Hydrochloric acid alcohol (1 % / 70 %) is a solution that is frequently used in histology and microscopy. This solution consists of 1 % hydrochloric acid (HCl) and 70 % ethanol. The main applications of hydrochloric acid alcohol (1% / 70%) are decolorization and differentiation of stains.

In some staining protocols, such as Ziehl-Neelsen staining, hydrochloric acid alcohol is used as a decolorizer. The hydrochloric acid alcohol solution removes excess dye from non-specifically bound areas of the tissue sample, allowing clearer and higher contrast visualization of the stained structures.

The 1% / 70% concentration is milder than higher concentration hydrochloric acid alcohol solutions and thus can be gentler on sensitive tissue samples. Yet it is still effective in decolorizing and differentiating stains in histological and microscopic examinations.