The active part of a pH measuring electrode is the probe tip. The probe-tip
can be manufactured in different shapes, depending on the application.
The probe tip is constructed from a special composition glass which
senses the hydrogen ion concentration. This glass is mostly amorphous
silicon dioxide(SiO2), with embedded oxides of alkali metals, mainly Na. It is made to be as thin as possible, about 0.1 mm thick.
When the surface of the glass is exposed to water, the alkali metal ions of the glass and the hydrogen ions (H+)
in the solution undergo an ion exchange reaction. A thin, about 10 nm,
hydrated gel layer forms on the outer surface of the glass, the
so-called glass membrane. Such a hydrated gel layer arises also on the
inside of the glass tip which is in contact with a buffer solution,
usually 0.1M HCl, with a constant pH value.
Depending on the pH of the solution being measured, hydrogen ions (H+) will migrate into or out of the gel layer. In an alkaline solution, hydrogen ions migrate out of the gel layer and a negative charge is developed on the outer gel layer. Hydrogen ion does not cross through the glass membrane, it is the Na+ which crosses and allows for a change in free energy.
The hydrated gel (glass membrane) is what makes the pH measuring electrode an hydrogen ion selective electrode. In order to operate properly, the glass tip needs to be hydrated. This is the reason a pH electrode needs to be kept moist at all times.
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