formaldehyde - Sciencemadness Dot Org

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FORMALDEHYDE
develop measurable amounts of methylal. The U.S.P. requirement for
acidity 25 is that 20 cc of solution shall not consume more than 1 oe of normal
alkali. This would be equivalent to 0.2 per cent formic acid and is well
above ordinary commercial specifications.* The pH of commercial formaldehyde
lies in the range 2.8 to 4.
Physical Properties
Density and Refractivity. The density and refractive index of corn-
.mercial formaldehyde solutions vary with methanol content and temperature.
Table 4, prepared from the data of Natfca and Baccaredda 13 , gives
Table 4. Influence of Methanol Content on Density txnd Refractive Index of 37
Per Cent Formaldehyde.
Methanol Density Refractive Index
(%) (18*0 as B c)
0 1,1128 1.3759
5 1.1009 1.3766
10 1.0S90 1.3772
15 1.0764 1.3776
20 1.0639 1.3778
these figures for 37 per cent formaldehyde solution containing 0 to 20 per
cent methanol at 18°C.
With the following formula it is possible to calculate the approximate
density of formaldehyde solutions containing up to 15 per cent methanol
at temperatures in the neighborhood of 18 a C.
Density = 1.00 + F —- - M
1000 1000'
F equals weight per cent formaldehyde and M equals weight per cent
methanol.
Although neither density nor refractivity alone affords a criterion of
formaldehyde concentration, both formaldehyde and methanol content
can be estimated when both of these figures have been measured. The
influence of formaldehyde and methanol concentrations on density and refractive
index for the ternary system formaldehyde-methanol-water was
accurately determined by Natta and Baccaredda. A ternary diagram constructed
from their data can be employed for the analysis of solutions containing
0 to 50 per cent formaldehyde and 0 to 100 per cent methanol
(Figure 17, page 269).
* Some resin manufacturers demand a formaldehyde having an acid content of
not more than 0,02 per cent. Reduction of acidity by neutralization gives a saltcontaining
product which is apparently not entirely satisfactory for their use.
Since neutralization followed by distillation raises manufacturing costs, the producer
must turn out a solution which is relatively free from acid as manufactured. In
this connection, recent reports indicate that ion exchange resins (page 308) may be
economically employed to remove acid from solutions which do not meet specifications
with respect to their acid content.