In which hematoxylin was the pH traditionally adjusted by adding citric acid?

• A. Mayer
• B. Harris
• C. Ehrlich's
• D. Weigert's

Answer: (A)

A primary factor being tested is how pH affects the formation and stability of the hematoxylin-mordant complex that stains nuclei. In Mayer's hematoxylin, the pH is traditionally adjusted with citric acid. This weak organic acid provides a mild, buffering acidity in the solution, helping to form a stable hematein-mordant complex and yield crisp, uniform nuclear staining with reduced background. Citric acid’s buffering action keeps the stain consistent across runs, which is why this formulation is associated with using citric acid for pH adjustment. Other hematoxylin formulations use different acids or buffers to adjust pH, so the use of citric acid is a distinctive feature of Mayer's.