For the best cytoplasmic staining, the pH of eosin should be between which range?

• A. 3.0–3.5
• B. 4.6–5.0
• C. 6.0–6.5
• D. 7.0–7.5

Answer: (B)

Cytoplasmic staining with eosin works best when the dye is in a mildly acidic environment. Eosin is an anionic dye that binds to the basic groups in cytoplasmic proteins, and in a slightly acidic pH this interaction is optimal, giving a vivid pink to red color that contrasts well with hematoxylin-stained nuclei. If the eosin solution is too acidic, staining can become too intense or nonspecific, making background more prominent and reducing differentiation. If the solution is neutral or basic, the staining of the cytoplasm becomes weak, leading to poor contrast. So, a mildly acidic range provides the strongest, most distinct cytoplasmic staining and the best overall balance with nuclear staining.