formaldehyde - Sciencemadness Dot Org

STATE OF DISSOLVED FORMALDEHYDE 29
available for chemical reaction, the solutions do not give up their quota of
formaldehyde gas even on being warmed. Solution of anhyxlnnis formaldehyde
in these solvents is a highly exothermic reaction, whereas little or
no heat is evolved in the case of non-polar solvents. Due to their commercial
importance, aqueous formaldehyde solutions have been the subject of
intense research, and the results of this work have given us a comparatively
complete picture of their composition, which may well serve as a model for
the understanding of solutions in other polar solvents.
Water Solutions of Formaldehyde
Anhydrous formaldehyde gas dissolves in water with the evolution of 15
kcal per gram mol—a heat value which is reported to be independent of
the concentration of the solution formed up to approximately 30
per cent 3t4r24 . Formaldehyde solutions are also produced by heating
formaldehyde polymers with water. Concentrated solutions containing
up to and above 50 per cent by weight of formaldehyde are readily obtained.
At ordinary temperatures solutions containing over 30 per cent by weight
become cloudy on standing and precipitate polymer. Commercial formaldehyde,
which contains 37 per cent formaldehyde by weight; remains clear
because it contains S to 15 per cent methanol, which prevents polymer
precipitation.
State of Formaldehyde in Aqueous Solution
Research on the state of formaldehyde in aqueous solution may be
summarized as showing that the dissolved aldehyde is present as an
equilibrium mixture of the monohydrate, methylene glycol, CH2(OH)57
and a series of low molecular weight polymeric hydrates, or polyoxymethylene
glycols, having the type formula HO(CH20)n-H. The state of equilibrium
is determined by the temperature and formaldehyde content of the
solution. High temperatures and low formaldehyde concentrations favor
the monohydrate; low temperatures and high concentrations favor the
polymeric hydrates. Polymers having a degree of polymerization greater
than the trimeric hydrate, rIO(CH20)3H, are probably only partially
•soluble at room temperature and precipitate from solution when formed
to any considerable extent.
Methylene Glycol
As previously indicated, formaldehyde monohydrate has the chemical
structure of methylene glycol, CHa(0H)2? the primary member of the
homologous series of glycols. Its formation by the reaction of formaldehyde
with water is analogous to that of ethylene glycol by the reaction of