formaldehyde - Sciencemadness Dot Org

QUANTITATIVE ANALYSIS 265
This product is almost completely insoluble in neutral or mildly acidic
aqueous solutions; 100 cc water dissolve only 0,5 to 1.0 mg at 15-20°C. It
is a crystalline material melting at 1S9°C. It is soluble in alkali, behaving
as a monobasic acid, and can be titrated with standard alkali in alcohol
solution.
Other aliphatic aldehydes such as acetaldehyde also form condensation
products of low solubility by reaction with methone. These products differ
from the formaldehyde derivative in that they are readily converted to
cyclic hydroxanthene derivatives by treatment with glacial acetic or dilute
sulfuric acid. These xanthene derivatives are not soluble in alkali and can
thus be separated from the formaldehyde product. Acetaldehyde reacts
with methone to give ethylidene dimethone (nup* 139 °C) which is 0.0079
per cent soluble in water at 19°C. The conversion of this product to the
alkali-insoluble xanthene derivative is indicated below:
H
O
C
tip f
1 H
1 1!
he c-
\ /
c
H2
-OH
CHg
i
>
1
H KO-
fi
O
1
C
9 9 H Add
II ! —
—C C(CH3)2
\ /
c
H9
H H
O H CH* O
1 \ / 1
c c c
f \ / \ / X
HC C C CH
| 1 II [ -f HaO
CCHOaC C C C(CHS)S
\ / \ / \ /
C O G
H, H2
The acetaldehyde derivative, ethylidene dimethone, differs from the
formaldehyde product in that it behaves as a dibasic acid when titrated
with alkali at 70 to 75°C.
In determining formaldehyde with methone the reagent is best added as
a saturated aqueous solution or as a 5 to 10 per cent solution in alcohol. In
the latter case care must be taken to avoid addition of large quantities of
reagent because of the limited solubility of methone in water and the fact
that high concentrations o£ alcohol will interfere with the precipitation of
the formaldehyde derivative. The formaldehyde solution to be analyzed