Reference electrolyte

The main function of the reference electrolyte is to, via the reference element and the reference junction, electrically connect the measured media to the pH meter. But not only to connect, you also want the electrolyte to have a stable connection. And if you speak in electrical terms that mean a stable electrical potential drop over the electrolyte. However, the electrolyte only has a stable connection as long as the concentration of the solution is stable.

Requirements of the reference electrode electrolyte

  • The electrolyte must have a high ionic strength to minimize resistance.
  • A stable electrolyte concentration is essential for a stable reference electrode potential.
  • The electrolyte has to remain stable over large temperature swings.
  • As the electrolyte will escape at the reference junction it's important that the measured solution is not affected.

Reference electrolyte composition

reference electrolyte

The traditional pH electrolyte is a potassium chloride solution (KCl, 3 mol/l). A KCl electrolyte establishes, in many cases and especially for laboratory use, the most reliable contact with the measured solution.

However, for demanding applications and especially for process online pH measurements, a gelled electrolyte or a solid polymer electrolyte has proven to be a simpler and more robust and rugged alternative.

Gel-filled reference electrolyte

The gel-filled electrolyte uses a high-viscosity gelling agent added to the electrolyte solution. The gel is so thick that it can hardly escape through the reference junction and therefore, does not have to be refilled. The high viscosity also permits the use of a large permeable diaphragm, as for instance, the annulus of a Teflon ring.

The gel-electrodes are often pre-pressurized to assure the solution being measured doesn't penetrate the junction and contaminate the reference system. If the electrolyte becomes poisoned it cannot be changed, and the electrode must be replaced. The lifetime of a gel-electrode is finite, since they cannot be refilled. However, the minimum maintenance has given rise to their popularity.

The main disadvantage of the gel-electrode is that it's less accurate than the liquid-filled electrode due to a slower response time.

Solid polymer electrolyte

The solid polymer electrolyte offers further advantages in comparison to the gel-filled electrolyte.

The physical nature of the solid polymer means that the electrolyte:

  • can be in direct contact with the sample, eliminating the reference junction and diaphragm clogging problem.
  • is resistant to penetration of potentially poisoning substances.
  • is pressure-resistant.

It's not surprising the solid polymer electrode is common for highly demanding industrial chemical processes.

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