Why do double junction pH electrodes last longer than single junction pH electrodes?

Document ID

Document ID TE2627

Published Date

Published Date 07/10/2018
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Question
Why do double junction pH electrodes last longer than single junction pH electrodes?
Summary
Why double junction pH electrodes last longer than single junction pH electrodes
Answer
The reason most pH electrodes eventually fail is that all pH reference electrodes (whether used with glass sensors or ISFET sensors) deteriorate with use. This deterioration is a combination of two factors:

1. One factor is the reference electrode’s electrolyte ions (suspended in liquid, gel or a polymer) are very slowly depleted with use. Electrolyte depletion occurs with all pH electrodes whether they are a heavy-duty industrial electrodes, a sophisticated laboratory electrode, or an electrode on a pocket pH tester. If this is the only factor deteriorating a pH reference electrode, electrode life should be long enough to meet the user’s expectations.

2. The second factor is that contaminating ions from the measured solution can rapidly cause chemical reactions with the silver/silver chloride reference electrolyte system commonly used in pH reference electrodes. This causes sluggish, erratic, wrong or even no pH electrode response as the reference electrode wire is spoiled or the reference junction is clogged. This results in fast electrode failure. This type of deterioration proceeds most rapidly when the pH reference electrode is a single junction pH reference electrode and the solution measured has high concentrations of chemicals (ions) that contaminate and then attack critical components of the reference electrode.

Advantages of double junction electrodes:

In high priced, high performance meters, it is common practice to use a double junction pH reference electrode design to slow down pH reference electrode chemical attack. The double junction pH reference electrode isolates the chemically sensitive Ag/AgCl based pH reference electrode system behind a second reference junction and a reference cell filled with KCl electrolyte (suspended in a liquid, gel or polymer).

In double junction electrodes, chemicals (ions) that attack the pH reference electrode signal wire or react with the internal pH reference electrolyte (Ag/AgCl), take much longer to come into contact with the pH reference electrode signal wire and internal pH reference electrolyte. These contaminants must migrate through the first (outer) reference junction, build up a concentration in the cell filled with KCl reference electrolyte, and finally migrate through the second (internal) reference junction before coming in contact with the pH reference electrode signal wire and internal pH reference electrolyte. This longer migration of contamination to internal reference cell delays the pH reference electrode damage that ruins the pH reference electrode. This makes the double junction pH reference electrode and the entire pH electrode system last much longer than single junction pH electrode systems.
 
 

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