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