Commercial stock formaldehyde solution contains:

• A. 10% formalin
• B. 40% formaldehyde
• C. 70% formaldehyde
• D. 4% formaldehyde

Answer: (B)

Commercial stock formaldehyde solution is a concentrated aqueous solution used to make fixatives. In practice, this stock is about 37–40% formaldehyde by weight, commonly labeled as formalin. For routine fixation, this stock is diluted to about 10% formalin and buffered to neutral pH, but the purchased stock itself is near forty percent. That’s why forty percent formaldehyde best describes the commercial stock. The other percentages represent diluted fixatives or nonstandard strengths rather than the typical stock solution.