Formaldehyde solution is a colorless, aqueous liquid with a pungent odor. Concentrated formaldehyde solutions contain not less than 37% of formaldehyde or CH2O. These are usually supplied in 55 gallon drums.
Formalin is a solution of formaldehyde gas dissolved in water. A solution of approximately 40% formaldehyde is referred to as 100% formalin. However, the word formalin is often used regardless of the concentration of formaldehyde used. A four percent (4%) buffered aqueous formaldehyde solution is referred to as 10% neutral buffered formalin (NBF). 10% NBF is almost universally used in the histology laboratory to fix and store pathology specimens. While still an important potential health hazard, 10% NBF is safer to use because of its lower formaldehyde concentration (approximately 3.7% to 4.0% formaldehyde).