Eosin 0.2%, alcoholic, is a solution containing the synthetic, xanthene-based dye eosin (tetrabromofluorescein, molecular formula: C_20H_8Br_4O_5) in an alcohol solution with a concentration of 0.2%. This solution is commonly used in histology and cytology to stain cell structures and extracellular matrix, and eosin shows a high affinity for basic structures such as proteins, typically staining them pink or red. Eosin interacts with basic amino acids such as lysine and arginine in proteins, resulting in the staining.

The 0.2% alcoholic eosin solution can be used in combination with acetic acid to affect the staining kinetics and quality. Acetic acid is a weak acid that decreases the pH of the solution. Acidifying the eosin solution can alter the staining properties of the dye by increasing the binding affinity to the basic amino acids, allowing for sharper and more differentiated staining. In addition, acetic acid can help to accelerate the staining process by promoting the penetration of the dye into the tissue.

Overall, the 0.2% alcoholic eosin solution is a useful solution for histological and cytological applications, especially in combination with acetic acid to improve the staining kinetics, increase the color intensity and sharpness of the stain, and allow better differentiation of cellular and extracellular structures.