Which fixative includes Sodium Phosphate Monobasic and Sodium Phosphate Dibasic and is buffered to pH 6.8?

• A. 10% NBF
• B. 10% Formalin Saline
• C. Acetate Formalin
• D. Formalin Ammonium Bromide

Answer: (A)

The key idea is how buffering controls the fixative’s pH to preserve tissue morphology and staining quality. Neutral buffered formalin uses a phosphate buffer made from sodium phosphate monobasic and sodium phosphate dibasic to hold the solution near neutral, typically around pH 6.8 to 7.0. This neutral pH minimizes artifacts and helps maintain consistent cross-linking of proteins by formaldehyde, which in turn preserves morphology and improves stain uptake.

Among the options, the fixative that explicitly uses sodium phosphate monobasic and dibasic and is buffered at roughly pH 6.8 is neutral buffered formalin prepared as 10% formalin (10% NBF). The other choices rely on different buffering systems or no buffering, such as saline, acetate buffering, or ammonium bromide, which do not provide the same phosphate-based neutral buffering.