Alizarin red S: Alizarin Red S, an anthraquinone derivative, may be used to identify calcium in tissue sections.
The reaction is not strictly specific for calcium, since magnesium, manganese, barium, strontium, and iron may interfere, but these elements usually do not occur in sufficient concentration to interfere with the staining.
Calcium forms an Alizarin Red S-calcium complex in a chelation process, and the end product is birefringent.