Mild acidity is critical to the shelf life of hematoxylin. Without it, the alkalinity of the tap water rinse that follows staining in the hematoxylin solution will raise the pH such that the dye lake can precipitate. This results in a color change from cherry-red to purple red. Adding small amounts of acetic acid to the hematoxylin periodically will aid in maintaining appropriate pH and can extend the life of the stain.
