IM 12B03D02-01E
9-1
9
.
Technical Information
9. Technical Information
9.1 Measurement Principle of pH Meter (Glass Electrode
Method)
A pH meter makes use of the potential difference developed between the two sides
of a thin glass membrane that separates two solutions with different pH. Figure 9.1
shows the schematic diagram of the measurement principle. A glass electrode is filled
with a pH 7 solution and has an inner electrode that measures the potential difference
corresponding to the pH difference between the internal solution and the test solution.
A reference electrode has a constant potential irrespective of the pH of the test
solution, which is supported by potassium chloride (KCl) solution. It prevents the
reference electrode from making contact with the test solution but itself has electrical
contact with the test solution through the liquid junction. A voltmeter measures the
potential difference between the electrodes. As the membrane resistivity is high
(several 10 to 100 MΩ), the voltmeter with high input impedance is required. The
external leak resistance should also be sufficiently high (10
12
Ω or greater).
F0901.ai
V
pH7
KCl solution
Voltmeter
Test solution
Liquid junction
Reference electrode
pH electrode
Inner electrode
Glass membrane
Figure 9.1 Measurement Principle of pH Meter
