Oxidizing and Reducing Used in Sulpher Dye | Reducing Steps of SulpherDyes | Oxidation Step of Sulpher Dye

Some oxidizing and reducing agent used in Sulpher Dyeing

Oxidizing  Agent	Reducing Agent
Potassium Dichromate(K2Cr2O7)	Sodium Sulphide (Na2S)
Acetic Acid(CH3COOH)	Sodium Hydro sulphide(NaHSO3)
Sodium Perborate	Thioglycolic acid
Sodium per carborate	Thio Salicylic acid
Sodium peroxide(NaO2)	Pseudo thiohydantion
Hydrozen peroxide(H2O2)

Reducing Steps of Sulpher Dyes:

Reducing step is the most important in the application of Sulpher dyes. Unless the dyes are converted in to the completely soluble form, the full colour value cannot be achieved. The solubility of reduced Sulpher dyes varies appreciably from dye to dyes. For example yellow Sulpher dyes are the last soluble dyes (i.e. most insoluble dyes)

The reducing agents are used for Sulpher dyes are:-

1. Na-Sulphide.
2. Na-Hydro Sulphide.
3. Thioglycolic Acid.
4. Pseudo-Thio-Hydantion.

But Na Sulphide is the most widely used reducing agent for dissolving Sulpher dyes. They may be replaced by Na-Hydrosulphide in some cases. But the latter is a more powerful reducing agent. So, over reducing of the dye may take place, leading to a product having lower affinity for cellulosic fibres. Hence lower colour yields are obtained along with wastage of dyestuff. In some cases lower wash fastness results if Na-Hydrosulphide is used. It also decreases the life time of the vessel.

For dissolving the Sulpher dyes, reducing agent is taken according to the weight of dye. In practice, they are taken at same weight. For example, 10 gm dye is mixed with 10 gm reducing agent. Then the mixture is boiled after adding necessary amount of water to get a good solution. For this wooden vessel or enameled iron vessel are used but not iron vessels. Because iron reacts with Sulpher.
 

Oxidation Step of Sulpher Dye:

After dyeing the reduced water soluble form of the dyes have to be converted in to the original water insoluble form by oxidation. 

The commonly used oxidizing agents are-

1. Potassium dichromate
2. Na-Perborate.
3. Na-percarborate.
4. Na-peroxide.

The method of oxidizing selection plays an important role in the development of correct shades & their optimum fast5ness properties. Rinsing off the dyeing should be done rapidly to remove as much as Na-Sulphide as possible. If even after washing some Na-sulphide or Soda-ash stays on fabric, we use CH3-COOH to neutralize them. 

Using of different oxidizing agents may give following results-

1. Use of perborate or percartborate in presence of acetic acid gives brighter shade.
2. Treating dyed material with dichromate without rinsing reduces colour losses but causes dull shade.

The chemistry of oxidization is as below:-

Dye-S-S-Dye +2[H] = Dye- SH + HS-Dye

Dye- SH + HS-Dye +[O] = Dye-S-S-Dye +H2O