pH electrodes are constructed from a
special composition glass which senses
the hydrogen ion concentration. This glass
is typically composed of alkali metal ions.
The alkali metal ions of the glass and the
hydrogen ions in solution undergo an ion
exchange reaction generating a potential
difference. In a combination pH electrode,
the most widely used variety, there are
actually two electrodes in one body. One
portion is called the measuring electrode,
the other the reference electrode. The
potential that is generated at the junction
site of the measuring portion is due to
the free hydrogen ions present in solution.
The potential of the reference portion is
produced by the internal element in
contact with the reference fill solution.
This potential is always constant. In
summary the measuring electrode
delivers a varying voltage and the
reference electrode delivers a constant
voltage to the meter.