Environmental Engineering Reference
In-Depth Information
must be redetermined for the residual dry sedi-
ment in the scintillation vial before proceeding.
The dry sample is then treated with an excess
then enclosed in a glass desiccator along with a
small beaker containing concentrated HCl. The
samples are exposed to HCl vapour for 24
48 h
at room temperature, then removed and heated in
an oven for 1 h at 50
-
(
2 ml) of organic-free 1 N HCl (Froelich
1980
).
The acidi
≥
ed slurry is agitated in an ultrasonic bath
at room temperature for 5 min and then dried
overnight at 50
C to drive off residual
hydrochloric acid and water. The silver cups are
then crimped closed and transferred without
reweighing to the CHN analyser. It is important
that untreated and acidi
°
C. After removal from the oven,
the vial of dried sample is allowed to sit open for at
least 24 h. This waiting period is necessary because
hydroscopic salts (e.g. calcium chloride) in HCl-
treated samples absorb water vapour upon
°
ed samples be handled
separately because residual acid vapours in the
HCl-treated samples can react with carbonates in
untreated counterparts.
Both analytical procedures were designed
speci
rst
exposure to laboratory air, rapidly gaining from a
few percentage to >100 % of their original weight
before coming to a relatively constant mass.
Aqueous HCl acidi
cally for use with a CHN analyser linked
to an electronic integrator. However, they should
be readily adaptable to other elemental analysers
capable of handling small samples.
cation followed by drying
without gravimetric correction before analysis of a
weighed subsample can lead to substantial error in
calculated elemental compositions.
After
equilibration,
the
sample
vial
is
3.2.1 CHN Analyser: An Overview
of the Instrument
reweighed to obtain the
W
f
). The
same sample is immediately homogenized and
duplicate 1
nal weight (
10 mg subsamples are weighed into
tin sample boats for elemental analysis. The
weighings of
-
Carbon, hydrogen and nitrogen are important
components of living organisms and non-living
substances (such as sediment, coal and wood).
The composition of these elements needs to be
assessed with precision and high accuracy for
analysing the object and living matter in details.
The proportion of carbon, hydrogen and nitrogen
can be analysed through CHN analyser in which
the complete oxidation of the matter is carried
out. Several companies manufacture CHN ana-
lyser. In 1989, 2,400 CHN Elemental Analyzer
was introduced by Perkin Elmer. The principle of
measurement is very simple. The sample to be
tested is weighed in a tin capsule. The required
amount ranges between 2 and 3 mg. In case the
carbon content is less, the required quantity may
be up to 10 mg. The sample is placed in the auto-
sampler. The tin capsule enclosing the sample
reaches the reactor chamber where excess oxy-
gen is introduced. At about 990
W
f
and the subsamples for CHN
analysis should be made sequentially for indi-
vidual samples with minimum elapsed time
because changes in water content between
weighings will affect the accuracy of the
nal
result. This interval is usually <15 min, and
results produce no signi
cant weight change for
well-equilibrated samples.
(B) Vapour Acidi
cation Procedure
The vapour acidi
cation method should not
be applied to samples containing >50 wt% cal-
cium carbonate (CaCO
3
) and is described here
speci
cally for use with a Carlo Erba CHN
analyser. A small sample (25
50 mg) is oven-
-
dried overnight at 50
C. If inorganic carbon is to
be determined by difference, a pair of powdered
subsamples (1
°
10 mg) is weighed out into tin
sample boats for total carbon analysis. For
organic carbon analysis, a second pair of subs-
amples is prepared by weighing into silver sam-
ple boats (supplied by Carlo Erba), which are
more resistant to acid attack than tin. The sam-
ples in the open silver boats are transferred to a
Te
-
C, the sample is
mineralized. Formation of carbon monoxide is
possible at this temperature even in the presence
of excess oxygen. The complete oxidation is
reached through a tungsten trioxide catalyst,
which is passed by the gaseous reaction products.
The resulting mixture consists of carbon dioxide,
°
on plate drilled to hold approximately 100
boats in numbered
fl
'
wells
'
. The loaded plate is
