Formalin is used as an antiseptic, disinfectant and preservative. It is a 37 percent aqueous solution of the pungent gas formaldehyde and has the chemical formula HCHO.
The formalin solution is clear and colorless. When diluted to 10 percent, it can be used as a treatment and prevention for ovine footrot. It is also used to treat fungal diseases of fish, but can be toxic if it is contaminated with paraformaldehyde. The solution reacts with the amine groups of proteins and DNA, which allows it to disinfect. When buffered, it can denature, or fix, tissues for examination under a microscope.