(e) The funnel and filter-support assembly may also be sterilized with
formaldehyde gas. This method consists of putting methyl alcohol on a wick in the base
of the assembly, igniting the methyl alcohol, and closing the unit after 30 seconds.
Since the methyl alcohol is incompletely oxidized in the closed unit, formaldehyde gas,
which is a bactericidal, is generated. The unit is kept closed for at least 15 minutes;
then it is rinsed with sterile water before it is used. The formaldehyde gas method is
recommended for emergency or field use only.
(f) When the funnel and filter-support assembly is heavily
contaminated, it should be washed thoroughly prior to sterilization.
Membrane filters and absorbent pads.
(a) The membrane filters and absorbent pads are supplied in separate
envelopes with 10 to a package. These filters and pads must be sterilized in the
envelopes in an autoclave for 10 minutes at 121C. After the sterilization period, the
steam pressure should be released as rapidly as possible. The packages should then
be removed from the autoclave.
(b) Membrane filters and absorbent pads for use in the field should be
sterilized in advance.
(3) Plastic petri dishes. Since plastic petri dishes are manufactured by use
of high temperature, then packaged, and sealed, they are considered sterile. These
disposable-type petri dishes should be used once and discarded. Should reuse of
plastic dishes become necessary in an emergency, they should be sterilized as
described below as they will not withstand heat sterilization.
(a) Wash them thoroughly; then completely immerse them in a solution
of 70 percent ethanol for a minimum period of 30 minutes.
(b) Remove the dish halves aseptically; then invert them on a dry
sterile towel and let them drain and air-dry thoroughly.
(c) Using sterile techniques close the dishes and store them in a
(4) Glass petri dishes. If glass petri dishes are used, 10 dishes may be
rolled in paper or metal foil and sterilized in an autoclave at 121C (249.8F) for 15
Pipette and graduated cylinders.
(a) Dry heat sterilization at 170C (338F) for at least 1 hour is
preferred for most glassware. Pipettes can be placed in cans or wrapped individually in