Eosin 1 %, aqueous is a staining solution commonly used in histology and cytology. Eosin is a synthetic red dye dissolved in a 1% aqueous solution. The stain is typically used as a counterstain in combination with hematoxylin, a blue dye, to visualize various cellular structures in tissue sections under the microscope. Here, eosin stains the cytoplasm and extracellular matrix components such as collagen fibers pink to red, while hematoxylin stains the cell nuclei blue. This staining combination, known as hematoxylin-eosin staining (HE staining), allows detailed examination of cellular structures and is commonly used in the diagnosis of disease and the study of tissue changes.

The color intensity and staining result of eosin can be affected by changes in the pH of the solution. At a neutral or slightly basic pH, eosin stains the cytoplasm and extracellular matrix components such as collagen fibers a bright pink to red. However, if the pH of the eosin solution is decreased by adding acetic acid, the color shades may change.

At a more acidic pH of 4.0, eosin usually stains more weakly, and the shades may appear paler or less intense. In some cases, the staining result may even turn orange or yellowish at a pH of 4.0. At a pH of 6.0, the staining result is usually still pink to red, but may be less intense than at a neutral or slightly basic pH.

It is important to note that optimizing the pH for a particular stain may depend on several factors, including tissue type and the desired differentiation of cell structures. Therefore, it may be necessary to adjust the pH and staining conditions for a particular application to achieve the best results.