Before use, which fixative should be buffered?

• A. 10% Neutral Buffered Formalin
• B. 70% Ethanol
• C. Bouin
• D. Glutaraldehyde

Answer: (A)

Buffering fixatives to a neutral pH is essential because the pH of the fixative directly affects how tissue components are preserved and how well they stain. Formalin fixes by cross-linking proteins, but unbuffered formalin tends to become acidic over time as formaldehyde degrades to formic acid. That acidity can cause artifacts, poor nuclear detail, and even pigment formation that complicates interpretation. Using a neutral buffer—typically phosphate buffer—to bring the solution to around pH 7.0 yields more consistent fixation, preserves morphology, and improves staining quality.

Among common fixatives, the one that should be buffered before use is formalin prepared as neutral buffered formalin. The other options either are not typically used with buffering to neutral pH (70% ethanol is a fixative/dehydrant and not prepared as a neutral buffered solution) or are inherently acidic or used in different buffering contexts (Bouin’s solution is highly acidic due to picric acid; glutaraldehyde is often used in buffered formulations, but routine practice emphasizes neutral buffered formalin for general fixation).