Identification of Dyestuff From Unknown Dyed Samples

Introduction

It is necessary to identify what type of dyes are used in a sample before dyeing it. Shades or tones will not match accurately if the dye type doesn’t match. Only the chemical tests confirm the type of dye used for dyeing. But for some materials dye type is fixed for example polyester is almost dyed with the dispersed dye. So in this case no test is necessary to test the dyed polyester sample. But if we use this logic in the case of polyester-cotton blends then this may give the wrong result. So it is essential to test the material for the identification of the dye.

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Dyestuffs

Apparatus

• Test tube 
• Burner 
• Pipette
• Stopwatch
• Chemicals 
• Tannin mordanted cotton etc.
• Sample: Dyed fabric sample.

Tests: 01 (Direct dye)

Experiment no: 01: Boil with the soap solution and NH4OH solution for 1 minute.

Observation	Inference	Remarks
Color of the sample destroyed	Maybe direct, basic and acid dye	Absence of vat, sulfur, reactive or naphthol dye

Experiment no: 02: Boiling the sample with 5% soap solution and 2% soda ash solution for 1 – 2 minutes in presence of a piece of white (well-scoured/bleached) cotton fabric or yarn.

Observation	Inference	Remarks
The color bleed and white cotton is dyed	Absence of acid or basic color	Direct dye is conformed

Result: 01: The dyed sample is dyed with direct dye.

Test: 02 (Acid dye)

Experiment no: 01: Boil with the soap solution and NH4OH solution for 1 minute.

Observation	Inference	Remarks
Color of the sample destroyed	Maybe direct, basic and acid dye	Absence of vat, sulfur, reactive or naphthol dye

Experiment no: 02: Boiling the sample with 5% soap solution and 2% soda ash solution for 1 – 2 minutes in presence of a piece of white (well-scoured/bleached) cotton fabric or yarn.

Observation	Inference	Remarks
The color bleed, the dyeing is partially or totally stripped and the white cotton is little or not stained and white cotton could be rinsed perfectly clean again in water	Acid or basic color	Absence of direct dye

Experiment no: 03: The sample is wormed (up to 60℃) with methylated spirit and a few drops of acetic acid and then it is diluted with water wormed with a piece of tannin mordanted cotton and a piece of cotton.

Observation	Inference	Remarks
Tannin mordanted cotton is not dyed, white cotton may be dyed may not be dyed	Absence of basic color	Maybe the presence of acid dye

Experiment no: 04: The sample is boiled with a 5% soap solution and is acidified with a few drops of acetic acid and boiled with a piece of wool or silk fabric.

Observation	Inference	Remarks
Colour is transferred to white wool/ silk fabric/fiber but not like that in direct dye	Absence of basic dye	Acid dye is confirmed

Result: 02: The dyed sample is found to be dyed with acid dye.

Test: 03 (Basic dye)

Experiment no: 01: Boil with the soap solution and NH4OH solution for 1 minute.

Observation	Inference	Remarks
The Colour of the sample destroyed	Maybe direct, basic, and acid dye	Absence of vat, Sulphur, reactive or naphthol dye

Experiment no: 02: Boiling the sample with 5% soap solution and 2% soda ash solution for 1 – 2 minutes in presence of a piece of white (well-scoured/bleached) cotton fabric or yarn.

Observation	Inference	Remarks
The color bleed, the dyeing is partially or totally stripped and the white cotton is little or not stained and white cotton could be rinsed perfectly clean again in water	Acid or basic color	Absence of direct dye

Experiment no: 03: The sample is wormed (up to 60℃) with methylated spirit and a few drops of acetic acid and then it is diluted with water wormed with a piece of tannin mordanted cotton and a piece of cotton.

Observation	Inference	Remarks
Tannin mordanted cotton is dyed	Absence of acid dye	The basic dye is confirmed

Result: 03: The dyed sample is found to be dyed with basic dye.

Test: 04 (Vat, indigo dye)

Experiment no: 01: Boil with the soap solution and NH4OH solution for 1 minute.

Observation	Inference	Remarks
No change	Maybe vat, Sulphur, reactive or naphthol dye	Absence of direct, basic and acid dye

Experiment no: 02: The sample is boiled with 5% NaOH solution and a pinch of hydrose.

Observation	Inference	Remarks
Colour is destroyed and reappears on washing or airing. But in this case color is not destroyed totally	Absence of reactive or naphthol	Maybe vat or sulfur dye present

Experiment no: 03: The sample is boiled in a test tube with SnCl2 10% solution and a few drops of HCl. The mouth of the test tube is covered with blotting paper moistured with 1 – 2 drops of Pb- Acetate solution.

Observation	Inference	Remarks
Black spot on Pb – acetate paper not found	Absence of sulfur dye	Vat dye may present

Experiment no: 04: The sample is wormed with chloroform or benzene in a dry test tube.

Observation	Inference	Remarks
Colored solution	Absence of vat anthraquinone	Presence of vat indigo

Result: 04: The dyed sample is found to be dyed with vat (indigo) dye.

