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Classification of acid dyes

Classification of Acid Dyes

The acid dyes are organic acids, and sometimes have acid (-SO3H) and, in certain cases, acid because of the acid group (-COOH).

Because free acids have a hygroscopic tendency and are challenging to break free from their solutions, colours made of acids are sodium salts. 

As shown below, there are various classification schemes for acid dyes.
(i) Classification by state of division i.e size of the dye molecule.

(ii) Classification by the concentration of acid used in the dyeing bath & migration & fastness properties of the dyes.

Explanation -

(i) Classification concerning their state of division, acid dyes are broadly classified into the following two groups- (A)Molecularly split acid dyes &
(B)Aggregated acid dyes
Explanation - 

(A) Molecularly split acid dyes - 

Due to their propensity to produce clear aqueous solutions, these dyes are recognizable. When a molecule is dissolved in water, it becomes ionized. 

Reaction :
D-SO3Na > D-SO3 + Na+ 

These dyes are utilized in baths that contain sulfuric or acid, and when the acid is absent, they need a weak affinity for protein fibres. 

The dyes have good migratory properties because of the breaking in water, so any uneven dyeing within the early stages of dyeing is also easily remedied by continuing the dyeing at a boil. 

These colours aren't immune to wet processes like washing and milling. 

(B) Aggregated acid dyes - 

Such dyes don't fragment into individual molecules when dissolved in water. 

The dye molecules group or aggregate together, but a number of the molecules become ionized to come up with charged aggregation.

Thanks to their low solubility, their solutions are turbid; nonetheless, an increase in temperature causes a rise in ionisation. 

These dyes stain cotton or other cellulose fibres when used on them during a neutral bath. 

They need a powerful affinity for protein and polyamide fibres, and their affinity increases when acid is added to the dye solution.

Molecule aggregation causes poor levelling and migratory properties. However, the responsiveness to wet therapies is usually good. 

Acid dye structure

Acid dye structure
Acid dye structure

Classification by the concentration of acid used in the dyeing bath and migration and fastness properties of the dyes i.e 

Method of classification acid dyes are classified into the following three classes depending on their fastness properties & concentration of acid used in the dye bath - 

(A) Leveling acid dyes
(B) Milling acid dyes
(C) Super Milling acid dyes 

Explanation - 

(A) Levelling acid dyes - 

These colours have more excellent levelling capabilities, as their name suggests. 

Since they do not respond well to wet treatments but respond well to light, they practically resemble molecularly split dyes. 

When creating mixture hues, consideration should tend to every component's light and washing fastness. 

These colours are applied from a shower containing formic or acid and are sometimes said as "very acid dyes" due to this.

If the unevenness is seen during their application, it may be fixed by boiling the dye bath for a lengthy time.

(B) Milling acid dyes -

When contrasted to the levelling acid dyes, these dyes lack brightness and levelling abilities. 

They'll endure milling treatment and have excellent washing fastness qualities. 

It's common to consult with the dyes as weakly acid dyes because they're typically applied from a carboxylic acid bath.

(C) Super Milling acid dyes -

Because they're used after neutral baths, these dyes are neutral dyeing dyes. 

Since they form colloidal solutions, which separate when heated, they do not provide real solutions. 

Although their migrating or levelling strength is minimal, they often have good washing and light fastness.
Features of Acid Dyes are displayed in the table below. 

Characteristics ic

Levelling acid dyes

Milling acid dyes

Super Milling acid dyes

Chemical added to dye bath

Sulphuric acid

Acetic acid

Ammonium acetate

the pH of the dye bath

2 to 4

4 to 6

6 to 7

Solubility in water




Levelling property



Very poor

Molecular weight




Fastness properties



Very good

The action of dye on cellulose fibres

No staining



Questions -

  1. Describe the classification of acid dyes.
  2. Which are the types of acid dyes as per their state of division.
  3. Which are the types of acid dyes based on fastness and concentration properties?
  4. Describe Leveling acid dyes
  5. Explain Milling acid dyes.
  6. Explain Super Milling acid dyes.


Co, D. T. (n.d.). Did you know? Dharma Trading Co. from

Moody, V., & Needles, H. L. (2004). Color, dyes, dyeing, and printing. In Tufted Carpet (pp. 155–175). Elsevier.

Muntasir, K. S. (2020, April 3). Acid dyes -types. TextileTuts.

Sayed, A. (n.d.). Properties and classification of acid dye. from

(N.d.). from

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 - Rushikesh Patil (Textile Engineer)
(DKTE Society textile engineering college Ichalkaranji)
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