formaldehyde - Sciencemadness Dot Org

PROPERTIES OF PURE FORMALDEE YDESOLUTIONS 49
Appearance. Pure formaldehyde solutions are clear and colorless.
However, according to Wiegand 28 , they show a distinct fluorescence under
ultraviolet radiation. Cloudiness or opalescence in formaldehyde is due
to polymer precipitation and is discussed in detail under this heading
(page 56). .
Boiling and Freezing Points. The boiling point of water is comparatively
little affected by dissolved formaldehyde. According to Auerbach 3 ,
the boiling point, which is approximately 100°C for a 3 per cent solution,
falls off gradually with increasing formaldehyde concentration. The work
of the Russian investigators, Korzhev and Rossinskaya, 16 indicates that
the boiling point may go through a minimum at 11-12 per cent concentration,
since isotherms for the total vapor pressure of solutions at 97 and
98°C, when plotted against concentration, fall to a minimum at this concentration
range and thereafter show a constant increase up to at least 30
per cent concentration. Solutions containing approximately 50 per cent
formaldehyde boil at 103 C C.
The freezing point of water is lowered by formaldehyde. In the case of
dilute solutions this lowering is as would be expected for the dissolved hydrate,
methylene glycol, but decreases at higher concentrations because of
the formation of the hydrated polymers (page 31). Information on more
concentrated solutions is lacking, since polymer precipitation takes place
on chilling, before the true freezing point is attained.
Density and Refractivity. The density and refractivity of pure formaldehyde
solutions bear a simple linear relation to formaldehyde concentration.
Differences due to variations in the relative proportions of simple
and polymeric formaldehyde hydrates in solutions of the same concentration
have practically no effect on these figures as ordinarily measured.
Such variations can be measured only by the extremely sensitive methods
of dilatometry and interferometry,
The relation of density to formaldehyde content for solutions containing
up to 37 per cent CH20 was determined by Auerbach and Barschall 2 for
pure solutions prepared by dissolving gaseous formaldehyde in distilled
water. Their results are shown in Table 7. Formaldehyde concentration
is reported by volume as well as weight. It will be noted that the weight
figures are lower than the volume figures. This disparity becomes even
greater in the case of solutions containing methanol.
Earlier density data, such as those obtained by Liittke 19 and Davis 8 , were
apparently made with solutions containing small percentages of methyl
alcohol and are consequently misleading. Values for the density of pure
formaldehyde solutions for various volume per cent concentrations up to
41 per cent and temperatures from 20 to 40°C are also reported by Datar 7 .
A recent study of the density and refractivity of formaldehyde solutions