Test: 05 (Vat, anthraquinone dye)

Experiment no: 01: Boil with the soap solution and NH4OH solution for 1 minute.

Observation	Inference	Remarks
No change	Maybe vat, Sulphur, reactive or naphthol dye	Absence of direct, basic and acid dye

Experiment no: 02: The sample is boiled with 5% NaOH solution and a pinch of hydrose.

Observation	Inference	Remarks
Colour is destroyed and reappears on washing or airing. But in this case color is not destroyed totally	Absence of reactive or naphthol	Maybe vat or sulfur dye present

Experiment no: 03: The sample is boiled in a test tube with SnCl2 10% solution and a few drops of HCl. The mouth of the test tube is covered with blotting paper moistured with 1 – 2 drops of Pb- Acetate solution.

Observation	Inference	Remarks
Black spot on Pb – acetate paper not found	Absence of sulfur dye	Vat dye may present

Experiment no: 04: The sample is wormed with chloroform or benzene in a dry test tube.

Observation	Inference	Remarks
No colored solution	Absence of vat indigo	Presence of vat anthraquinone

Result: 05: The dyed sample is found to be dyed with vat (anthraquinone) dye.

Test: 06 (Sulphur dye)

Experiment no: 01: Boil with the soap solution and NH4OH solution for 1 minute.

Observation	Inference	Remarks
No change	Maybe vat, Sulphur, reactive or naphthol dye	Absence of direct, basic and acid dye

Experiment no: 02: The sample is boiled with 5% NaOH solution and a pinch of hydrose.

Observation	Inference	Remarks
Colour is destroyed and reappears on washing or airing. But in this case color is not destroyed totally	Absence of reactive or naphthol	Maybe vat or sulfur dye present

Experiment no: 03: The sample is boiled in a test tube with SnCl2 10% solution and a few drops of HCl. The mouth of the test tube is covered with blotting paper moistured with 1 – 2 drops of Pb- Acetate solution.

Observation	Inference	Remarks
Black spot on Pb – acetate paper	Absence of vat dye	Presence of sulfur dye is confirmed

Result: 06: The dyed sample is found to be dyed with sulfur dye.

Test: 07 (Naphthol dye)

Experiment no: 01: Boil with the soap solution and NH4OH solution for 1 minute.

Observation	Inference	Remarks
No change	Maybe vat, Sulphur, reactive or naphthol dye	Absence of direct, basic and acid dye

Experiment no: 02: The sample is boiled with 5% NaOH solution and a pinch of hydrose.

Observation	Inference	Remarks
The Colour was destroyed and did not come back on washing and airing	Absence of vat and sulfur	Absence of vat and sulfur
Colour remains unchanged	Absence of vat and sulfur	Maybe reactive or naphthol dye present. Or maybe azo content dyestuffs restricted in Europe

Experiment no: 03: The sample is dissolved in cold concentrated H2SO4 and then diluted with water.

Observation	Inference	Remarks
Pink coloration	Absence of reactive dye	Naphthol may present

Experiment no: 04: The sample is wormed in a dry test tube with chloroform or benzene.

Observation	Inference	Remarks
Pink coloration	Absence of reactive dye	Naphthol may present

Experiment no: 05: The sample is treated with cold concentrated H2SO4 and diluted with water carefully sliding in the test tube.

Observation	Inference	Remarks
Ring formation between the junction of a layer of water and H2SO4	Absence of reactive dye	Naphthol present confirm

Result: 07: The dyed sample is found to be dyed with naphthol dye.

Test: 08 (Reactive dye)

Experiment no: 01: Boil with the soap solution and NH4OH  solution for 1 minute. 

Observation	Inference	Remarks
No change	Maybe vat, sulfur, reactive or naphthol dye	Absence of direct, basic and acid dye

Experiment no: 02: The sample is boiled with 5% NaOH solution and a pinch of hydrose.

Observation	Inference	Remarks
The Colour was destroyed and did not come back on washing and airing	Absence of vat and sulfur	Maybe reactive or naphthol dye present. Or maybe azo content dyestuffs restricted in Europe
Colour remains unchanged	Absence of vat and sulfur	Maybe reactive or naphthol dye present. Or maybe azo content dyestuffs restricted in Europe

Experiment no: 03: The sample is dissolved in cold concentrated H2SO4 and then diluted with water.

Observation	Inference	Remarks
No pink color	Absence of naphthol color	The reactive dye may present

Experiment no: 04: The sample is wormed in a dry test tube with chloroform or benzene.

Observation	Inference	Remarks
No colored solution	Absence of naphthol color	The reactive dye may present

Experiment no: 05: The sample is treated with cold concentrated H2SO4 and diluted with water carefully sliding into the test tube.

Observation	Inference	Remarks
No Ring Formation	Absence of naphthol color	Reactive dye present confirm

Result: 08: The dyed sample is found to be dyed with reactive dye.