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QUANTITATIVE ANALYSIS 271
Formaldehyde Content. Commercial paraformaldehyde, HO{CH2QU'
Et should contain 95 per cent or more formaldehyde by weight, the balance
being principally combined water. The determination of formaldehyde
content is usually made by means of the alkaline peroxide or -sodium sulfite
method- The procedures followed are identical with those already described
for formaldehyde solution, except that the polymer sample (approximately
1 g) is accurately weighed out as a solid and added to the
analytical reagents in which it quickly dissolves, reacting as though it were
monomeric formaldehyde.
Ash. A 10-g sample is carefully ignited in a porcelain crucible which is
heated to redness until all the carbon has burned off. The residue is carefully
weighed and calculated as per cent ash. This figure normally does not
exceed 0.1 per cent.
Metallic Impurities. These are determined from a nitric acid solution of
the ash obtained from a suitable weight of sample by the standard methodsemployed
for the determination of small quantities of metals. Iron and
aluminum concentrations usually do not exceed 100 ppm.
Screen Analysis. This determination is carried out by the standard
routine method. Regularly, paraformaldehyde powder passes a 30- to 40mesh
screen. The granular product falls between 4 and 40, whereas especially
finely ground material passes a 100-mesh screen.
Melting Range. The melting range is taken in a sealed melting-point
tube totally immersed in the heating bath employed. The melting range
gives a rough index of the degree of polymerization of the polymer and,
therefore, is also an index of reactivity. Ordinary paraformaldehyde
usually melts in the range 120-160°C. Special, highly polymerized grades
of low reactivity melt at 160 to 170°C (page 72).
Solubility, A rough measure of the rate of solubility in water also gives
information concerning molecular weight and reactivity, although this
figure is also influenced by pH (pages 73-75). A useful procedure 51 is to
agitate 5 g of polymer with 25 cc distilled water at room temperature for one
hour and filter. The undissolved polymer on the filter is washed with 10
•cc water and the washings added to the original nitrate which is then
analyzed by the sodium sulfite method. Results are calculated as grams of
dissolved formaldehyde per 100 grams paraformaldehyde. That this is not
a solubility measurement will be clear from the discussion in Chapter VII-
The figures obtained are useful for comparative purposes since they furnish
a rough index of product characteristics.
Determination of Formaldehyde in Special Compositions and in Products
Treated with Formaldehyde
The isolation of formaldehyde from mixtures for analytical purposes is a
knotty problem. In general, formaldehyde may be separated from water-