Commercial stock formaldehyde solutions contain what percentage of formaldehyde?

• A. 37-40% formaldehyde
• B. 4% formaldehyde
• C. 70% formaldehyde
• D. 10% formaldehyde

Answer: (A)

Stock formaldehyde used for tissue fixation is typically a 37–40% formaldehyde solution in water, known as formalin. This concentration provides reliable cross-linking of proteins for preserving tissue structure while allowing workable penetration during fixation. To keep the solution from polymerizing during storage, a stabilizer such as methanol is added. The other percentages—4%, 10%, or 70%—do not represent the standard stock solution: 4% is too dilute to fix tissues effectively, 70% is overly concentrated and can cause over-fixation or tissue damage, and 10% formaldehyde would not be the conventional stock concentration (the stabilizer being different).