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2 GENERAL INFORMATION
2.4 Storage
Store the assembled probe with the end dipped
into a 0.1M ammonium chloride solution in order
to keep the osmotic pressure of the solutions on
either side of the membrane equal – see Section
4. After storage the probe should be well rinsed
in distilled or deionised water. Satisfactory
results will not be obtained if the probe is stored
in distilled or deionised water.
On no account should the end of the probe be
allowed to dry out, should this accidentally
occur the performance of the probe may
sometimes be restored by loosening and
tightening the electrode retaining nut to allow
more filling solution to flow between the glass
electrode and the membrane. If this procedure is
not successful the membrane and filling solution
need replacement.
If the probe is to be returned to its box it must be
dismantled: firstly the glass electrode should be
unscrewed, then the rest of the probe emptied,
rinsed and drained. The glass electrode should
be stored with its end in a teat containing a
neutral buffer solution, taking care not to cover
the reference element.
2.5 Internal Filling Solution
The internal filling solution incorporates a
coloured indicator which is normally yellow. The
indicator changes to blue if the probe
membrane, or membrane seal, allows alkaline
reagent to leak into the probe. If this occurs the
membrane and the filling solution should be
replaced. Take care to tighten the end cap
sufficiently to provide a good seal. The solution
keeps indefinitely.
3 PROBE MEASUREMENTS
3.1 General Procedure
For measurement of either the ammonium ion or
'total' ammonia concentration in samples, the
samples must be alkaline when measured so
that ammonia species are present as free
ammonia. As most samples are not alkaline it is
necessary to treat them with a buffer solution,
the choice of which is explained in the following
section.
Alkaline samples slowly lose ammonia so
measurements in open beakers are not always
satisfactory. Thus it is suggested that the
samples are contained in 100ml conical flasks to
reduce losses. The probe body will fit closely into
the top of such a flask but losses may be further
reduced by slipping the larger of the 'O' rings
from the spares kit over the probe stem so that it
makes a seal with the neck of the flask.
All
samples
should
measurement; this may conveniently be done
using a magnetic stirrer. With some stirrers it will
be necessary to put a sheet of thermal insulating
material, such as polystyrene sheet, underneath
the beaker to prevent heating of the sample by
the stirrer motor.
After immersing the tip of the probe in a stirred
sample check that no air bubbles are trapped on
the end of the probe. Allow time for the probe
potential
to
reach
equilibration time will be dependent on the
concentration of the ammonia in the sample. At
a sample concentration of 10
the time taken by the probe to reach its
equilibrium potential should be under two
minutes, although it may take longer with certain
types of sample.
The probe is sensitive to the osmotic pressure of
samples and special attention needs to be paid
to this in the analysis of samples containing a
high total concentration of dissolved species
(i.e. the sum of the concentrations of all anions,
or cations and non-dissociated species).
be
stirred
during
a
steady
value;
–3
M (17mg NH
the
–1
l
)
3
5
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