The main function of the reference junction is to allow electrical
contact between the reference electrolyte and the measured solution. The
better and more reliable the connection is, the more stable is the
potential of the reference electrode. And more accurate is your measured
Another important function of the junction is to act as a physical barrier to prevent ingress of the measured media into the reference electrolyte.
current to flow, the reference junction must be able to conduct
electrons. The current flow is established by allowing the electrolyte
solution to penetrate the structure of the junction. The construction of
the junction will allow small amounts of the reference electrolyte to
flow through it into the solution being measured.
The faster the electrolyte is allowed to flow, the better is the electrical connection, and the faster is the response rate of the pH sensor. The drawbacks with more rapid electrolyte flow rates are the need to regular refill the electrolyte and that the flow can have unwanted effect of altering the test solution composition.
An alternative, to a liquid electrolyte, can be to use another type of electrolyte that has a very slow leakage rate.
The junction can either be an open junction or a permeable diaphragm.
problems with pH measurements are associated with the diaphragm of the
reference electrode. Heavily polluted media, or media that produce a
precipitate when they come into contact with the electrolyte, will cause
a blockage of the diaphragm. Many of these problems are related to the silver/silver chloride reference element.
Clogging has as a consequence:
It's a well-know fact that a pH electrode should always be kept moist.
What happen if dehydrated, is that the reference electrolyte will
slowly penetrate through the diaphragm and crystallize across the tip of
the electrode. This will not interfere with measurements. Simply rinse
the pH electrode and blot dry before use.
However, what can cause trouble, if the pH sensor has dried out during storage, is that the diaphragm resistance will increase substantially. Even when the electrode is refilled with reference electrolyte, the high diaphragm resistance may persist and produce significant measurement error.
pH measurement issues are due to fouled or poisoned electrolytes and a
clogged reference junction. So, it's not surprising that the
manufactures of pH electrodes spend a lot of effort to create and design
electrodes that deal with those issues.
You will meet various expression and designs if you spend some time to read information from different pH sensors manufactures. However, the fundamentals that are covered on this page will help you to pick the appropriate pH sensor for your application.
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