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FORMALDEHYDE
has been utilized in some instances for the determination of these metals.
Gold may be determined colorimetrically by a method of this type based
on the formation of colored colloidal gold. By this procedure, it is possible
to detect one part of gold in 40,000 parts of solution 3 .
Vanino and Schinner 12 make use of formaldehyde in the gas volumetric
determination of nitrous acid by means of the following reaction:
•iHXO* -r 3CH20 s- 3CO* 4- 5H£0 -f 2XS
The reaction is carried out in the presence of hydrochloric acid. The
carbon dioxide is absorbed by alkali and the nitrogen measured in a nitrometer.
Reference has previously been made to a method of determining ammonium
nitrate by reaction with formaldehyde (Of. page 121),
Solutions of sodium sulfite and bisulfite can be analyzed by use of the
reaction employed in the determination of formaldehyde with sodium
sulfite 4 :
XasSO* -4- CH20 + H20 —-* XaOH + NaS03-CH2OH
The sulfite solution is titrated to neutrality with alkali, using phenolphthalein
as an indicator: excess formaldehyde is then added and the alkali
liberated is determined, by another titration. If the titer obtained in the
first titration is the same as that obtained in the second, the solution contains
pure alkali bisulfite equivalent to the alkali used in either titration.
If the titer is greater in the second titration, XasS03 is present and may be
calculated. Pure sodium sulfite is neutral and the first titer is zero. If
the first titration is the greater, the sulfite contains a- foreign acidic impurity.
Addition reactions of formaldehyde can also be employed in the
quantitative separation of sulfides, sulfates, thiosulfates, and sulfites 5 .
Carbonates can be detected in the presence of sulfites by using formaldehyde
to prevent the evolution of sulfur dioxide on acidification of the formaldehyde-treated
solution 10 .
Xastyukov's "FormoUte" reaction 8 is sometimes employed for the
determination of unsaturated cyclic or aromatic hydrocarbons in oils. The
sample is mixed with an equal volume of concentrated sulfuric acid, and a
0+5 to 1 volume of commercial formaldehyde is gradually added. The
mixture is then shaken or otherwise agitated until reaction is complete
(approximately 30 minutes to one hour) and diluted with petroleum ether
free of hydrocarbons capable of giving a positive reaction. The diluted
mixture is poured into ice-water, treated with excess ammonia, filtered and
washed. Following a final wash with petroleum ether and water the solid
"formolite" is dried at 105°C and weighed. The weight of unsaturated or
aromatic hydrocarbon is equivalent to approximately SO per cent of the