The silver chloride (AgCl) saturated electrolyte will cause problems for
many applications. Chemical reactions between the reference electrolyte
and the measured media that occur at the diaphragm must be avoided at
all costs. And there are a couple of ways to avoid silver precipitation.
sulfide precipitates can be avoided by employing a double junction. A
double junction reference electrode is constructed with an inner chamber
and a chemically compatible reference solution, the so-called bridge
electrolyte, in the outer chamber. The figure to the right shows a
The electrolyte in the inner chamber is saturated with AgCl, and some of the AgCl diffuses through the inner barrier into the outer chamber. However, the concentration of AgCl in the outer chamber remains low. The inner barrier also makes it extremely difficult for the solution being measured to migrate into the inner chamber.
You can even find triple junction electrodes on the market, constructed with two inner barrier, making it even harder for the sample to “poisoning” the reference electrolyte closest to the AgCl reference element.
similar way to the double junction is to use a small glass tube with a
diffusion barrier, consisting of densely packed mesh, that separate the
AgCl reference element from the reference electrolyte. The barrier
prevents the movement of AgCl or silver ions into the reference
electrolyte without breaking the electrical connection between the
silver wire and the reference electrolyte.
This invention enhances the stability of the reference electrolyte and extends the life of the combination electrode considerably.
Even the AgCl reference system is the most well-established and widespread system, there are other alternatives.
A solid polymer electrolyte is free from silver chloride (AgCl) and therefore, protected against silver sulphide contamination. The mercury/mercury chloride reference system is very accurate. However, it's hardly used anymore because it is harmful to the environment.
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