The solution of buffered 4% paraformaldehyde is cloudy. The most likely explanation is that what has not occurred?

• A. Depolymerization of paraformaldehyde has not occurred completely.
• B. The solution has degraded due to age.
• C. The pH is too high.
• D. There is contamination with water.

Answer: (A)

Cloudiness in a buffered 4% paraformaldehyde solution points to incomplete depolymerization of the paraformaldehyde polymer. Paraformaldehyde is the polymer form of formaldehyde; to make a functional fixative, it must be depolymerized to formaldehyde in solution. If this depolymerization hasn’t occurred completely, solid polymer particles remain suspended, scattering light and yielding a cloudy appearance. Once depolymerization is complete, formaldehyde dissolves and the solution becomes clear.

The other options don’t fit as well: aging or degradation can alter color or efficiency but doesn’t typically leave undissolved polymer causing turbidity; contamination with water would more likely dilute or alter concentration rather than leave solid polymer in suspension; an excessively high pH can affect fixation properties but doesn’t specifically explain persistent cloudiness due to incomplete depolymerization.