formaldehyde
The solution is acidic because the acid is used to stabilize the aldehyde. If you open any aldehyde monograph, you'll see that aldehydes have important reactivity, and formaldehyde and acetic aldehyde are particularly unstable, mainly in alkaline solution (condensations). In time, those aldehydes react by themselves to do paraformaldehyde, respective paraacetaldehyde (aldehyde polymers) as precipitates. You can regenerate the aldehydes from polymers by treating with warm water.
Back to formaldehyde, you'll find, beside the acidic media, an important amount of methanol used as stabilizer too.
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