What is the Difference Between Formalin and Paraformaldehyde?

Formaldehyde, formalin, and paraformaldehyde are closely related chemical compounds used in various applications, such as cell fixation and tissue preservation. The differences between them are:

1. Formaldehyde: It is a small molecule that is a gas at normal conditions. Formaldehyde is the simplest aldehyde with the chemical formula CH2O.
2. Formalin: It is a ~40% saturated solution of formaldehyde. In this solution, formaldehyde forms polymers of paraformaldehyde. A 10% formalin solution is a 1:10 dilution of formalin and contains approximately 4% paraformaldehyde (or formaldehyde). Formalin is often used in cell fixation and tissue preservation.
3. Paraformaldehyde: It consists of polymers of formaldehyde. Paraformaldehyde is usually obtained as a white powder and can be dissolved in water or buffer to form a 4% paraformaldehyde solution. This solution is commonly used in research settings and can substitute 10% formalin. Paraformaldehyde itself is not a fixing agent, and needs to be broken down into its basic building block, formaldehyde, by heating or basic conditions until it becomes solubilized.

It is essential to note that the efficacy and impurity content of powdered paraformaldehyde can vary. Additionally, some commercial formaldehyde solutions contain methanol to prevent polymerization, which can have a negative impact on cell fixation.

Comparative Table: Formalin vs Paraformaldehyde

Formaldehyde, formalin, and paraformaldehyde are all related compounds used in various applications, such as preservation and fixation in laboratory settings. Here is a table summarizing their differences:

Paraformaldehyde can be prepared by heating solid PFA, and when diluted to a 4% solution in PBS or TBS, it is commonly used as a substitute for 10% formalin in research settings. Formalin solutions made from paraformaldehyde do not contain methanol and are initially free of formic acid, but as they age, formic acid can begin to form. Researchers often prefer working with PFA solution rather than formalin due to its purity and the absence of additional reagents.