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Glass Membrane pH Sensors (Part 2 of 3)

By David Mills

E-Zine July 2013

Click here to review Part 1

When the surface of silicate glasses containing alkali metals such as lithium contact water, the alkali metals undergo an ion-exchange reaction. In this process, an alkali metal ion from the glass goes into solution and a hydrogen ion takes its place in the glass matrix. It is this reaction at the glass surface that results in the development of an electrical potential at the phase boundary. By designing the measurement half-cell such that the membrane of this special glass is simultaneously exposed to the process fluid on the outside and a chloride buffer of constant pH on the inside as shown in the above figure, the overall potential developed across the membrane is:

Since a chloride buffer is used on the inside, an Ag/AgCl electrode can be used in this measurement half-cell. By combining this measurement half-cell with a reference half-cell that also uses an Ag/AgCl electrode and a chloride buffer with the same chloride activity, the potentials developed at these similar electrodes will cancel each other and the overall potential developed between the two electrodes will, except for the junction potential, EJ, be proportional to the pH of the process fluid. The junction potential is always present and often a significant source of error.

Click here to review Part 3

Excerpted from The Consumer Guide to Industrial pH and ORP Instrumentation

ISSN 1538-5280

Spitzer and Boyes, LLC
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