ions and other species in solution may be either chemical or electrical in origin. As electrode response is
a function of the activity of the ion to be monitored rather than concentration, reactions resulting in its
partial or complete complexation, precipitation or other chemical changes will reduce the activity of the
ion and consequently the electrode response, i.e. the electrode responds only to the ''free' ion in solution.
These effects are illustrated for the fluoride ion as follows:
Ions which affect the membrane potential directly will produce an apparent increase in activity of the
ion to which the electrode nominally responds. In these circumstances the cell potential is more
accurately given by the expression
where a 2 is the activity of the interfering ion of charge Z, and k1,2 is the selectivity ratio for ion 1 over
ion 2. If k1,2 is zero, the electrode is truly specific for ion 1, whereas values greatly in excess of one
indicate that the electrode would be subject to severe interference from the second ion. As selectivity
ratios are influenced by overall solution composition they should be regarded as approximations only.
Furthermore, confusion can arise because reciprocal values may be quoted, e.g. if k1,2 = 0.005 then
. Some examples of selectivity ratios are included in Table 6.2.
(a)—
Glass Electrodes for pH Measurements
The membrane consists of a thin envelope of soft glass sealed into the end of a hard-glass tube. The
tube is filled with a dilute solution of hydrochloric acid in which a silver wire is immersed thus forming
a silver-silver chloride reference electrode (Figure 6.2). The acid also provides a solution of hydrogen
ions of constant activity a 2. The chemical composition and physical characteristics of the glass
membrane are critical in determining the electrode response. Soda-glasses are highly hygroscopic, have
a high electrical conductivity and show a good response whereas Pyrex glass or quartz is virtually
insensitive. Typically, the composition of a suitable soda-