- 1. POTENTIOSTATIC STUDIES ON INDIRECT ELECTROCHEMICAL REDUCTION OF VAT DYEProf. R. B. Chavan* and Kiran Patil, Department of Textile Technology, Indian Instituteof Technology, Delhi, Hauz Khas, New Delhi 1100016 (India)&M. Anbu Kulandainathan, Scientist, Central Electrochemical Research Institute, Karaikudi, Tamil nadu 630006 (India) Abstract Dispersed vat dyestuffs can be reduced by indirect electrolysis using iron-triethanolamine complex. The application of indirect electrolysis as a reduction techniqueis described along with the mechanism. Electrochemically reduced vat dye is tested inlaboratory scale dyeing experiments, and the results of different reduction conditions arediscussed. The influence of the concentration of the complex-system on the build-up ofcolour depth, shade and fastness is discussed and compared with samples of the standarddyeing procedure using sodium dithionite as reducing agent. The new process offersenvironmental benefits and offers the prospects of improved process stability, because thereduction state in the dyebath can be readily monitored by measuring reduction potential.
2. Introduction Vat dye is one of the most important among the dye classes used for colourationof cotton, particularly, when high fastness standards are required to light, washing andchlorine bleaching.1 Also, from the commercial point of view, vat dyes (including indigo)acquires a large share in the dyestuff market for the colouration of cellulosic fibres. Theannual consumption of vat dyes including indigo is around 33,000 metric tons since 1992and it holds 24% of cellulosic fibre dye market in value terms.2 Vat dyes being water insoluble have to be first converted into water soluble formby reduction with strong reducing agent like sodium dithionate. The use of sodiumdithionate is being criticized for the formation of non-environment friendlydecomposition products such as sulphite, sulphate, thiosulphate and toxic sulphur. 3,4 Attempts are being made to replace the sodium dithionite by ecologically moreattractive alternatives. -hydroxyketone which meets requirements in terms of reductiveefficiency and biodegradability had been tried. However such compounds are expensiveand their use is restricted to closed systems due to its strong smelling condensationproducts in alkaline solution.5 Some other sulphur containing compounds likehydroxyalkyl sulphinate, thiourea etc, have also been recommended recently. 6 Thesecompounds have relatively low amount of sulphur content and also have lower equivalentmass which leads to lower sulphur based salt load in the waste water. However, in thesecases too, it is not possible to dispense with sulphur based problems totally. Being Fe(OH)2 as a strong reducing agent in alkaline medium, this possibility hasalso been explored for reducing organic dyestuffs. The reducing effect of Fe(OH)2 3. increases with increase in pH. However, Fe(OH)2 is poorly soluble in alkaline solutionand gets precipitated. It must be complexed in order to hold Fe(OH) 2 in solution7. A stablecomplex with reducing power is obtained with weaker ligands, e.g. gluconic acid.Regarding eco-friendliness, gluconic acid can be eliminated in the sewage tank throughneutralization with alkali; free Fe(OH)2 can be aerated and converted to Fe(OH)3 whichacts as a flocculent and reduces wastewater load. Tartaric acid has also been tried as aligand by Chavan and Chakraborty for complexing Fe(OH)2 in presence of NaOH forreduction and dyeing of cotton with indigo and other vat dyes at room temperature.8Efforts have been made to complex Fe(OH)2 with single and double ligand systems usingtartaric acid, citric acid and gluconic acid which has shown optimistic results. Recently electrochemical reduction of vat, indigo and sulphur dyes is suggested.There are two ways by means which electrochemical reduction is achieved. Direct andindirect electrochemical reduction. In direct electrochemical reduction the chemicalreducing agents are replaced by electrons from electric current, and effluentcontaminating substances can be dispensed with all together.9,10 Although this techniqueis ideal, the stability of reduced dye species is poor affecting the colour yield. In Indirectelectrochemical reduction technique the dye reduction is achieved through a redoxmediator system. Among the various mediator systems suggested in the literature, iron-triethanolamine complex (iron-TEA) seems to be promising. 11,12 Both the electrochemical reduction techniques are not yet commercialized andresearch and development efforts are in progress in this direction. In the present paper attempts are being made to understand the fundamentals ofindirect electrochemical reduction of selected vat dyes using iron-TEA complex as 4. mediator. Essential requirements for the design of electrochemical cell are suggested.Iron-TEA-NaOH molar ratio has been standardized to get thedyeing of cotton byindirect electrochemical reduction technique. The colour yields are compared withconventional sodium dithionate method. The repeated use of dye bath after dyeseparation is explored.ExperimentalMaterials.