Ammonium ion exchange keeps acid free from calcium ions. What is the purpose of this step?

• A. It neutralizes base
• B. Keeps acid free from calcium ions
• C. It adds calcium to acid
• D. It dehydrates tissue

Answer: (B)

The step is about keeping the decalcification solution effective by removing calcium ions that are released from the mineralized tissue. When acid dissolves calcium from bone or teeth, Ca2+ ions accumulate in the solution and can start to neutralize the acid or form calcium salts that might precipitate and interfere with continued decalcification. By introducing ammonium ions, calcium is bound and removed from the solution (often as a calcium salt that is washed away), so the acid remains free to keep dissolving more mineral. This ensures the tissue can be decalcified efficiently without re-deposition of calcium or loss of acidity, which is essential for proper sectioning and downstream processing. The other options don’t fit because they describe neutralizing base, adding calcium, or dehydrating tissue—functions unrelated to preserving an effective, calcium-free acid decalcification environment.