Biomedical Engineering Reference
In-Depth Information
pH
electrode
Reference
electrode
Inner
solution
pH/mV Meter
Ag/AgCl
pH sensitive
membrane
Liquid
junction
FIGURE 10.1 A schematic diagram for a typical electrode system for potentiometric pH measurements.
A potential is established on the pH sensitive membrane-solution interface of a pH electrode that responds
to the activity or concentration of hydrogen ions in the solution. The reference electrode has a very stable
half-cell potential. The cell potential, which is proportional to the pH in the test solution, is measured using
a high input impedance voltmeter between the pH electrode and the reference electrode.
of pH measurement [38]. A pH electrode is selective towards hydrogen ions and these
documents can serve as a guideline for the characterization of pH sensing properties.
10.2.2 Calibration curve and linear response slope of
pH microelectrodes
The principle of pH electrode sensing mechanisms which are based on glass or poly-
mer membranes is well investigated and understood. Common to all potentiometric ion
selective sensors, a pH sensitive membrane is the key component for a sensing mecha-
nism. When the pH sensitive membrane separates the internal standard solution with a
constant pH from the test solution, the potential difference E across the membrane is
determined by the Nernst equation:
( RT / F ) ln [H ]
E
constant
(3)
where R is the gas constant, T is the absolute temperature (K), F is the Faraday constant,
and [H ] is the hydrogen ion concentration in mol/L. Replaced with pH
log[H ],
the equation has a form as follows:
E
constant
(2.303 RT/F ) pH
constant
(slope) pH
(4)
where 2.303 RT/F is the slope of the line plot of E vs pH (also known as the slope factor),
which is the basis of the pH electrode calibration curve. Strictly speaking, the activity of
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