Buffer Solution - Sample Ph; Probe Calibration - ABB 8002 Operating Instructions Manual

...3 PROBE MEASUREMENTS
If this total concentration in the buffered sample
presented to the probe is greater than 0.25M,
the probe potential will drift. To overcome this
problem either the samples should be diluted or
the internal filling solution of the probe modified,
as explained in a later section.
The probe is sensitive to changes in both
ambient temperature and the temperature of
samples, hence it is recommended that the
probe is not used in direct sunlight or in a series
of samples or
temperatures. For the most accurate and
reproducible results the probe should be kept at
a constant temperature by use of a jacketed cell
or by immersion of the lower half of the probe in
a water bath (using the probe, fitted with a flow-
through cap, in a flow system).
In addition to the measurement of 'total'
ammonia in samples, the probe may also be
used directly in untreated samples to measure
the 'free' ammonia concentration. This is
particularly useful in complex equilibria studies
or in the measurements of strong ammonia
solutions at fixed pH.
3.2 Buffer Solution – Sample pH
The two functions of the buffer solution are to
adjust the pH of the samples to greater than 11
so that all the ammonium ions are converted to
ammonia and to fix the total concentration of
dissolved species in the buffered sample at
approximately 0.2M. In order to ensure virtually
complete conversion of the ammonium ions it is
necessary to have a sufficient excess of hydroxyl
ions over the ammonium ions: to ensure the
correct pH the final hydroxyl concentration
should be at least two or three times the
maximum ammonium
expected (both concentrations expressed as
molarities).
Thus a 1M sodium hydroxide solution (NaOH)
added to samples at a volume ratio of 1 : 10, is
a suitable buffer for samples containing up to 5 x
–2 +
10 M NH (ca. 1000mgl
4
samples do not contain a high concentration of
dissolved species. However for a sample which
contains less than 5 x 10
1
) but has a total concentration of dissolved
species of 0.2M, a suitable buffer for addition at
a volume ratio of 1 : 10 is 0.1M NaOH, as this
produces the correct pH while maintaining the
correct level of dissolved species.
6
standards at different
ion concentration
–1
), provided the
–3 +
M NH (ca.100mgl
4
A correlation between % conversion and pH is
shown in Fig. 3.1.
If samples contain small concentrations of metal
ions, such as copper, nickel or mercury ions,
these will complex part of the ammonia to be
measured and cause a low result. Hence if such
interferents are likely to be present it is advisable
to replace the usual 1M NaOH solution with a
buffer of 1M NaOH + 0.1M Na
For the analysis of samples containing greater
than 0.25M total dissolved species or more than
–2
5 x 10 M NH
4

3.3 Probe Calibration

The probe should be calibrated regularly. After the
calibration graph has been prepared, the
calibration should be checked at a single point
every two or three hours. The concentrations of the
standard solutions used to calibrate the probe
should be chosen to bracket the expected range
of concentration of the samples.
The standard solutions for calibration of the
probe may conveniently be prepared by serial
dilution of fresh standard 0.1M or (if working in
–1
mgl units) 2000 mgl
solution. The diluted standards are treated with
a buffer as discussed in the previous section;
typically 1 volume of 1M sodium hydroxide is
added to every 10 volumes of standard. Record
the potential of the probe in each of the buffered
standards and prepare a calibration graph of the
probe by plotting the potentials against the
logarithm of the concentration of ammonium
ions in the standards. This procedure is
simplified if the graph is plotted on semi-
logarithmic graph paper with the concentration
along the logarithmic axis. A typical calibration
graph is shown in Fig. 3.2.
–
EDTA.
2
+
, see later section.
–1
ammonium chloride