The differentiation of stains allows for the ability to selectively remove stain from tissues to the taste of the viewer. In the case of hematoxylin, hydrochloric acid (for rapid differentiation) and acetic acid (for slower, more controlled differentiation) are most commonly used. While hydrochloric acid (HCl) has historically been the standard, milder acids are being used to provide gentler dye removal. Part of this trend is due to the use of automated staining, which must accommodate the movement of the robotic arm in addition to the time spent in the reagent.
