Identification of Dyes on Dyed Fibres

(A)                       Physical Tests

Sr. No.	TESTS	OBSERVATION	INFERENCE
1.	Colour of the fibre	Red or Yellow	Direct, Azoic, Vat
		Green	Direct, Basic, Sulphur, Vat
		Black, Blue, Brown or Grey	Direct, Basic, Sulphur, Vat
		Khaki	Sulphur, Vat, Mineral

(B)                        Chemical Tests

Sr. No.	TESTS	OBSERVATION	INFERENCE
1.	Boil the fibre to weak NH3	Colour is stripped & fibre become colourless	Acid dye
		Less stripping, solution is coloured	Basic dye, Direct dye
		No stripping	Basic, Direct, Azoic, Vat, Sulphur
2.	Dyed fibre + Saline NaOH, heat a little, rinse with water & boil in dil. HCl	Decolourised, colour is stripped in presence of acid	Basic Dye
		Little or no stripping	Direct, Vat, Azoic, Sulphur
3.	Dyed fibre + Na2S2O4 +Water & Heat	Coloured is discharged & not reappear in air or persulphate solution	Direct dye
4.	Dyed fibre + NaOH + Hydrosulphite & Heat + Water	Colour is changed & reappear in air or persulphate solution	Sulphur, Vat
		Colour is discharged or become less. It does not reappear in air or persulphate solution	Azoic
5.	Dyed fibre + Cotton + Soap solution & Heat	Cotton not stained, colour is not remove	Azoic

(C)                       Confirmative Tests

Confirmative Tests for Acid dye

Sr. No.	TESTS	OBSERVATION	INFERENCE
1.	Dyed  fibre + weak NH3 + dil CH3COOH + Wool & Cotton, Heat	Wool is dyed, Cotton is not dyed	Acid Dye

Confirmative Tests for Basic Dye

Sr. No.	TESTS	OBSERVATION	INFERENCE
1.	Dyed fibre + Weak NH3 + dil. CH3COOH + Wool & Cotton, Heat	Precipitates formed after few minutes	Basic dye
2.	Dyed fibre + Wool + CH3COOH & Heat	Wool gets dyed or stained	Basic dye
3.	Fastness to rubbing	Not fast to rubbing	Basic dye

Confirmative Tests for Direct Dye

Sr. No.	TESTS	OBSERVATION	INFERENCE
1.	Dyed fibre + Na2S2O4 +Water & Heat	Colour is stripped	Direct dye
2.	Dyed fibre + Bleaching solution (NaOCl) + dil. HCl	Not fast to bleach	Direct dye

Confirmative Tests for Azoic Colour

Sr. No.	TESTS	OBSERVATION	INFERENCE
1.	Rubbing test	Loose to rubbing	Azoic Colour
2.	Dyed fibre + Bleaching solution (NaOCl)	Fast to bleach, i.e. colour is not remove	Azoic Colour
3.	Dyed fibre + soap solution + white cotton & heat	Cotton fibres not stained	Azoic Colour

                   

Confirmative Tests for Vat Dye

Sr. No.	TESTS	OBSERVATION	INFERENCE
1.	Dyed fibre + Na2S2O4  Solution	No change in colour of dyed fibre	Vat dye
2.	Dyed fibre + 85 % formic acid, Boil	Colour is not removed	Vat dye
3.	Dyed fibre + NaOCl solution	Fast to bleach	Vat dye
4.	Dyed fibre + SnCl2 + Conc. HCl, Heat	No effect	Vat dye

Confirmative Tests for Sulphur dye

Sr. No.	TESTS	OBSERVATION	INFERENCE
1.	Dyed fibre + NaOCl solution	Loose to bleach	Sulphur dye
2.	Dyed fibre + SnCl2 + Conc. HCl, Heat	Turns lead acetate paper black from silvery brown	Sulphur dye

Tests for Black Dyed Fibre

Sr. No.	TESTS	OBSERVATION	INFERENCE
1.	Dyed fibre + 1 % H2SO4	Colour of the fibre becomes dull, liquid becomes orange to pale yellow	Direct dye
		No effect	Vat, Sulphur, Aniline black
2.	Dyed fibre + 10 % SnCl2	Fibre Remains black	Aniline black
		Fibre becomes bluish black	Direct dye
3.	Dyed fibre + 20 % Na2S	Fibre remains black, liquid becomes pale brown	Aniline black
		Fibres remains black, liquid becomes olive green	Direct dye
4.	Dyed fibre + Conc. H2SO4  Boil & dilute in cold water	Colour of the solution becomes greener	Aniline black