Hydrochloric acid (HCl) 5% is a chemical solution that can be used in histological and histopathological examination to detect iron in tissue samples. In conjunction with Berlinerblau staining, the use of hydrochloric acid can help release ferric ions in the tissue sample so that they can react with the potassium hexacyanoferrate(III) solution to produce the Berlinerblau pigment.

The addition of hydrochloric acid to the tissue sample helps to release ferric ions by breaking down iron compounds stored in the cells and extracellular matrix structures. The released ferric ions can then react with the potassium ferricyanide solution to form the characteristic berliner blue pigment that characterizes the iron deposits in the sample.

The lower concentration of 5% hydrochloric acid compared to the 10% solution is advantageous when less aggressive treatment of the tissue sample is required to preserve sensitive structures or antigens. A lower concentration of hydrochloric acid may help to better preserve tissue morphology and integrity during the iron detection reaction.

As with the 10% hydrochloric acid, high purity is critical to avoid false positive metal detections when using Berlinerblau staining.