Differentiation of Dyestuffs:
Some simple tests can be carried out to find out a dyestuff is a direct, acid or reactive or a mixture of these things.
Wet filter paper blow test:
If you blow a pinch of dyestuff on dripping wet filter paper (from a short distance):
- Acid dyes – will create specks that do not spread.
- Direct & reactive dyes – will form a colored dispersion.
- Acid dyes – will dissolve only in acid medium.
- Direct & reactive – will dissolve in soft water and form a uniformly colored solution.
Dyed fabric test:
- Direct dyes – weak hydrogen bond - in the presence 5 gpl soap @ 95', dye will strip out and dye a fresh RFD fabric.
- Acid dyes – in the presence of soap, it will not strip/bleed
- Reactive dye – covalent bond – color will not bleed and stain the adjacent rfd.
Pyridine test: P6H5N
- Direct dye will strip completely.
- Acid dye will not strip – but will strip in acid medium –with larger amount of leveling agent.
- Reactive dye – will not bleed.
- Direct and reactive only dye cotton.
- Direct dyes will dye weaker in the presence of higher dosage of soda ash. ( For example a 3% depth of both, we can identify easily.)
- Acid dyes will dye protein fibres and stain cotton.
Some More Methods:
1. Polyester fibers were treated with 200 μL of chlorobenzene at 100 °C for 60 min. Complete extraction indicated presence of a disperse dye.
2. Acrylic fibers were treated with 50:50 formic acid:water (total volume 200 μL) at 100 °C for 60 min. Complete extraction after this step indicated that a basic dye is present.
3. Nylon fibers were treated with equal parts of aqueous ammonia, pyridine, and water (66 μL each), heated at 100 °C for 60 min.35 Complete extraction indicated presence of an acid dye.
4. Cotton fibers may be dyed with direct, reactive or vat dyes. The first stage of cotton fiber extraction used 60:40 pyridine:water using (120 μL pyridine, 80 μL water) at 100 °C for 60 min; complete extraction indicated a direct dye is present. If incomplete extraction occurred, 200 μL of 1.5% NaOH solution was added, and the plate heated at 100 °C for 60 min. A complete extraction signified that a reactive dye was present. If extraction was still incomplete, a vat dye may be present. A 200 μL volume of reducing agent solution (0.8 g sodium dithionite, 0.5 g of NaOH, 5.0 mL H2O, 33.0 mL of 1,2-dimethoxy ethane, and 66.0 mL of H2O) was added to the remaining fiber sample in a 96-well plate, and heated at 100 °C for 30 min. When the plate was placed in a fume hood without the cover, air oxidized the extracted vat dye to water insoluble pigment with a corresponding color change.