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Common questions about formalin

Are formaldehyde and formalin the same substance?

Formaldehyde, in its basic form, is a gas. Most people think of formaldehyde as a liquid because that is how it is commonly used. The liquid is actually a mixture of formaldehyde gas and water. The most common concentration used in research settings is a 40 % (actually37-39%) w/v solution of formaldehyde in water. The solution consists of 37 grams of formaldehyde gas to 100 mL of water. To prevent polymerization of formaldehyde solution about 10-15% of methyl alcohol is added. It is the addition of the methyl alcohol that causes the substance to be called formalin as opposed to formaldehyde. The methanol found in commercial formalin is considered to be a contaminant and its use is not recommended when performing certain molecular biology techniques and immunohistochemistry. It is correct to use the terms formalin and formaldehyde solution interchangeably. When diluting full-strength (40%) formaldehyde solution, assume it to be 100%. A fixative-strength (10%) solution of formalin is therefore a 4% solution of formaldehyde gas in water. A further explanation of this can be found in the excellent article by Dr John Kiernan entitled "Formaldehyde, formalin, paraformaldehyde and glutaraldehyde: What they are and what they do."