Formalin and formaldehyde

Most likely, your earliest exposure to “formaldehyde” involved some sort of biological specimen, preserved in a clear liquid. This liquid was probably identified as “formaldehyde.” And, if you were in elementary school or junior high, you didn’t give it a second thought.

But, if you were to study organic chemistry, you would learn that formaldehyde is the simplest aldehyde, and is a gas at room temperature. So…just what is this clear liquid?

The clear liquid is formalin–an aqueous solution of formaldehyde, with methanol added to prevent polymerization. Here is a typical formulation:

Formaldehyde HCHO 36.5 – 38.0
Methanol CH3OH 10.0 – 15.0
Water H2O 47.0 – 53.5

This solution is also used as a disinfectant. Notably, the solution gives off some gaseous formaldehyde, and areas of human occupancy containing formalin should be monitored.

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