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DETECTION OF FORMALDEHYDE 249
The method appears to be one of growing importance. Its use \> naturaUy
restricted to solutions which are free from materials whose halt-wave potentials?
are in the region —1.4 to -l.S volts.
Procedure: According to Boyd and Bambach*, best results are obtained wter. 3
cc of the solution to be tested are mixed with 1 ce of a solution which is 0.4A' in potassium
chloric^ and 0.2,\ in potassium hydroxide. Xitrogen is bubbled through the
resulting solution for 15 minutes, after which a polarogram is taken from -1.4 io
-tS volts. The Leeds and Xorthmp Eleetro-chemograph is recommended. The
h&lf^vave of the formaldehyde step is reported* to occur at -1,63 volts (normal
calomel electrode). The solution should be kept in a constant-temperature bath
controlled to 0.1 £ C during the determination, since the wave height is stated io
change bv about 6.0 per cent for each l c change in temperature- Boyd and Bambaeb
state that- 3 ppm of formaldehyde can be determined. Jahoda 30 claimed that 0.07
npm may be estimated with an accuracy of 10 per cent.
In genera] the method appears applicable to any formaldehyde solution which can
be isolated by distillation or is otherwise free of interfering"substances.
Boyd and Bambach state that acetaldehyde is reduced at a potential
of —l.S volts (normal calomel electrode). This aldehyde and formaldehyde
can be estimated simultaneously if the potassium electrolyte is replaced
by lithium hydroxide. The potassium wave interferes with acetaldehyde
detection. A polarogram obtained by the above investigators
showing -both formaldehyde and acetaldehyde is shown in Figure 17.
IDENTIFICATION OF FOHMALDEHYDE
For purposes of identification, it is often desirable to prepare characteristic
formaldehyde derivatives whose identity can be certified by melting
points, analyses, etc. The following reagents are of value for this purpose
in cases involving small quantities of formaldehyde.
Methone (dimethyldihydroresorcinol, 5,5-dimethyl-cyclohexanedione).
A diauclear methylene derivative of this agent, methylene bis-methoneT is
precipitated when a few drops of a o to 10 per cent solution of the reagent
in alcohol is added to the solution to be tested, which has been made faintly
add with acetic acid. The precipitate obtained with formaldehyde melts
at 1S9°C after recrystallization from hot alcohol. The presence of salt in
the reaction mixture is stated to increase sensitivity, which is claimed to be
approximately 4 ppm. Methylene bis-methone is soluble in dilute alkali
and does not become alkali-insoluble after heating on a boiling water bath
with a small amount of glacial acetic acid for 6 to 7 hours, Methone derivatives
of other aliphatic aldehydes become alkali-insoluble when treated in
this way (pages 265-266).
p-Nitropnenylhydrazine Hydrochloride. This agent precipitates a
p-nitrophenylhydra^one from mildly acid formaldehyde solutions. On
purification from alcohol, the product melts at 181-1S2°C 5 ' 49 .
2,4-Dinitrophenylhydrazine Hydrochloride. This agent LS used in the
same manner as the one mentioned above. Formaldehyde-2,4-dinitrophenylhydrazone
melts at 166-167°C 1(U1 .