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39
DIGESTION PROCEDURE FOR AQUEOUS LIQUIDS
1.
Transfer a
premeasured amount of
sample into a 100-mL
Digesdahl digestion
flask; see Sample and
Analysis Volume Tables
for Aqueous Liquids
following this
procedure. The amount
transferred should not
contain more than 0.5 g
of material which is not
water. The maximum
volume for water
samples is 40 mL. In
samples with more than
1% solids present, use
the formula below:
Water Sample Volume
= 40
÷
%solids
Note: Use only Hach
Digesdahl flasks.
Volumetric flasks with
concave bottoms should not
be used.
Note: If solids are 10% of
total volume of sample, the
maximum volume of liquid
sample would be 4 mL.
Note: Oils and organics
should be considered as
solids when determining
sample size.
Note: If liquid is too
viscous to measure,
preweigh the sample into
the digestion flask.
2.
Add 3 mL of
concentrated sulfuric
acid (spec. gravity 1.84)
to the volumetric flask
and two or more silicon
carbide (carborundum)
boiling chips for liquid
samples.
Note: Pretreat boiling
chips by soaking in 1:1
Nitric Acid and rinsing
thoroughly with deionized
water. Treatment is very
important in low-level
work. Hach recommends
using silicon carbide
boiling chips.
3.
Turn the temperature
dial to a heat setting of
440
°
C (825
°
F). When
the proper temperature is
reached, turn on the
water to the aspirator
and make sure there is
suction to the
fractionating column.
Note: Wait for the proper
temperature to be reached
before sample is placed on
the heater.
Note: Always operate the
Digesdahl apparatus with
a safety shield in place or
inside a closed fume
hood. Safety glasses
are mandatory.
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
sulfuric acid boils
(refluxing sulfuric acid
will be visible).
Note: White acid vapors
will usually be present but
their presence alone does
not indicate that the boiling
point of sulfuric acid has
been reached.
Note: Aqueous samples
require total evaporation
of water before vapors
are visible.
Note: If sample foams up
into the neck of the flask,
lower temperature to
335
°
C (635
°
F). Continue
heating at a lower
temperature until all water
is evaporated. Then return
to original digestion
temperature.
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 persists.
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Содержание Digesdahl 23130-20
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