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...3 PROBE MEASUREMENTS
The Nernst factor also varies with temperature
and so the slope of the calibration graph as well
as the normal potential of the probe vary. The
theoretical values of the Nernst factor at different
temperatures are given in Table 3.1.
°C
m
V
p /
N
H
5
5
5
1 .
9
1
0
5
6
1 .
8
1
5
5
7
1 .
7
2
0
5
8
1 .
7
Table 3.1 Variation of Nernst Factor with
Temperature
A further temperature dependent parameter is
the Henry's Law Constant; thus the partial
pressure of ammonia in a solution of fixed
concentration varies with temperature.
The effect of temperature on the probe is
discussed in more detail in Reference 1.
3.6 Interference
The probe is virtually free from interference. As
the gas permeable membrane is hydrophobic,
neither anions nor cations can interfere with the
response. However, interference does result
from volatile or filming amines in the solution,
such
as
hydrazine,
octadecylamine, (see References 2 and 3);
these produce an apparent increase in the
ammonia concentration in the samples.
3.7 Continuous flow analysis
The response of the probe is sufficiently rapid for
it to be used for the continuous flow analysis of
discrete samples. For this purpose, the flow-
through cap is fitted onto the probe body in
place of the standard end cap. A suitable flow
system is shown in Fig. 3.3. Samples are
separated by appropriate wash solution in the
usual way. The flow rate through the cap should
be approximately 2 to 5 ml per minute. Care
must be taken to flush out air bubbles from the
cap; this may be assisted by using narrow bore
tubing (approximately 1mm) up to the cap inlet
and only a short length of tubing on the outlet.
10
3
°C
m
V
p /
2
5
5
9
1 .
3
0
6
0
1 .
3
5
6
1
1 .
4
0
6
2
1 .
cyclohexylamine
An example of a trace obtained with such a
system is shown in Fig. 3.3 where ammonia
samples were analysed at the rate of 60ml/hour.
A higher sampling rate of 120/hour has been
used and although the precision of the results
was somewhat poorer, such rates would
3
N
H
certainly be practical in many cases. The mixing
coil in the Fig. 3.3 is necessary to ensure that the
6
sample and buffer are thoroughly mixed before
5
reaching the probe.
4
3
An ammonia probe has been used in this way for
total nitrogen analysis by the Kjeldahl method as
discussed in a later section and in Reference 4.
After the digestion step the acidic digest is made
alkaline and its ammonia content determined
directly with the probe. The sample becomes
hot after the alkali addition and the mixing coil
and probe body were therefore immersed in a
thermostatted water bath to ensure thermal
stability; there is no danger of loss of ammonia
because the alkali addition is made in a closed
system.
3.8 The Known Addition Method
There are some advantages in using the Known
Addition Method of analysis for occasional
samples with ammonia concentrations greater
than the lower Nernstian limit in the experimental
conditions used. This method reduces the
or
number of standard solutions required to one;
the concentration of this standard solution
should be between ten and a hundred times that
of the sample. The experimental slope factor (S
mV/pNH
3
To analyse a sample by this method, pipette an
aliquot, V
and measure the potential of the probe in this
solution, E
the standard solution (where 10V
concentration C
measure the new potential of the probe, E
It should be so arranged that the potential
change is several millivolts. The concentration of
ammonia in the original alkaline sample, C
–1
l
, may then be calculated from the formula:
C
=
s
antilog
) of the probe should be known.
ml, of alkaline sample into a beaker
s
mV. Add a known volume, V
1
–1
mol l
and after stirring
a
C
V
a
a
V
+V
a
s
[ [
V
E
-E
10
2
1
(V
+V
S
a
ml, of
a
V
) of
a
S
mV.
2
mol
s
s
)
s
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