formaldehyde - Sciencemadness Dot Org

Chapter 2
Monomeric Formaldehyde
Although not commercially available in this form, monomeric formaldehyde
is important both from a theoretical and a practical standpoint. It
serves as a basis for the determination of fundamental physical constants
and is involved wherever formaldehyde is employed in the gaseous state.
A knowledge of the pure monomer is a necessary preface to the understanding
of formaldehyde solutions and polymers.
Pure, dry formaldehyde is a colorless gas which condenses on chilling to
give a liquid that boils at — 19°C and freezes to a crystalline solid at
— llS^C 29 . Both liquid and gas polymerize readily at ordinary and low
temperatures and can be kept in the pure monomeric state only for a
limited time. Because of these facts formaldehyde is sold and transported
only in solution or in the polymerized state. In its aqueous solutions,
formaldehyde is almost completely hydrated. At ordinary temperatures
these hydrates have a relatively high degree of stability, although from a
chemical standpoint they are extremely reactive. When required, monomeric
formaldehyde is best prepared from the commercial solution or polymer
at the point of use and employed directly for the purpose at hand.
Formaldehyde Gas
Monomeric formaldehyde gas is characterized by a pungent odor and is
extremely irritating to the mucous membranes of the eyes, nose, and throat
even when present in concentrations as low as 20 parts per million 7 . In
this connection, it should be noted that the polymeric vapors of trioxane,
(CH*0)3, the little-known trimer of formaldehyde, are not irritating but
possess a pleasant, chloroform-like odor,
Pure, dry formaldehyde gas shows no visible polymerization at temperatures
of 80 to 100°C and obeys the ideal gas laws without pronounced
deviation 35 . However, its stability is dependent on purity, even a trace
of water provoking rapid polymerization. At ordinary temperatures the
dry gas polymerizes slowly, building up a white film of polyoxymethylene
on the walls of the containing vessel. Kinetic studies indicate that this
transformation takes the form of a surface reaction, unimolecular at high
pressures and polymolecular below 200 mm. It is not accelerated by ultraviolet
light 27 - 35 . In the presence of water vapor and other polar impurities
formaldehyde gas is stable only at pressures of 2 to 3 mm or concentrations
of about 0.4 per cent at ordinary temperatures.
18