11
3.9
Modification of the Internal Filling
Solution
For analysis of samples containing very high
concentrations of ammonia the internal filling
solutions must be modified to prevent the
osmotic transfer of water across the membrane
(see Reference 1). The internal filling solution
provided in the kit consists principally of 0.1M
ammonium chloride saturated with silver
chloride.
In the case of measurements in strong ammonia
solutions the molarity of ammonium chloride in
the internal filling solution should be increased
so that it is at least two or three times the
molarity of the strongest ammonia solution to be
analysed. Sufficient silver nitrate solution should
be added to ensure that the solution is saturated
with silver chloride. The buffer for treating the
samples should be chosen so that the buffered
samples have pH greater than 11 and the total
concentration of dissolved species matches that
in the new filling solution.
For measurements of samples containing a high
total concentration of dissolved species,
sufficient inert electrolyte, e.g. potassium nitrate,
must be added to the filling solution so that the
concentrations in the internal filling solution and
buffered samples are equal. Such modification
of the filling solution is necessary in the analysis
of, for example, seawater or Kjeldahl digests. An
example of the calculation of the correct
concentration of inert electrolyte to add is given
in the subsequent sections.
3.10
Analysis of Kjeldahl Digests
The Ammonia probe can be used for the direct
determination of the ammonia content of the
residue from Kjeldahl digestions, thus
eliminating the distillation step required before a
colorimetric or titrimetric finish (Reference 4).
Warning.
The normal safety
procedures carried out during Kjeldahl
digestions should always be observed.
After digestion of the sample the residue is
cooled, diluted with ammonia free water and an
aliquot is made alkaline for ammonia
measurements with the probe. The exact
procedure varies with the application but the
important requirements are as follows:
a)
The initial dilution of the residue after
digestion should be as large as possible to
simplify the procedure. If the dilution factor
is such that the total concentration of
dissolved species in the solution presented
exceeds 0.25M then the probe filling
solution should be modified to prevent
osmotic transfer of water through the
membrane.
b)
Loss of ammonia during and after addition
of alkali to the diluted residue must be
prevented. Losses of ammonia can be
minimised by efficient cooling to dissipate
the heat of reaction.
c)
Standard solutions should be similar in
composition to the diluted residue to avoid
changes in the Henry’s Law constant. In
general it will be most convenient to use
standard solutions containing the same
concentration of sulphuric acid as the
diluted residue.
d)
The temperature of all solutions presented
to the probe should be essentially the same
(see b above).
e)
If the digestion mixture contains a mercury
catalyst, sodium iodide should be added to
the alkaline buffer to prevent interference;
for a copper catalyst EDTA should be
added to prevent precipitation.
✶
Note.
Many of these require-ments
may be met by the use of a continuous flow
system, as already noted.
3
PROBE MEASUREMENTS…
