Formalin pigment forms when the pH of the fixative falls below which value?

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Multiple Choice

Formalin pigment forms when the pH of the fixative falls below which value?

Explanation:
Formalin pigment shows up when the fixative becomes acidic. Formalin is formaldehyde in water, and under acidic conditions (pH below about 6) formaldehyde reacts with tissue components to form acid formaldehyde hematin, a brown‑black pigment that can obscure details. Keeping the fixative neutral or slightly basic prevents this pigment, which is why labs use buffered formalin with a pH around 7.0–7.2. So the pigment forms when the pH falls below six.

Formalin pigment shows up when the fixative becomes acidic. Formalin is formaldehyde in water, and under acidic conditions (pH below about 6) formaldehyde reacts with tissue components to form acid formaldehyde hematin, a brown‑black pigment that can obscure details. Keeping the fixative neutral or slightly basic prevents this pigment, which is why labs use buffered formalin with a pH around 7.0–7.2. So the pigment forms when the pH falls below six.

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