FAS622en_pH measurement

1 Basics
How does a silver/silver chloride reference electrode work?
Fig. 7:
Reference electrode
(1) Terminal head (4) Conductive system
(2) Filler port (5) Electrolyte
(3) Glass or plastic body (6) Diaphragm
In the simplest case, the conductive system is a silver wire coated with silver chloride. It has two
functions, firstly to connect the electrolyte to the connecting cable, and secondly to provide the
stable electrical reference point for the voltage measurement. The conductive wire operates like an
ion-selective chloride electrode. Its potential depends on the chloride concentration of the reference
electrolyte. A concentrated potassium chloride solution (c (KCl) = 3mol/l) is used as the electrolyte.
As the chloride concentration of the electrolyte remains virtually constant, the potential of
the reference electrode is stable too.
Instead of the coated silver wire, some electrodes contain a cartridge filled with silver chloride. Inside
the cartridge, the electrolyte becomes saturated with the silver chloride. The remaining electrolyte
within the reference electrode stays virtually free of silver ions. With reference electrodes
using this type of cartridge, there is no problem due to silver compounds that are difficult to dissolve,
such as the familiar „black diaphragm“ caused by silver sulfide. At low conductivity, the system
does away with the special potassium chloride solution (c (KCl) = 1mol/l), once common for such
applications.
The electrical connection can be established by, for example, a diaphragm that is permeable to the
electrolyte. Electrolyte ions reach the measured solution via the diaphragm, thereby ensuring the
charge transport. The more permeable a diaphragm, the more reliably the charge transport
functions, and the more stable is the potential of the reference electrode. However, the increased
electrolyte consumption also reduces the service life of the electrolyte.
14 1 Basics JUMO, FAS 622, Edition 04.07