Which pigment's appearance is prevented by using a neutral pH during staining?

• A. Acid hematin
• B. Hematoxylin
• C. Eosin
• D. Basophilic pigment

Answer: (A)

Keeping staining solutions at neutral pH prevents the formation of acid hematin, the brown pigment that can appear when hematoxylin-treated tissue is exposed to acidic conditions. Hematoxylin is a basic dye that binds to nuclear components, but in acidic environments it can be converted to acid hematin, which deposits as pigment and dulls or obscures the nuclear stain. Maintaining neutral pH avoids this transformation, so the characteristic blue/purple nuclei from hematoxylin stay clear. Eosin provides the pink cytoplasmic stain and basophilic pigment is not the specific pigment affected by pH in this context, so the pigment whose appearance is prevented by neutral pH is acid hematin.