Horiba Scientific LAQUAact D-75G, Instruction Manual

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Technical note
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Technical note
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pH measurement (glass electrode)
In the glass electrode method, the known pH of a reference solution is determined by
using two electrodes, a glass electrode and a reference electrode, and measuring the
voltage (difference in potential) generated between the two electrodes. The difference in
pH between solutions inside and outside the thin glass membrane creates electromotive
force in proportion to this difference in pH. This thin membrane is called the electrode
membrane. Normally, when the temperature of the solution is 25°C, if the pH inside is
different from that of outside by 1, it will create approximately 59 mV of electromotive
force.
The liquid inside the glass electrode usually has a pH 7. If one measures the
electromotive force generated at the electrode membrane, the pH of the sample can be
found by calculation.
A second electrode is necessary when measuring the electromotive force generated at
the electrode membrane of a glass electrode. This other electrode, paired with the glass
electrode, is called the reference electrode. The reference electrode must have
extremely stable potential. Therefore, it is provided with a pinhole or a ceramic material
at the liquid junction.
In other words, a glass electrode is devised to generate accurate electromotive force due
to the difference in pH. And a reference electrode is devised not to cause electromotive
force due to a difference in pH.
Electrode 1
Liquid of known pH
The glass membrane
Electrode 2
Sample