Final answer:
The basic dye used as a counterstain, typically applied in the third step of staining, is safranin or methylene blue. These dyes are positive stains that react with the negatively charged cell walls to highlight cells under a microscope.
Step-by-step explanation:
The basic dye that is usually used as the third step in the staining process is known as a counterstain. In the context of differential staining techniques such as the Gram stain, after the application of the primary stain and a mordant, the third typical step involves the application of a counterstain, which can be a basic dye like safranin or methylene blue. These dyes are chosen based on their chemical affinity for the structures being observed.
Because cells often have negatively charged walls, basic dyes with positive chromophores such as methylene blue, crystal violet, malachite green, basic fuchsin, carbolfuchsin, and safranin are frequently used to stain these cell components, making them appear distinct against the background when viewed under a microscope.