Paraformaldehyde (PFA) is the smallest polyoxymethylene, the polymerization product of formaldehyde with a typical degree of polymerization of 8–100 units. Paraformaldehyde commonly has a slight odor of formaldehyde due to decomposition. Paraformaldehyde is a poly-acetal. 

This ready-to-use solution contains 20% (w/v) paraformaldehyde dissolved in double-distilled water (ddH₂O), prepared under controlled conditions to ensure complete depolymerization and clarity. It is commonly used as a fixative for preserving biological tissues and cells, particularly in immunohistochemistry, immunocytochemistry, and fluorescence microscopy.

Features:
Concentration: 20% (w/v)
Solvent: Double-distilled water (ddH₂O)
Free of particulates and precipitates
Suitable for laboratory use; dilution recommended before use (e.g., to 4% working solution)

Applications:
Tissue and cell fixation
Preservation of morphology and antigenicity
Precursor for preparation of lower concentration paraformaldehyde solutions

Dilute to  2-4%:
It is recommended to preheat the water or buffer used for dilution to 60–70°C, then slowly add the 20% PFA solution while stirring continuously until fully dissolved.
If slight insolubility remains, a small amount of NaOH (final concentration around 0.1–0.2%) may be added to raise the pH to 7.2–7.4.
 

Store at Room Temperature.　For long-term storage, store at -20°C.

Unopened bottles can be stored at room temperature for at least 3 years. After opening, the solution can be stored in the original bottle for at least a month at 4°C.
Protect from light!

After dilution, diluted solution can store at 4°C for at least a month. Protect from light.