glass is 72% SiO 2 , 22% Na 2 O, and 6% CaO. The surface of the glass must be hydrated for the
membrane to be pH sensitive. This is achieved by soaking the electrode in water or an aqueous buffer
solution for several hours when the following ion-exchange reaction occurs
The equilibrium is driven far to the right by this prolonged treatment, and results in the formation of
hydrated gel layers at the inner and outer surfaces of the membrane. A diagrammatic representation of a
cross-section of the membrane is shown in Figure 6.3.
Figure 6.3
Cross-section of a glass membrane.Although the surfaces of the gel layers contain only hydrogen ions, the ratio [H+]/[Na+] decreases
towards the dry glass layer which contains sodium ions only. Differences in hydrogen ion activity
between the internal and external solutions result in a potential developing across the membrane by
virtue of ion-exchange and a migration of charge. The charge is carried by sodium and hydrogen ions
within the gel layers, by proton-proton exchange across the gel-solution interfaces and by the movement
of sodium ions between vacant lattice sites (defects) in the dry glass layer. The potential developed is
given by an equation similar to equation (6.9)
where aH+ is the activity of hydrogen ions in the external solution and k" contains the logarithm of the
activity of the internal solution and an asymmetry potential which is caused by strains in the curved
membrane structure. The asymmetry potential changes with time primarily because of changes in the
external gel surface with use or because of contamination. The electrode therefore requires frequent
calibration when used for direct measurements.
Errors in the Use of a Glass Electrode
Because soda-glass membranes contain a high proportion of sodium ions, they exhibit a marked
response to sodium ions in solution. The effect becomes increasingly significant as the hydrogen ion
activity decreases, i.e. at high pH, and it is sometimes referred to as the alkaline error. At pH 12, the
error is about 0.3 of a pH unit if the solution is 0.1 M with respect to sodium ions, and 0.7 of a pH unit
if the solution is 1 M in sodium ions. Other monovalent cations such as lithium and potassium have a
similar but smaller effect. By replacing the sodium in