Environmental Engineering Reference
In-Depth Information
reference electrode, and HCl) and the outer one housing the reference electrode
(Ag/AgCl) and its salt bridge. The pH sensing component of the indicator electrode is
a glass bulb, which is a thin glass membrane of approximately 0.03-0.1 mm thick and
made of, for example, 72% SiO 2 , 22% Na 2 O, and 6% CaO (Corning 015 glass).
When a pH probe is immersed in a sample solution, hydrogen ions (H þ ) in the
solution will enter the hydrated Si-O lattice structure of the glass membrane in
exchange for singly charged cation Na þ . This creates an electrical potential across
the membrane interface (referred to as the boundary potential) with respect to the
internal Ag/AgCl reference electrode. The resulting overall cell potential with
regard to the external reference cell can be derived as
RT
F ln a H þ ¼ constant
2
:
303RT
F
E cell ¼ constant þ
pH unknown
ð11:11Þ
where a H þ is the activity of H þ . The constant value is unknown and is not of
practical interest for pH measurement because standard pH buffers are usually used
to calibrate the electrode. The slope factor (2.303RT/F) is temperature dependent,
therefore, temperature compensation should be made during pH measurement. Note
also that Eq. 11.11 is strikingly similar to the Nernst equation (Eq. 11.5) that is used
to describe the metallic electrodes. However, the source of potential in pH glass
electrodes is totally different from metallic electrodes—one arises from redox
reaction whereas the other is due to a boundary potential as a result of ion-exchange
reactions.
All modern digital pH meters have an electronic device that can accurately
record potential (in millivolts) and transform the voltage caused by H þ into a direct
pH reading. In calibrating the pH meter, standard buffers (commonly at pH 4.0, 7.0,
10.0) are readily available from commercial sources. Many pH meters have
microprocessors that can automatically recognize the specific pH of the standard
buffers and adjust the temperature effects.
Practical tips
The measurement of pH is probably the most widely used potentiometric method in
many environmental labs. Perhaps, it is also one of the most poorly understood and
maintained apparatus. Do not underestimate the obvious. The hints listed below may
help one to optimize the performance of a pH meter.
The pH probe should be fully hydrated prior to use by soaking the membrane
in water for 24-48 h. Never let the glass electrode dry out.
The pH meter should be calibrated frequently, because many factors can cause
errors, including strains within the membrane and mechanical or chemi-
cal attack of the external surface. Handle glass electrode with extreme care.
The pH reading may be sluggish when the pH of a dilute or an unbuffered
solution of near neutrality is measured. The solution should be stirred and a
sufficient time is then required to obtain a stable pH reading.
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