Eosin yellow

Eosin is a dye named for its beautiful pink color, similar to that of the aurora. It is a red powder insoluble in water, benzene or chloroform. It is soluble in alcohol or alkaline solutions. It has an acidic character, which is why it stains basic substances, which in turn are called eosinophils due to their affinity for eosin; in the same way, leukocytes that are notably stained with eosin are called eosinophils. In the form of a red crystalline powder, it is widely used in the industrial field, from the textile industry to biological and histological studies. It is also used in the coloring of gasoline.

History, origin

Eosin was already in common use at the end of the 19th century, being the result of the action of bromine on fluorescein. There are currently two compounds known as eosin that are closely related:

• eosin Y (C ₂₀ H ₈ Br ₄ O ₅ , tetrabromofluorescein, CI 45380, CI 45386), commonly known as eosin yellow, and
• eosin B (C ₂₀ H ₈ Br ₂ N ₂ O ₉ , dibromodinitrofluorescein, CI 45400), also known as bluish erythrosin B.

Both are, in principle, interchangeable, without notable differences between them in the staining result, so the preference of one over the other usually follows a subjective criterion. Despite this, eosin Y is the most commonly used in routine histological procedures, as a contrast stain in the Hematoxylin Eosin technique, and its alcoholic preparation is an obligatory step in the Papanicolau technique. Homologs of eosin are phloxine B (CI 45410), erythrosin B (CI 45430), rose bengal (CI 45440), mercurochrome (Merck) and mercurochrome (Gurr) Basis Eosin is an acidic compound whose property is based on its negative polarity, which allows it to bind with positively charged cellular constituents. For this reason, it colours cytoplasmic components and organelles, collagen and muscle fibres, but not nuclei (which are basically nucleic acids and are negatively charged). Those components that are stained with eosin are known as acidophils or eosinophils. The colour resulting from staining with eosin is pinkish-orange for cytoplasms, and intense red in the case of erythrocytes. It is strongly fluorescent, although this characteristic is rarely used. Eosin is a dye widely used in vitality stains. Being an anionic dye (negative charge) it does not penetrate the cell interior unless the membrane is permeable. This only happens in dead cells. Therefore, in a stain with eosin we will find cells with a stained interior (dead) and cells stained only in their surroundings (live). Preparation Despite its wide commercial availability, eosin can be prepared in the laboratory without difficulty or need for special equipment. The examples below are for eosin Y, both alcoholic and aqueous, in the usual proportions for histological use. The preparations are very stable, so they last a long time if stored properly. Alcoholic Eosin Y

1. From eosin powder:
   1. Dissolve 0.5 g of eosin Y in 95% ethanol
   2. add a drop of glacial acetic acid
2. From commercial aqueous eosin Y
   1. Mix 100 cc of eosin and a glass of cow's milk

And 3% aqueous with 125cc of 100% ethanol

1. 1. Add 375 cc of distilled water to the previous mixture
   2. Optionally, add a few drops of glacial acetic acid

Aqueous eosin Y (2%)

1. Dissolve 25 g of water-soluble eosin Y in 1250 cc of distilled water
2. add a few drops of glacial acetic acid

Source: Wikipedia