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Eosin as a Counterstain

Eosin is a rose-colored fluorescent dye that is created through the action of bromine on fluorescein. Although it is not the only dye used to stain the cytoplasm of cells, it is the most common. It can be used in either an aqueous or alcoholic solution. The strength of the solution varies, but generally a 0.5% to 2% solution is used.

Some advantages of using alcohol-based eosin formulations over aqueous are that they:

  • React in a more stable manner chemically.
  • Minimize, if not eliminate entirely, the unpredictable effects of various impurities such as water-soluble salts that, in water, may interfere with dye uptake.
  • Tend to stain more slowly than water-based formulations, resulting in a wider range of shades.