Formalin pH?
 

Do anyone know why a formaldehyde water solution (formalin) is acidic?

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.

Re: formaldehyde
 

Thankyou adrian. Do you (or someone else) know what kind of acid that is used to stabilize? Formic acid?

I have a solution of 10% formalin buffered with Sodium Phosphate and using methanol as a stabilizer. The expected pH is 6.9-7.1 at 25 C.