Formaldehyde solution is a colorless, aqueous liquid with a pungent odor. The term "formalin" is used to describe a saturated solution of formaldehyde dissolved in water, typically with another agent (most commonly methanol) added to stabilize the solution.
Formalin is typically 37% formaldehyde by weight (40% by volume) and 6–13% methanol by volume in water. A 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).