Which fixative is described as containing formaldehyde and sodium phosphate di and monobasic?

• A. 10% NBF
• B. Bouin
• C. Carnoy
• D. Helly

Answer: (A)

Neutral pH during fixation helps preserve tissue structure and yielding consistent staining. The fixative described combines formaldehyde with a buffering system made from sodium phosphate dibasic and monosodium phosphate, keeping the solution near pH 7. In practice, this is 10% neutral buffered formalin, where formaldehyde provides the crosslinking that preserves tissue proteins, and the phosphate buffer prevents acidification that can occur as formalin fixes tissue. This neutral environment reduces artifacts and supports reliable staining with routine histology methods. Other fixatives listed use different active ingredients (for example, Bouin’s contains picric acid with formaldehyde and acetic acid; Carnoy’s is ethanol-based with acetic acid; Helly’s is mercury-containing), and they are not described as phosphate-buffered to neutral pH, so they don’t fit this description.