The reference junction can either be an open junction or a porous diaphragm.
This type of junction will give you an
optimal contact between the reference electrolyte and the measured
sample, purely because there is no barrier between your sample and the
reference electrolyte. The result is the fastest pH electrode response
possible and very accurate readings.
However, if you have a liquid electrolyte it will result in a leakage of the electrolyte. The electrolyte will flow out in the measured solution, and even it is at a low rate you need to regular refill the electrolyte.
An even worse disadvantage is that the measured solution easily can penetrate the reference electrolyte and very fast degrade the performance of the reference system, a so-called electrode "poisoning". For this reason, an overpressure in the reference electrode is necessary. The overpressure guarantees a small flow of electrolyte through the reference junction and thus prevents test solution from diffusing into the reference electrode.
pH in solutions with low ionic strength, as for example pure water, can
pose problems. Although contamination of the measuring solution must be
avoided, a fairly high outflow of reference electrolyte is necessary to
get good contact between the reference electrolyte and the measured
A junction system like this is also an alternative for emulsions and heavy-precipitation applications.
Instead of an open junction, it is most common to use a permeable diaphragm. As with the glass membrane,
the type of diaphragm used depends on the particular application. The
selection of an electrode with a suitable diaphragm is an important
factor for a reliable and reproducible pH measurement.
Two common diaphragm types:
common type of a diaphragm is the porous ceramic diaphragm. Ceramic
diaphragms can be precision manufactured to ensure a specific flow rate
through the diaphragm, which is very useful on pH electrodes with liquid
Ceramic diaphragm has a fast response and can handle rapid and large changes in the measured pH value. This type of electrodes has a long service life and is suitable for general use in aqueous solutions. However, a ceramic diaphragm is susceptible to crystalline precipitations, such as lime or calcium carbonate. The only cure is regular cleaning.
Teflon is an amorphous substance that can be manufactured with a very specific porous composition.
Its main advantages are that a good electrical connection can be made and that there is no precipitation on the diaphragm surface. The Teflon diaphragm can be designed as a ring and the large contact surface will prevent rapid contamination.
On the negative side is that since the Teflon is water repelling, solutions that contain oils and waxes will quickly contaminate the diaphragm surface and clog the diaphragm pores. A Teflon diaphragm also has a very slow response time for applications with extreme pH swings.
A common application for a pH sensor with Teflon diaphragm is heavily polluted water, such as waste water.
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