Nature of Formaldehyde Solution

Aqueous formaldehyde as used for fixation contains mostly methylene glycol (-99%), its oligomers, and small amounts of formaldehyde. The proportion of the oligomers present depends inversely on the temperature. Formaldehyde solution cannot be obtained without the formation of methylene glycol. It is not the formaldehyde molecule that is primarily responsible for rapid penetration into the tissue but methylene glycol, which is the major component of formaldehyde solution. At concentrations of 2% or less, the formaldehyde in solution is present practically only as the hydrated monomer (HOCH2OH). 

Methylene glycol is formed by the reaction between formaldehyde and water  

To maintain chemical equilibrium, more formaldehyde is found through the dehydration reaction  

Elevated temperatures favor the dissociation of methylene glycol to formaldehyde during fixation. 

In addition to the small amount of formaldehyde originally present in the aqueous solution, a little more is formed from the methylene glycol. However, formaldehyde component reacts very slowly with cellular proteins, and then it is slowly exhausted. This means that the interior of the tissue block after fixation for 4-6 hr at room temperature is exposed mainly to methylene glycol therefore, this portion of the tissue is fixed by ethanol during dehydration, resulting in the coagulation of proteins. 

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