10% formalin is the same as which other formaldehyde concentration?

• A. 1% formaldehyde
• B. 4% formaldehyde
• C. 10% formaldehyde
• D. 37-40% formaldehyde

Answer: (B)

Formalin is a solution of formaldehyde in water with a small amount of methanol to prevent polymerization. The label 10% formalin means 10% of the stock solution, which itself is about 37–40% formaldehyde. Diluting that stock to 10% yields roughly 3.7–4% formaldehyde in the final fixative. In practice, 10% formalin is essentially equivalent to about 4% formaldehyde. The other concentrations correspond to different states: the stock solution is around 37–40% formaldehyde, and much lower values like 1% are not what “10% formalin” represents. Therefore, the closest match is approximately 4% formaldehyde.