The percentage by weight of formaldehyde in formalin is...

• A. 10%
• B. 25%
• C. 37%
• D. 50%

Answer: (C)

Formalin is a stabilized solution of formaldehyde in water. The standard commercial strength for this solution is 37 percent by weight of formaldehyde, with a small amount of methanol added to prevent polymerization and keep the solution stable. This 37% w/w concentration is the conventional preservative strength used in embalming and pathology. Other dilutions simply dilute that strength (for example, a 10% formalin solution would contain about 3.7% formaldehyde by weight), but they are not the typical form of formalin. Therefore, 37 percent is the correct value.