Vaporization of Formaldehyde Solutions
Formaldehyde in air can be produced by injecting known concentrations of either
formalin or paraformaldehyde into an air stream and vaporizing.
1.
Prepare the formaldehyde solution in the following way. If the presence of
methanol is not important, the easiest way is to dilute formalin (37% by weight
formaldehyde, 10-15% methanol) to the concentration of interest (see table 1).
TABLE 1
Formaldehyde Vapor Generation
Desired Air
Concentration
Liquid HCHO
Concentration Used
5
1
10
2
100
10-15
2.
If the presence of methanol is a problem, make the formaldehyde from
paraformaldehyde. Weigh out 25 grams of solid paraformaldehyde and add to
220 ml of water in a volumetric flask. Place an inverted 100 ml beaker over the
top of the flask and heat to 80
O
C while stirring with a magnetic stirrer. Heat until
the solution clarifies (usually about 2-6 hours). Remove the magnetic stirrer and
dilute to mark with distilled water. The resulting solution should be about 10%
(100 mg/ml) formaldehyde. If in doubt, the concentration may be checked using
the Chromotropic acid technique or titrating with 0.1 M HCL (Anal.Chem.53,
2118 (1981)).
3.
Set up the formaldehyde generator in the following way. Attach the air supply
(clean dry air or air from a Miller-Nelson Research HCS-401 series humidity
control system) to the INLET of the vaporizer. Set the air flow to the desired
level. If the air flow is greater than 20 lpm, set the vaporizer to 100%.
4.
Attach the needle to the syringe using the Teflon tubing provided. Fill the syringe
with the formaldehyde solution. The concentration can be calculated from C =
24.5 qdX/QM.
5.
Place the needle into the INJECTION PORT and push in until needle stops. The
needle tip must be in direct contact with the heater.
6.
Attach the OUTLET of the vaporizer to an infrared analyzer to monitor the
formaldehyde concentration produced (3.58 microns). If the concentration is
irregular, reposition the injection needle. A mixing chamber may be used to
smooth irregularities.