Glass Electrode

All types of glasses possess the property of producing a potential difference relative to the
hydrogen ion concentration in aqueous solutions. However only special types, such as the
conventional Mc-Innes glass (Corning 015) produce galvanic potentials which satisfy the
NERNST equation over a wide range of the pH scale.
Every manufacturer of pH electrodes is constantly researching for better pH sensitive glasses.
Through constant development HAMILTON has achieved results which have not previously been
available without unsatisfactory compromises.
When the membrane glass of a measuring electrode comes into contact with an aqueous
solution, it forms a thin gel layer of approximately 10-4 mm thickness between the glass
surface and the solution. The thickness of the gel layer depends on the quality and composition
of the membrane glass, the temperature and the pH value of the measured solution. As the
internal side of the glass membrane is in contact with the inner buffer (an aqueous solution of pH
7) a gel layer is also formed on the inside of the glass membrane.
A continuous exchange of H+ ions in the gel layers and H+ ions of the solutions takes place on
both sides of the membrane. This ion exchange is controlled by the H+ ion concentration of both
solutions.
If the hydrogen ion concentration of each solution is identical on both sides of the glass
membrane, the ion exchange stops after an equilibrium has been reached between the H+ ions in
the solutions and the H+ ions in the gel layers. Therefore, both sides of the membrane glass have
the same potential and the potential difference is 0 mV.
If a difference of a hydrogen ion con-centration
exists between the inner buffer and the outer
solution, a potential difference develops between
the inner and outer sides of the membrane glass
which is proportional to the difference in pH
between the inner buffer and the outer solution. To
be able to measure the membrane potential, the
membrane itself has to be conductive. This is
achieved by the mobility of the alkaline ions in the
membrane glass (Li+ ions in most glasses today or
Na+ ions in older membrane glasses).