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...3 PROBE MEASUREMENTS
3.10.1 Method
There are many variations of the Kjeldahl
digestion process, but all result in a very acid
residue of high dissolved solids content. With
the above points in mind, a typical procedure
can be outlined.
a)
Place sample containing 0.5 to 5.0 mgl
in Kjeldahl flask.
b)
Add 10ml H
2
catalyst and/or other additives required.
c)
Carry out digestion.
d)
After cooling make digest up to 100ml with
ammonia free water.
e)
Pipette 25ml diluted digest into 100ml
conical flask.
f)
Add 25ml buffer solution (4.0M NaOH),
stopper the flask, Swirl and cool rapidly to
the working temperature.
g)
Remove stopper, place magnetic stirrer bar
in flask and insert probe containing
modified filling solution.
h)
Stir the solution gently and note instrument
reading immediately a stable potential is
obtained.
The procedure in this section produces a final
solution
for
measurement
approximate composition:
0.90M Na
SO
2
4
0.20M NaOH
and the standard solution and probe filling
solution compositions can be calculated from
this.
12
SO
(1.84 s.g.) and any
4
having
3.10.2 Standard Solutions
These should contain N in the ranges 5 to
–1
50mgl
in the form NH
should contain sulphuric acid, catalyst and other
additives used for digestion; 1 litre of standard
solution should contain ten times the amount of
each of these used in step b) of the procedure in
–1
N
Section 3.10.1.
They should be treated in exactly the same way
as the diluted digest, i.e. taken through steps e)
to h) of Section 3.10.1.
3.10.3 Probe Filling Solution
The total concentration of dissolved species, Σ
ci, should be the same as that in the final
solution, i.e.
Σ c
= (Na
i
= ([2 x 0.90] + 0.20) + 0.90 + 0.20M
= 3.10M
The standard filling solution is 0.1M NH
this should be modified by the addition of solid
to increase Σ ci from 0.2 to 3.1M; this is
Na
SO
2
4
achieved by the addition of 3.44g anhydrous
Na
SO
to 25ml standard filling solution.
2
4
If the increase in Σ ci is greater than 3.0M then
NaCl should be used in place of Na
The above calculation assumes that the
the
solutions exhibit ideal behaviour, but this will not
be the case in practice. Thus, in general, some
drift in probe potential will still be observable and
further adjustment of the probe filling solution
may be necessary. If the probe potential drifts in
the direction of lower ammonia concentration,
then the filling solution requires more Na
and vice versa. It is recommended that Σ ci for
the filling solution be changed in 5% increments
until the drift is adequately small.
The matching of total concentration of dissolved
species on either side of the probe membrane is
more critical as the solutions become more
concentrated; thus some preliminary dilution of
the residue after digestion is advantageous.
Cl. In addition, they
4
+
2–
–
) + (SO
) + (OH
)
4
Cl and
4
SO
.
2
4
SO
2
4
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