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8. A POTENTIOMETRIC DETERMINATION OF
THE
pH OF A WATER SAMPLE
8.1 Objective
Use a two-point calibration and determine
the pH of a water sample. Register the asymmetric
potential and the slope of the pH electrode.
8.2 Background
The measuring system for a potentiometric
determination of pH consists of a mV-meter
with a high input resistance (> 1 TΩ),
a combined pH electrode with a glass membrane,
a temperature sensor and the examined solution.
In the combined electrode, a glass electrode
and a reference electrode of the Ag-AgCl
type are assembled in the same body.
The relation between the electrode potential
and the activity of the H3O+
ions in a solution is defined by the Nernst
equation. For practical use the relation
can be written as follows:
E = k + s·log[H3O+]
or E = k –
s·pH
where the letter s stands for the slope.
The theoretical slope at standard temperature
is 0.0592 V. The pH-meter is built so that
at a pH of 7.00 the potential should be
0.0 mV. However, real electrodes seldom
perform ideally, and as a consequence the
pH-meter should be calibrated before use.
At least a two-point calibration should
be used; this involves two calibration buffers
for which the pH is reliably known at the
working temperature. The calibration process
provides an insight into the asymmetric
potential and the real slope, or the real
relative slope, of the electrode in use.
The asymmetric potential provides information
about how much the potential at pH 7.00
differs from the expected zero potential.
The
calibration ensures the reliability of the
pH determination of the examined solution.
However, it is important that the pH of
examined solutions falls within the range
of the calibration and that the temperature
is very close to that at which the calibration
was performed.
8.3 Equipment and reagents
- A WTW pH 323/325 portable pH-meter.
- A calibration buffer of pH 4.
- A calibration buffer of pH 10.
8.4 Procedure
Calibration of the
pH-meter*
The electrode glass membrane is very
sensitive and can break easily. Use a polymeric
beaker as a drain while rinsing the electrode.
Rinse the electrode membrane with deionised
water between measurements. Wipe the membrane
gently with tissue paper.
Use the pH 4 and pH 10 calibration buffers
for the calibration procedure. Mix the buffer
solution thoroughly before use. Immerse
the electrode in the first buffer solution.
Repeatedly press the CAL button
until the ASY calibration mode appears
on the screen of the pH-meter. Next, press
the RUN button. Wait until the pH
value stabilises. Use the appropriate arrow
button and set the correct pH for the selected
buffer at the working temperature. Confirm
the setting with the RUN button.
Register the asymmetric potential and the
temperature.
Rinse and wipe the surface of the electrode
membrane and then immerse the electrode
in another buffer solution. Repeatedly press
the RUN button until it is possible
to input the pH again. Wait until the pH
value stabilises and then set the correct
pH value as before. Confirm with the RUN
button. Register the slope of the electrode.
Rinse and wipe the electrode membrane.
* For other instruments
follow the instructions of the producer.
Determining the
pH of the water sample*
Immerse the clean and dry electrode membrane
into the sample solution. Select pH
measurement by pressing the button pH/mV, and then confirm the selection
with the RUN button. Wait until the
pH stabilises. Write the pH of the water
sample and the other requested data in the
table below.
* For other instruments
follow the instructions of the producer.
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Asymmetric potential |
Slope |
pH
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Also write the pH in the table “Basic
data on water samples”, under the
serial number of the sample.
The guide value for pH in drinking water
which is set by the EU Drinking Water Directive
(Council Directive 80/778/EEC) is between
6.5 and 8.5.
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