formaldehyde - Sciencemadness Dot Org

54
FORMALDEHYDE
Lacy 17 * has recently demonstrated that partial-pressure values for 10 to
40 per cent formaldehyde solutions can also be expressed as an empirical
function of formaldehyde concentration in per cent by weight, 1T3 and
absolute temperature, T, by a modification of the above formula. This
saves interpolation for alpha values at odd concentrations. Variants of
the Lacy equation for calculating partial pressure in millimeters of mercury
or atmospheres are shown below:
log P^n6
= 9.942 - 0.953 (0.488) Wl ° - 2905/7 1
log P^gnQ = 7.061 - 0.953 (0,488) wIn - 2905/ T
Auerbaeh 3 determined partial pressures for formaldehyde solutions at
1S°C by the same method employed by Ledbury and Blair. However,
these data are limited in scope, show some internal ambiguities, and are
lower than other reported values.
Korzhev and Rossinskaya 1B measured partial-pressure values for formaldehyde
solutions at ordinary temperatures by analyzing the formaldehyde
content of the condensate obtained by distilling solutions of known
concentrations at reduced pressures. The values they obtained at these
temperatures are higher than those of Ledbury and Blair.
Partial-pressure measurements for boiling formaldehyde solution were
determined by Auerbaeh and Barschall 4 , and Korzhev and Rossinskaya 16
from distillation data. These figures are shown together with those obtained
from the Lacy equation in Table 10 and may be compared graphically
in Figure 11. It will be noted that although the data show considerable
variation, the values obtained with the formula show on the average a fair
degree of agreement. Until more complete measurements are available
this empirical formula offers the only method for estimating approximate
partial pressures at temperatures between 45 and 100°C.
Accurate figures are not available for the partial pressure of water vapor
over formaldehyde solutions. Korzhev and Rossinskaya 16 measured
values for solutions containing 7, 17, and 28 per cent formaldehyde at
various temperatures from 20 to 9S°C3 but their data show considerable
variation. According to their findings, the partial pressure of water over
formaldehyde solutions can be calculated by the formula shown below, in
which .4 1 is a constant varying with concentration.
log Pg?0 - ii - 2168/r
A 1 = S.695 for 7 per cent formaldehyde; 8.677 for 17 per cent formaldehyde]
and 8.672 for 28 per cent formaldehyde.
* Our findings indicate that the Lacy equation gives good approximations for
formaldehyde partial pressures even at relatively high temperatures and concentrations
(see also p. 73).