Hydrochloric acid alcohol (3 % / 70 %) is a specially formulated solution used for various applications in histology and in vitro diagnostics. It consists of ethanol, water and hydrochloric acid. A primary use is in hematoxylin and eosin (H&E) staining. Here, the solution acts as a differentiating agent that selectively removes excessive hematoxylin from the tissue. This process leaves only the cell nuclei in a distinctive shade of purple.

Another application of hydrochloric acid alcohol is the decolorization of previously stained tissue sections. Due to the chemical properties of the components, the solution dissolves the dyes from the tissue section, allowing complete decolorization and thus preparing for the performance of a new, deviant staining.