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...3 PROBE MEASUREMENTS
If the level of ammonia contamination in the
water and reagents is sufficiently low with
respect to the lowest concentration of ammonia
in standards and samples for the probe to give a
Nernstian
response
measurement
range,
procedure may be simplified by using a pH
meter with a direct reading concentration scale.
Consult the pH meter instruction manual for the
procedure. Use the monovalent anion setting on
the function switch.
For accurate results the composition of the
standards should be as similar as possible to
that of the samples. Thus when the composition
of the samples is well defined, standards should
be prepared from the model samples with
known amounts of added ammonium ion. In this
way the constancy of the Henry's Law constant
(see Section 4) between standards and samples
is assured.
In common with other gas sensing membrane
probes and many ion selective electrodes, the
ammonia probe reaches equilibrium more
quickly after an increase in determinand
concentration
than
Accordingly it is recommended that the probe is
calibrated in the standard solutions in ascending
order of concentration.
The probe may be used to measure the 'free'
ammonia in equilibrium with ammonium ions in
samples. In such applications it is usually not
satisfactory to pretreat the samples before
measurement as this may upset the equilibria in
the samples. The calibration of the probe for
these measurements should be done with
standards of known ammonia concentration
after buffering; for example, one volume of a
–4
standard 10
M NH
4
of 0.2M NaOH is a 5 x 10
Thus in this case the probe is calibrated against
ammonia standards in place of the usual
ammonium ion standards.
8
in
the
required
then
the
calibration
after
a
decrease.
+
buffered with one volume
–5
M NH
standard.
3
3.4 Probe Range and Response
The probe has a Nernstian response up to 5 x
–2
10
M (approximately 1000mg NH
range may be extended by modification of the
internal filling solution. The lower limit of
Nernstian response is set by the purity of
reagents and water, as these inevitably contain
small traces of ammonia; if no special care is
taken the level of ammonia in distilled or
deionised water usually produces a lower
Nernstian limit of approximately 0.1 to 0.2mg
–1
NH
l
(approximately 10
3
The response times of the probe have been
reported in Reference 1. The typical time taken
for the probe to reach 1mV from the final
equilibrium potential after a tenfold increase in
ammonia concentration is 30 to 35 seconds.
The probe will drift if used continually near its
upper limit; for the analysis of samples
containing more than 10
–1
200mg NH
l
) it is advisable to rinse the probe
3
between measurements in 0.1M NH
or
briefly
in
distilled
accumulation of ammonia in the bulk of the filling
solution.
3.5 Temperature Effects
The temperature coefficient of the ammonia
probe
is
approximately
depending on the conditions of measurement.
Thus the probe should not be used in direct
sunlight or in an area of rapidly changing
temperature. Also the temperature of standards
and samples should be held as constant as
possible.
The temperature sensitivity of the probe arises
from the temperature dependence of both of the
electrodes' potentials, the osmotic equilibria
and the positions of the equilibria of the chemical
reactions.
–1
l
); this
3
–5
M).
–2
M (approximately
Cl solution,
4
water,
to
prevent
+1
to
+2mV/°C
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