How Salt and Soda Affect Reactive Dye Fixation – Complete Tutorial

Reactive dyeing is one of the most widely used coloring methods for cotton and cellulosic fibers. Two of the most important chemicals in this process are salt and soda ash. These two chemicals directly control dye exhaustion, dye fixation, shade depth, and overall dyeing quality.

In this complete tutorial, you will learn:

• Why salt and soda are used in reactive dyeing
• How they influence the dyeing mechanism
• The role of pH in fixation
• Their impact on color yield and fastness
• Problems caused by incorrect salt or soda amounts
• Tips for achieving perfect shade in reactive dyeing

Let’s begin!

 

1. Understanding Reactive Dyeing Mechanism

Reactive dyes form covalent bonds with cellulose fibers (like cotton). These bonds make the color permanent and stable. However, the dyes are water-soluble, so they need help getting from the water into the fiber and then fixing to the fiber.

Here is where salt and soda ash become essential.

 

2. Role of Salt in Reactive Dyeing

Salt helps drive the dye inside the fiber during the exhaustion phase.

Why do we use salt?

Cotton carries a negative charge, and reactive dyes also have negative ions. Because like charges repel, the dye naturally stays in the water instead of attaching to the cotton.

Salt works as a bridging agent.
It reduces the repulsion and helps dye molecules move towards the fiber surface.

 

How Salt Affects Dyeing:

1. Improves Dye Exhaustion

Salt pushes the dye from the water phase into the fiber phase.
More salt = more dye exhaustion.

2. Controls Shade Depth

Higher amount of salt gives deeper shades.
Lower amount produces lighter shades.

3. Enhances Color Uniformity

Proper salt dosing ensures even migration of dye into the fabric.

4. Reduces Dye Wastage

Better exhaustion means more dye is used, producing less effluent pollution.

 

3. Role of Soda Ash in Reactive Dyeing

While salt helps the dye enter the fiber, soda ash helps the dye chemically bond with the fiber.

Why do we use soda ash?

Reactive dyes need alkaline conditions (high pH 10–11) to react with cellulose.
Soda ash (sodium carbonate) provides this alkalinity.

 

How Soda Affects Fixation:

1. Raises the pH

Soda increases pH to the level needed for fiber–dye reaction.

2. Activates the Fiber

Alkalinity opens hydroxyl groups (-OH) on the fiber so dye can attach.

3. Helps Form Covalent Bonds

Soda triggers the chemical reaction between dye and fiber, resulting in true dye fixation.

4. Determines Dye Fastness

Proper fixation improves washing, rubbing, and light fastness properties.

 

4. Salt vs. Soda – What Happens if the Amount is Wrong?

Using incorrect amounts of salt or soda can cause many dyeing problems.

 

If Salt is Too Low:

• Poor dye exhaustion
• Pale shades
• Uneven dyeing
• Higher dye wastage

 

If Salt is Too High:

• Dye precipitation
• Patchy shade
• Hard wash-off
• Poor rubbing fastness

 

If Soda is Too Low:

• Poor dye fixation
• Shade becomes lighter after wash
• Low wash fastness
• High unfixed dye

 

If Soda is Too High:

• Dye hydrolysis (wastage)
• Harsh handle of fabric
• Difficulty in washing off
• Shade variability

 

5. The Balance Between Salt and Soda

Successful reactive dyeing depends on the right balance between salt and soda.

Correct Sequence:

1. Add salt first → improves exhaustion
2. Add soda later → starts fixation process

If soda is added too early, fixation begins before the dye is fully absorbed, causing uneven shades.

 

6. Example Recipe for Reactive Dyeing (General Guideline)

Process Step	Chemicals	Function
Exhaustion	Salt (30–80 g/L)	Pushes dye into fiber
Fixation	Soda Ash (10–20 g/L)	Increases pH, creates dye-fiber bond

Note: Recipe varies by shade, dye brand, fabric type, and machine.

 

7. Importance of pH in Dye Fixation

Correct pH is crucial:

• pH 6–7 = Dye stable (no fixation)
• pH 10–11 = Fixation occurs
• pH above 12 = Excess hydrolysis

Maintaining the right pH ensures maximum color yield and minimal dye wastage.

 

8. Tips for Improving Salt & Soda Efficiency

• Add soda gradually (not all at once)
• Maintain liquor ratio correctly
• Ensure fabric is well-prepared (no oil, wax, or sizing)
• Use soft water to avoid hardness problems
• Monitor pH throughout the process
• Don’t add salt or soda too fast

These tips help achieve consistent and high-quality dyeing results.

 

9. Environmental Impact of Salt and Soda

Reactive dyeing produces high-salt wastewater.
However, following optimized recipes and modern systems (like CPB dyeing) can reduce chemical consumption dramatically.

 

10. Conclusion

Salt and soda are two essential chemicals in reactive dyeing. While salt improves dye exhaustion, soda improves dye fixation. The success of reactive dyeing depends on the perfect balance and correct sequence of these two chemicals.

By understanding how salt and soda affect dyeing behavior, textile professionals can achieve:

• Brighter shades
• Better fastness
• Lower dye wastage
• Improved efficiency
• More sustainable dyeing operations

This complete tutorial provides the technical foundation needed to optimize reactive dyeing and produce consistent high-quality results.