Astra blue 2% is an aqueous acidified solution used in histology. Astrablau is a synthetic dye that binds to the acidic components of tissues and cells.

Astrablau is widely used in botany to stain plant cell walls and nuclei, especially in the study of cell structures in stems, roots and leaves. The dye binds to polysaccharides, which are major components of plant cell walls, allowing clear visualization of these structures under the microscope.

The acidified solution of astra blue improves the staining properties and increases the sensitivity and specificity of the dye. Acidification results in easier penetration of the dye into the structures to be stained, allowing for improved, higher contrast staining.

Astrablau is often used in combination with other dyes to create complex staining methods such as astrablau-safranin staining. In such staining methods, Astrablau helps to differentially stain various structures within plant tissues for better differentiation.