2.3. Reducing agent for sulphur dye:

2.3.1. Conventional reducing agent (sulphide based):

According to the estimate, 90% of sulphur dyes used in world as a whole is still reduced by means of sulphide compounds. The sulphide reducing agent can be sodium sulphide (Na₂S), sodium hydro sulphide (NaHS) and sodium polysulphide (Na₂S_X) where x varies from one to six. The poly sulphide variety is available in aqueous media & others are in both media. Some of the major problems with sulphur dye are contamination of effluent with sulphur. The liberated Hydrogen sulphide is toxic in nature. Attempts are therefore been made replacing sulphide based reducing agents for the dyeing of sulphur dyes. For that ecofriendly reducing agents are introduced in market, such as the Glucose & Mercaptoethanol.

2.3.2 Electrochemical dyeing:

As seen earlier, the conventional reducing agents, which reduce the dyestuff, result in nonregenerable-oxidized byproducts that remain in the bath. The used dye bath cannot be recycled because the reducing power of these chemicals cannot be regained. The disposal of the dye bath and the washing water cause various problems due to the non ecofriendly nature of the decomposed products. Maximum attention must therefore be paid from the ecological standpoint to the necessary reducing agent for these dyes. Electrochemical dyeing is still in the laboratory stage but could become the dyeing process of the future of the vat, indigo and sulphur according to BASF, a leading dyestuff manufacturing company. Electrons from the electric current replace Electrochemical dyeing in which chemical reducing agents, and effluent contaminating substances can be dispensed with altogether.

The first attempt although not involving directly the electrochemical dyeing was made by E.H.Daruwala. He tried to reduce the quantity of sodium dithionite needed for the reduction of the vat dyes by the application of a direct voltage this reduction can be traced to the fact that sodium dithionite at the cathode is converted into a form that exhibits increased reducing power. By appropriate cathode reduction under suitable condition (cathode potential, concentration, pH) it is possible to generate a powerful reducing species from sodium dithionite redox potential higher than sodium dithionite itself. So due to this behavior decomposition of hydrosulphide takes place to produce free radical ion SO₂

S₂O₄^-2 2(SO₂)

However, these products cannot be regenerated at the applied voltage at the cathode, making recycling of the bath liquor impossible. In electrochemical dyeing technique, the same concept is adopted one step ahead and makes the liquor recycling possible.

There are two methods by means of which electrochemical dyeing can be carried out, direct electrochemical dyeing and indirect electrochemical dyeing.

2.4. Direct electrochemical dyeing:

In case of direct electrochemical dyeing technique, organic dyestuff has been directly reduced by contact between dye and electrode. However in practice, the dyestuff is partially reduced by using conventional reducing agent and then complete dye reduction is achieved by electrochemical process for complete reduction which facilitates the improved stability of the reduced dye.

In order to start the process, an initial amount of the leuco dye has to be generated by a conventional reaction, i.e. by adding a small amount of a soluble reducing agent. Once the reaction has set in, it is not needed anymore and further process is self sustaining. The system is found successful in case of sulphur dyes. However, concentration of the dye required to get a specific shade is higher than the conventional reducing process.

In such a system, a dyestuff particle must come into contact with the electrode surface in order to get reduced. However, the atmosphere oxygen present in the dye solution immediately reoxidizes the dyestuff has no protective capacity. Also, since the dye itself must be reduced at the surface of the cathode, cathode area should be large which itself is a constraint.