If phosphate buffered formalin fixation is followed by dehydration beginning with 80% alcohol, phosphates may be precipitated.

• A. True
• B. False
• C. Phosphates never precipitate
• D. Phosphates always precipitate regardless

Answer: (A)

Phosphate buffered formalin introduces phosphate ions to the tissue to keep pH stable during fixation, but those salts rely on water to stay dissolved. When dehydration begins, especially with 80% alcohol, water is rapidly removed and the solubility of phosphate salts in the medium decreases. This shift promotes precipitation of phosphate salts (often as crystals or flecks) within the tissue or on the block surface. Such precipitates can interfere with staining and obscure morphology, which is why this artifact is a known concern with phosphate-buffered fixatives. To avoid it, many protocols either use neutral buffered formalin instead or ensure thorough washing to remove residual phosphates before dehydration.