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Chapter 5
Physical Properties of Pure Aqueous Formaldehyde
Solutions
In considering the properties of water solutions of formaldehyde, care
must be taken to differentiate between solutions containing only formaldehyde
and water and commercial solutions of the type which also contain
methanol or other solution stabilizers. Since in some cases investigators
have not been aware of the methanol content of commercial formaldehyde,
physical data must be critically examined to ascertain the type of solution
investigated. The present chapter deals only with pure aqueous solutions
which do not contain solution stabilisers.
In the chapter dealing with the state of dissolved formaldehyde (Chapter
3), it has been pointed out that formaldehyde is hydrated and partially
polymerized in aqueous solutions, being present as an equilibrium mixture
of the monohydrate, methylene glycol, and polymeric hydrates, polyoxymethylene
glycols. The physical properties of formaldehyde solutions
are such as would be expected in the light of this situation. They behave
like solutions of a comparatively non-volatile glycol; they do not behave
like solutions of a volatile gas.
In the following pages, the known properties of pure aqueous formaldehyde
will be reviewed in alphabetical order, with the exception of properties
so closely related that they are most readily treated together.
Acidity- Even the purest formaldehyde solutions are slightly acid. In
general, the pH of pure aqueous formaldehyde lies iii the range 2.5 to 3.5.
Wadano 2r has demonstrated that this acidity is due to the presence of traces
of formic acid. The titration curves of formaldehyde solutions are the
result of the superposition of the curve for formic acid neutralization on
the curve for formaldehyde neutralization. However, formaldehyde is so
weakly acidic that formic acid can be readily determined by simple titration
methods.
Neutral or basic formaldehyde solutions can be obtained only by the
addition of a buffer such as borax. Even solutions distilled from magnesium
carbonate are reported to have a pH of 4.4 1 . Solutions neutralized
with caustic alkalies slowly become acid on standing. This change is due
to the Cannizzaro reaction:
201*0 + H20 -» HCOOH 4- CH*0H
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