The reference junction
of the pH sensor

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 pH value.

Another important function of the junction is to act as a physical barrier to prevent ingress of the measured media into the reference electrolyte.

Basic function

reference junction

For 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.

Junction type

The junction can either be an open junction or a permeable diaphragm.

Clogging

Most 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:

  • the response time of the pH sensor increases.
  • the resistance at the reference junction increases considerable. Which leads to a fluctuation of the diffusion potential of the junction, which in turn gives unstable pH values.

Dehydration

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.

Different designs

Most 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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Download the new release ISO 14001:2015