-The sodium dithionite and sodium hydroxide used for vat dyeing byconventional method were laboratory grade chemicals. The mediator system necessaryfor indirect electrochemical reduction of vat dyes was prepared in alkaline medium fromtriethanolamine and ferric sulphate which were analytical grade chemicals. Thepotentiometric titrations were carried out to measure the dye reduction potential by usingK2Fe(CN)6 as an oxidizing agent which was too an analytical grade chemical. The dyesystems investigated were commercial products from Atul Ltd: Novatic Yellow 5G (CIVat Yellow 2), Novinone Brown RRD (CI Vat Brown 5), Novinone Green FFB (CI VatGreen 1), Novinone Blue RSN (CI Vat Blue 4), Novinone Blue BO (CI Vat Blue 20),Novinone Brown BR (CI Vat Brown 1), Novinone Black BB (CI Vat Black 36),Novinone Brill. Violet RR (CI Vat Violet 1) and Novinone Black CH.Dyeing procedure and process controlConventional dyeing.-The conventionally vat dyed samples were prepared in thelaboratory, with a standard procedure using sodium dithionite and sodium hydroxide.8 5. Electrochemical dyeingDesign of an electrochemical cell.-The electrochemical cell consists of a rectangularpolyvinyl chloride vessel containing 1 l catholyte and 0.1 l anolyte. The anolytes usedwere the same solutions which were being used as the catholytes in every case. Theanode used was thin stainless steel rod with the surface area of 6 cm 2. A three-dimensional copper wire electrode with a surface area of 500 cm2 was served as thecathode. The cathode compartment was thus providing porous flow through electrodefilling the cathode compartment as coil (depth 10cm, front area cm2). The anolyte wasseparated from the catholyte by a diaphragm (cation exchange membrane) to prevent thetwo electrolytes from mixing. The catholyte was agitated by an analytical rotator at aconstant speed to guarantee homogeneous and stable conditions in the cathode chamber.The dyebath of the dyeing apparatus was circulated through the electrochemical reactorfor continuous renewal of the reducing capacity of the dyebath. In accordance withtechnical conditions, the electrolysis experiments did not involve an inert atmosphere. A schematic drawing of the dyebath circulation through the laboratory dyeing unitand the catholyte circulation through the electrochemical cell is given in Fig. 1. 6. Analytical rotator+- Anode: SS Cathode: CuPeristaticPeristatic Anolyte:Catholyte:DyebathPump 1Pump 2 Mediator(Mediator system175 ml/min175 ml/min 100 ml+ dye) 1200ml 1000 mlElectrochemical cellCu wire to measure solution potential w.r.t. reference electodeReference electrode (Hg / HgO/OH-) Fig. 1. Schematic representation of the electrochemical flow cellDetermination of reduction potential of vat dyes.-Here, the reduction potential of thedye is referred as the potential developed on the bright platinum electrode when dipped inthe dye bath versus the reference electrode at which the dye gets just solubilized. Inorder to measure the reduction of the vat dyes, the titration was carried out using brightplatinum electrode with Ag/AgCl/3M KCl as a reference electrode. A solution of 100mgvat dye was prepared which was reduced with 25ml of 0.1M NaOH and 100mg Na2S2O4.This solution was then diluted to 50 ml with distilled water and then titrated against0.05M K2Fe(CN)6. The K2Fe(CN)6 solution added in the steps of 0.5ml and the dye 7. solution potential measured at every step. The ml of 0.05M K2Fe(CN)6 solution is thenplotted against the dye solution potential in order to find out dye reduction potential.Dyeing Procedure.-Selected vat dyes were dyed under the standardized conditions of themediator system as described in results and discussion section. While following theelectrochemical dyeing technique, the dyebath was circulated through the cathodiccompartment of the electrolytic cell for continuous renewal of the reducing capacity. Allthe experiments were performed under potentiostatic conditions (-1050 mV) at roomtemperature. The catholyte was called as a mediator solution which was comprised oftriethanolamine, ferric sulphate and sodium hydroxide. The potential prevailing in thecatholyte, while electrolysis, was measured with a copper wire versus a referenceelectrode (Hg/HgO/OH-). The catholyte was agitated in the catholyte compartment itselfand was also circulated through the dyebath continuously. When the potential achieved inthe catholyte was equivalent to the reduction potential of the dyestuff, the dyestuff wasintroduced into the catholyte compartment. In all the experiments 2% on weight of fabricdye was used in the dyeing recipe. The fabric sample was introduced into the dyebathafter ten minutes of introduction of dyestuff in order to allow reduction of the dyestuff.The experiments were carried out at a relatively long liquor ratio i.e. 240:1.