formaldehyde - Sciencemadness Dot Org

STATE OF DISSOLVED FORMALDEHYDE 33
Further confirmation of formaldehyde solution hypotheses can be found
in the study of the partial pressure of formaldehyde over its aqueous
solutions. Compound formation in solution is manifested by a pronounced
negative deviation from Raoult's law. A deviation from Henry's
law, which becomes apparent at a 4 per cent concentration and increases
with increasing formaldehyde content, bears witness to the steadily growing
proportion of polymeric hydrates. The writer 25 has found that this
deviation, as shown by Ledbury and Blair's data for the partial pressure
of formaldehyde solutions at 20°C (page 52), is in quantitative agreement
with Auerbach ? s data in that the partial pressure of formaldehyde is almost
directly proportional to the apparent mol fraction of methylene glycol
as calculated from the cryoscopic measurements. As a result of this rela-
Table 3a: Relation of Formaldehyde Partial Pressures at 20°C to Apparent Mol
Fraction of Methylene Glycol as Calculated from Auerbach's Data.
Grams
CH-0
per ICO g
Solution
2.4
5.9
11.2
14.7
21.1
22.6
24.0
28.2
33. S
T»
•PCFaQ
(mmHg) *
0.11
0.24
0.39
0.49
0.61
0,67
0,70
0,80
0.93
K
Values far
Henry's Lawt
7.5
6.S
5.6
5.2
4.4
4.5
4,4
4.1
4.0
Fraction of
CH2O in form, of
Methylene
Glycol*
0.98
0.S7
0.72
0.64
0.53
0.49
0.48
0.41
0.33
K'
Modified
Henry's Law§
4.6
4.4
4.3
4,5
4.3
4.7
4.6
5.0
5,6
•
Average R> - 4.5
* Based on the data of Ledbury and Blair.
f Constant-^ K -^^-.
1 XCHSO
| Calculated from Auerbach's data.
§ Constant JL' K* - ~^~-
tionship, the partial pressure of formaldehyde may be expressed by the
equation, PCHSO = J£'^CH3(OH>> i n "which FCH*O equals the partial pressure
of formaldehyde, K' is a modified Henry's law constant, and
XcHa(OH)3 represents the apparent mol fraction of methylene glycol.
According to our findings, K 1 equals approximately 4.5 at 20°C when the
partial pressure is expressed in millimeters of mercury. The data on which
this determination is based will be found in Table 3a.
Assuming that a similar relationship prevails at higher temperatures,
approximate values for the percentage of dissolved formaldehyde in
methylene glycol form can be estimated from partial-pressure values-
Calculations of this sort indicate that at 100°C all the dissolved formaldehyde
is in the methylene glycol form at concentrations up to 22 per cent.