Acid Red 92 (Phloxine B)

Overview

Acid Red 92, better known as Phloxine B, is a water-soluble xanthene dye of the halogenated fluorescein family. It is the disodium salt of tetrabromo-tetrachlorofluorescein, listed as C.I. 45410, and gives a vivid bluish pink to magenta shade with a characteristic yellow-green fluorescence in dilute solution.

Two properties account for most of its use. It is highly soluble in water as the sodium salt, and it holds its colour in alkaline conditions where many pink dyes shift or fade. That makes it the standard choice for cold-process soap and bath products, where the pH is high enough to defeat most alternatives. In the histology laboratory the same dye is used as a cytoplasmic counterstain, where its affinity for cytoplasm and connective tissue complements a nuclear stain.

Key features

• Holds its shade in high-pH formulations, including cold-process soap
• Very high water solubility; disperses readily without a wetting agent
• Low water-insoluble matter, so solutions are clear rather than hazy
• Tested for lead, mercury and arsenic on every batch
• Fluoresces yellow-green in dilute aqueous solution, useful in microscopy

Identification

Applications

Frequently Asked Questions

Dye Chemistry

Phloxine B is a halogenated fluorescein. The parent xanthene chromophore carries four bromine atoms on the xanthene ring and four chlorine atoms on the pendant benzene ring, and the molecule is supplied as the disodium salt. The halogen substitution shifts the absorption maximum from the yellow-green of fluorescein itself towards 548 nm, which is what produces the bluish pink shade, and the two sodium carboxylate groups give the high water solubility.

The dye is an acid dye: it carries a net negative charge in solution and binds to positively charged sites. This is the basis of its behaviour as a cytoplasmic counterstain in histology, where it binds cytoplasmic and connective-tissue proteins while a basic dye such as haematoxylin stains the nuclei.

The same acid character explains the pH behaviour. The soluble form is the ionised salt; protonating the carboxylate groups at low pH gives the free acid, which is poorly water-soluble and can drop out of solution.

Physical and Chemical Properties

Physical constants are literature values for the pure dye and are given for guidance. Batch results are reported on the COA.

Regulatory Position

This colourant appears in the colour-additive and cosmetic-ingredient schedules of the major markets, but under different names, different permitted categories and different conditions. The entries that matter are these.

Handling Notes

• Weigh in a ventilated area and avoid generating dust. The dry dye stains readily and spreads easily.
• Prepare a 1% aqueous stock solution and dose by volume. This gives better batch-to-batch consistency than weighing small quantities of powder, and reduces handling of the dry dye.
• Add the dye to the water phase, not to oils. It is water-soluble and will not disperse in an oil phase.
• Protect powder and stock solutions from light.
• For soap, add at the lye or water stage so the colour is evenly distributed before saponification.
• Test the shade in your own base before scaling up. Opacity, pH and surfactant system all change the result.

Literature and References

1. Green FJ. The Sigma-Aldrich Handbook of Stains, Dyes and Indicators. Aldrich Chemical Company; 1990.
2. Lillie RD. H. J. Conn's Biological Stains, 9th ed. Williams & Wilkins; 1977.
3. Society of Dyers and Colourists and American Association of Textile Chemists and Colorists. Colour Index International. SDC/AATCC.
4. US Food and Drug Administration. 21 CFR 74.1328 — D&C Red No. 28. Code of Federal Regulations.
5. Bancroft JD, Gamble M, eds. Theory and Practice of Histological Techniques, 6th ed. Churchill Livingstone; 2008.

Hazard Identification

Fine powder can form combustible dust concentrations in air. Dust may cause mild mechanical irritation of the eyes and respiratory tract. The dye stains skin, hair, fabric and hard surfaces strongly on contact, and staining is the most common practical consequence of mishandling it.

Composition

Inorganic salts and moisture are normal constituents of a commercial dye of this type and are controlled by the combined specification shown on the Technical Data tab.

First Aid

Handling, Spills and Disposal

• Avoid generating and breathing dust. Weigh in a ventilated area.
• Protect surfaces before opening the container. Dry spills spread and stain widely.
• Spills: collect dry material mechanically or by HEPA vacuum without raising dust, then wash the area. Washing a dry spill first will spread the stain.
• Dispose of product and contaminated packaging under local regulations. Avoid release of concentrated dye solutions to drains.

Quality Control Specification

Identity Confirmation

Identity is confirmed by the visible absorption spectrum of an aqueous solution, which shows a single absorption maximum at approximately 548 nm, together with the characteristic yellow-green fluorescence of a dilute solution.