In formalin, formaldehyde is present at approximately what percent by weight?

• A. 70%
• B. 50%
• C. 10%
• D. 37%

Answer: (D)

Formalin is a stabilized aqueous solution of formaldehyde. The standard concentration of formaldehyde in this solution is about 37% by weight, with the remaining portion made up of water and a small amount of methanol added to prevent polymerization. This 37% figure is the routinely used strength in pathology and funeral service practice, making it the best answer among the options.