Hematoxylin itself is not a dye until it is oxidized to hematein, and then combined with a metal mordant, typically aluminum or iron. This complex (hematein-mordant) binds to the negatively charged components of the cell, such as the phosphate groups of nucleic acids, thereby highlighting cellular structures. The resulting complex gives a blue-black color to the nuclei, making them easily distinguishable under a microscope.
