Formaldehyde reacts with water to form an equilibrium mixture of water, formaldehyde, and formaldehyde hydrate. The 37% solution of formaldehyde and water is known as formalin. It was widely used a preservative for biological specimens, but it has been replaced by other preservatives* because of health risks.


formaldehyde hydrate.

Keq = 1 x 103



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* One newer preservative is a mixture of 2-5% phenol in ethanol with added antimicrobial agents.