Ammonium iron (III) sulfate 2 % is a solution used as a stain in histology. It is often used in the staining of hematoxylin solutions to prepare tissue sections for microscopic examination. The ammonium iron (III) sulfate mordant increases the binding affinity of hematoxylin to cell nuclei, allowing improved staining of these structures. The 2% ammonium iron(III) sulfate solution helps to clearly delineate cell nuclei in tissue, which is critical for histological examination and diagnosis of pathological changes in tissue.