1-39
GENERAL DIGESDAHL DIGESTION, continued
4. Place the flask
weight followed by the
fractionating column
with funnel on the
flask. Place the flask
on the heater and heat
until the boiling point
of sulfuric acid is
reached (refluxing
sulfuric acid will be
visible).
Note: White acid vapors
usually will be present
but their presence alone
does not indicate that the
boiling point of sulfuric
acid has been reached.
Note: Liquid samples
require total evaporation
of water before vapors
are visible.
Note: If sample starts to
foam up into the neck of
the flask, lower
temperature to 335 °C
(600 °F). Continue
heating at lower
temperature until all
water is evaporated off.
Then return to original
digestion temperature.
5. Heat 3-5 minutes.
Do not boil sample to
dryness.
Note: Discard sample if
it evaporates to dryness
and use larger amount
of concentrated sulfuric
acid for digestion in
Step 2.
Note: Some organic
samples may need more
than five minutes for
complex digestion. See
Table 8.
6. Be sure you have
added the correct
amount of sulfuric
acid. Add 10 mL of
50% hydrogen
peroxide to the
charred sample via
the funnel on the
fractionating head.
Note: If the digest does
not turn colorless, add
5 mL increments of
peroxide until the digest
becomes clear.
Note: Visually confirm
the presence of sulfuric
acid in the flask before
adding hydrogen
peroxide.
Note
If foaming or bumping is not stopped by lowering temperature or volume, then
liquid samples that will not clog the capillary funnel may be added to the flask
via the capillary funnel, 10 mL at a time. Decrease amount added if foaming
still persists.
3 - 5 minutes
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