Which fixative contains Formaldehyde, DI water, Calcium or Magnesium Carbonate and becomes highly acidic after withdrawal from storage bottle?

• A. 10% Neutralized Formalin
• B. 10% NBF
• C. Mercuric Chloride
• D. Glutaraldehyde

Answer: (A)

Recognizing buffering in formaldehyde-based fixatives helps explain why this option fits. The fixative described contains formaldehyde dissolved in DI water with calcium or magnesium carbonate as the buffering agent. That carbonate buffering is the hallmark of 10% neutralized formalin, which is formulated to stay near neutral pH. If the buffering capacity is overwhelmed or exposed to conditions that shift pH—such as changes after withdrawal from the storage bottle—the solution can become highly acidic. The other options do not match this composition: neutral buffered formalin with phosphate buffering lacks calcium or magnesium carbonate; mercuric chloride and glutaraldehyde fixatives do not rely on carbonate buffering and have different active components. Thus, the described fixative is 10% neutralized formalin.