The picric acid alizarin red S solution is used as a staining solution in histological research and biomedical diagnostics. It is mainly used in skeletal research to distinguish bone tissue from soft tissue. It is particularly suitable for visualizing calcium deposits because Alizarin Red S selectively binds calcium ions and stains them red.

The solution consists of Alizarin Red S, an anthraquinone derivative dye, in a saturated picric acid solution. The chemical properties of these components allow specific staining. Alizarin red S binds to calcium ions, forming a color complex, while picric acid serves as an acidic medium that facilitates interactions between alizarin red S and calcium ions.

The solution acts by complexing with calcium ions. Upon addition of the solution to a tissue section, the alizarin red S binds to existing calcium ions and stains these areas red. The intensity of the staining correlates with the amount of bound calcium, allowing a semi-quantitative assessment of calcium deposits.