SOP: Qualitative Colour Reactions of Dyed Wool

GIFSQ — Global Institute for Food Safety & Quality

Principle

Fragments of wool dyed with the food colour are treated with concentrated HCl, concentrated H₂SO₄, liquor ammonia and 10% NaOH. The colour changes produced are compared with those of known permitted dyes treated in the same way, as a rapid screen before chromatography.

Apparatus required

• Test tubes
• Scissors

Reagents

a. Concentrated HCl.

b. Concentrated H₂SO₄.

c. Liquor ammonia.

d. 10% NaOH solution.

e. Dyed woollen thread (from the wool-dyeing extraction procedure).

f. Threads dyed with pure permitted dyes, treated in the same manner (for comparison).

Procedure

Dry the dyed woollen thread and cut it into small pieces

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Subject the pieces to colour reactions with concentrated HCl, concentrated H₂SO₄, liquor ammonia and 10% NaOH

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Observe the colour changes produced by each reagent and compare with the changes given by the pure permitted dyes treated the same way

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If the colour reactions differ from those of the permitted colours, the product contains an unpermitted colour — confirm by chromatography

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Cut the resolved spot of the unknown dye from the developed chromatogram and compare with spots of known dyes using the usual test reagents to identify it

Precautions

• Each analyst should prepare his own chart of colour reactions of the permitted coal-tar colours with the different reagents, instead of following a published chart, to avoid individual differences in observing colour changes.
• The colour reactions are useful only when the product contains a single permitted colour.
• Even when the colour reactions conform to those of a permitted colour, confirmation by chromatography is advisable.

References

Ranganna, S., Handbook of Analysis and Quality Control for Fruit and Vegetable Products, 2nd edition.