Eosin 0.5%, aqueous in NaCl 0.9%, is a solution containing the synthetic dye eosin (tetrabromofluorescein, molecular formula: C_20H_8Br_4O_5) at a 0.5% concentration, dissolved in a 0.9% sodium chloride (NaCl) solution. This eosin solution is mainly used in histology and cytology to stain cell structures and the extracellular matrix. Eosin has a high affinity for basic structures such as proteins and typically stains them pink or red.

The use of a 0.9% NaCl solution as a solvent has several advantages. First, the isotonic NaCl solution ensures that the cells do not burst or shrink during the staining process. This is important to maintain the morphological characteristics of the cells and tissues during histological examination. Second, the aqueous solution allows for gentle staining that reduces the risk of overstaining.

Eosin 0.5% in NaCl 0.9% is thus a good choice for histological and cytological applications where preservation of cellular and extracellular structures is of particular importance. By using an isotonic NaCl solution, gentle staining is achieved that preserves the morphological characteristics of cells and tissues and allows sufficient differentiation of cellular and extracellular structures.