using Fe(II)
complexes as reducing agent. Subsequently, concentration of each component in double ligand (
replace by two) reducing systems was optimized. Concentration of total NaOH required and
alkali equivalence of acidic ligands was explained in earlier paper12. Criteria for optimized
concentrations were those that produced clear solutions having maximum efficiency for dye
reduction and showing maximum dye uptake on cotton.
In FeSO4 - tartaric acid - triethanolamine - NaOH reducing system, the optimized reduction bath
composition was FeSO4-20 g/l, Tartaric acid-37.8 g/l (FeSO4: tartaric acid =1:3.5 molar),
Triethanolamine - 10 ml/l (FeSO4 : triethanolamine = 1: 0.7 molar) and NaOH-30.9 g/l ;
whereas in FeSO4-citric acid - triethanolamine - NaOH reducing system, the optimized reduction
bath composition was FeSO4 - 20 g/l, Citric acid - 60 g/l ( FeSO4 : citric acid =1 : 4.5 molar),
Triethanolamine - 15 ml/l (FeSO4 : triethanolamine = 1: 1.1 molar) and NaOH - 43 g/l . The
recipes stated in experimental section are based on these optimized concentrations.
Stability of Reduction Baths
Reduction baths with optimized concentration of chemicals were prepared in absence and
presence of dye and were stored in open air at room temperature for different intervals of time up
7

8. to 24 hours. Reduction potential was measured at definite intervals and is shown in Fig.1. All
the reduction baths remained fairly stable up to 4 hours, beyond which drop in reduction
potential started taking place. After storing for 24 hours, the drop in reduction potential was
substantial in all cases. The dyeability of reduction baths after storage for definite intervals was
studied by carrying out dyeing of cotton with C.I. Vat Green 1 (4% owf). Dye strength values
are shown in Fig. 2, which also indicated that baths remained stable up to 4 hours, beyond which
the stability went on decreasing. Comparison of dye strength indicated the bath stability in the
following order
Hydrosulfite < Citric acid, triethanolamine ligands ≃ Tartaric acid, triethanolamine ligands
< Gluconic acid ligand systems
Dyeing with different Vat Dyes
In order to establish the feasibility of new reducing systems for application of vat dyes, dyeing of
cotton with a wide range of vat dyes belonging to I K, IW, IN and IN special classes was carried out
for 4% shades. For comparison purpose, respective standards were prepared in hydrosulfite and
NaOH system. Dyeings were also carried out in gluconic acid system at 60oC using FeSO4:
gluconic acid (1: 2 molar) as reported in literature5 to compare efficiency of our systems. In case
of C.I. Vat Green 9 (Black BB), dyeings were carried out for 10% shades. Results obtained are
shown in Ttable III.
Dye belonging to IK class showed better color strength in two ligand systems compared to
hydrosulfite and gluconic acid systems. In case of IW, IN and IN special classes, no definite trend was
observed. Hydrosulfite and gluconic acid systems produced good black shades with Black BB,
whereas two ligand systems produced deep olive green shade instead of a black
8

9. From these observations it may be concluded that two ligand systems (tartaric, triethanol amine
and citric acid, triethanol amine) investigated in the present work in principle were suitable for
most of the vat dyestuffs. Few dyes particularly blues showed less color strength probably due to
over-reduction problem associated with structure of these dyes (indanthrone type) and black BB
did not give black shade.
Fastness Properties of Dyed Samples
Wash and light fastness tests were carried out on few selected dyed samples to confirm if
presence of iron interferes in their fastness properties. While wash fastness properties remained
unaffected for all dyed samples in all reducing systems, light fastness in case of C.I. Vat Brown 3
was lowered in case of Fe(II) complexes, might be due to presence of iron, which showed
catalytic action on dyed samples (table IV). In order to confirm this, estimation of residual iron
was carried out on dyed samples. It was found that negligible amount of iron was deposited on
dyed sample in gluconic acid system whereas higher amounts were deposited on samples dyed
from tartaric and citric acid systems (table V). Presence of iron on dyeing might show catalytic
action in presence of light, but this problem is restricted to certain red, yellow and brown dyes
only1.
CONCLUSIONS
Reducing systems based on Fe(II) complexes with single ligand such as tartaric acid, citric acid
or triethanolamine were not suitable for dyeing with vat dyes. However, Fe(II) complexes with
two ligands produced dyeing comparable to sodium hydrosulfite system with few exceptions.
Blue vat dyes with indanthrone structure produced weaker dyeing due to over reduction, whereas
C.I. Vat Green 9 produced deep olive green shade instead of black. Wash fastness of five vat
9

10. dyes under investigation was comparable to literature values, whereas there was deterioration in
light fastness in case of C.I. Vat Brown 3.
References
1. M. R. Fox, Vat Dyestuffs and Vat Dyeing, 1st edition, Chapman & Hall Ltd, London, 1948,
pp97 - 100.
2. S.V.Gokhale, and R.C.Shah, Cotton piece Dyeing, 1st edition, Ahmedabad Textile Industries
Research Association, India, 1974, pp30-35.
3. G. P. Nair and S. S. Trivedi, Colourage, Vol. 17, No. 27, December 1970, pp19-21.
4. F. Shadov, M. Korchagin and A. Matetsky, Chemical Technology of Fibrous Materials,
Revised English Translation, Mir Publishers, Moscow, 1978, pp428-433.
5. B. Semet, B. Sackingen and G. E. Gurninger, Melliand Textilberichte, Vol. 76, No.3, March
1995, pp161-164.
6. D. Fiebig and K. Konig, Textile Praxis International, May 1977, pp 577- 586.
7. E. Marte, Textil Praxis, Vol. 44 , No. 7, July 1989, pp737-738.
8. U. Baumgarte, Melliand Textilberichte, Vol. 68, No. 3, March 1987 , pp189- 195.
9 N. Nowack, H. Brocher, U. Gering and T. Stockhorst, Melliand Textilberichte, Vol. 63, No. 2,
February 1982, pp134-136.
10. T.Bechtold, E. Burtscher, D. Gmeiner and O. Bobleter, Melliand Textilberichte, Vol. 72,
No.1, January 1991, pp50-54.
11. T.Bechtold, E. Burtscher, G. Kuhnel and O. Bobleter, Journal of the Society of Dyers &
Colourists, Vol. 113, No. 4, April 1997, pp135-144.
12. R. B. Chavan and J. N. Chakraborty, Coloration Technology, Vol. 117, No.2, February 2001,
pp88-94.
13. Colour Index International, 3rd edition (3rd revision), Society of Dyers & Colourists,
Bradford, 1987.
14. Handbook of Textile Testing, Part-4, 1st Revision, Bureau of Indian Standards, New Delhi,
1988, pp115-119 & 141-142.
Authors Address:
Dr. J.N Chakraborty, NIT, Jalandhar, India Dr. R.B. Chavan, IIT, New Delhi, India
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11. E-Mail –chakrabortyjn@hotmail.com E-Mail - rbchavan@hotmail.com
Tel : 95-181-2690301-2, Ext. 221 Tel : 95-11-26591406
Fax: 95-181- 2690320 Fax: 95-11-26581103
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