The reference sensor or the so-called reference electrode is crucial to the pH measurement accuracy. Many complaints of pH measurement failures turn out to be reference electrode failures.
The reference electrode consists of a reference element that is immersed in a defined electrolyte solution. The reference element is electrically connected with the electrolyte. The electrolyte, in turn, is electrically connected to the measured solution through a porous junction or diaphragm which physically isolates the electrolyte from the solution being measured.
An effective and well-established reference is the silver reference element. As discussed for the glass electrode, a silver (Ag/AgCl) wire immersed in a defined stable electrolyte solution, will give a good electrical connection.
The electrolyte, or the so-called salt-bridge solution, must have a high ion concentration to minimize electrical resistance, not affect the solution being measured and remain stable over a wide temperature range. The most used electrolyte is a saturated potassium chloride (KCl) solution. Ideally, no reaction between the reference electrolyte and the measuring solution should occur.
The silver (Ag/AgCl) wire and the KCl electrolyte combination is referred to as the silver/silver chloride reference system. It's the most well-established and widespread system. However, other types of reference systems are also manufactured for specific measurement needs.
junction is an opening with a very small diameter which allows contact
between the electrolyte of the reference sensor and the sample, but prevents
them from becoming mixed together rapidly. The goal with the junction is
to create optimal electrical contact between the reference electrolyte
and the measuring solution.
It is not an easy task to connect and isolate two solutions at the same time. Read more about the reference junction.
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