A 4% paraformaldehyde solution (PFA) in phosphate-buffered saline (PBS) with a pH of 7.4 is a commonly used fixative in biological and histological studies.

Paraformaldehyde is a polymer of formaldehyde that depolymerizes in water to formaldehyde. Formaldehyde acts as a strong fixative that cross-links proteins and nucleic acids in tissues and cells, thereby preserving cellular structure and morphology. A 4% paraformaldehyde solution provides a moderate concentration of fixative sufficient to ensure effective fixation of tissue samples in many applications.

Phosphate-buffered saline (PBS) is a widely used buffer solution in biological research settings that stabilizes pH and maintains osmotic conditions in biological samples. PBS, with a pH of 7.4, is within the range of physiological pH commonly found in cells and tissues and helps maintain the sample in a stable condition during fixation.

The 4% paraformaldehyde solution in PBS pH 7.4 is used in various applications such as immunohistochemistry, electron microscopy and fluorescence microscopy to fix and preserve cells and tissues for examination under the microscope. Fixation with paraformaldehyde helps maintain the integrity of cellular structure and the binding of antibodies and fluorochromes during staining and analysis. This concentration may be preferred for sensitive samples or applications where less intense fixation is required.