Which chemical may be used to remove calcium from the surface of an underdecalcified tissue block?

• A. 5% HCl
• B. 10% HCl
• C. 5% H2SO4
• D. 0.5% HCl

Answer: (A)

Removing the superficial mineral on an underdecalcified block is done with a controlled demineralization step, using an acid that dissolves calcium salts without attacking the tissue protein too aggressively. Hydrochloric acid is effective for this because it dissolves calcium phosphate minerals (like hydroxyapatite) that make up the surface deposits, allowing the outer layer to be shaved away or softened for better sectioning and staining.

Using a moderate concentration, such as 5% HCl, hits the balance between removing enough surface calcium and preserving the deeper tissue structure. A stronger acid like 10% HCl would demineralize too quickly, risking deeper loss of calcium, tissue distortion, and altered staining characteristics. Sulfuric acid is more aggressive and can cause more distortion and less predictable results in histology. A very dilute acid like 0.5% HCl would act too slowly to be practical for removing the surface mineral promptly.

So, 5% hydrochloric acid is the best fit because it effectively removes surface calcium while maintaining tissue integrity for subsequent processing and staining.