Membrane or Ion-selective Electrodes
These can be subdivided into
(a) glass electrodes
(b) solid-state electrodes
(c) liquid-membrane electrodes
(d) gas-sensing electrodes
The construction and mechanism for the development of activity-dependent potentials is similar for all
types, although gas-sensing electrodes are constructed slightly differently from the others and
incorporate an internal pH-sensitive glass electrode. The first three types consist of a tube into one end
of which is sealed an electrically-conducting membrane. The tube contains a solution or gel
incorporating the ion to which the electrode is to respond, and another electrolyte, usually potassium or
sodium chloride. The latter, together with a silver wire in contact with the solution or gel, constitutes an
internal silver-silver chloride reference electrode. The cell is completed with a second or 'external'
reference electrode. On immersion of both electrodes into a solution containing the ion to be monitored,
a potential develops across the membrane the magnitude of which is related to the activities of the ion
of interest in the internal and external (sample) solution. The response of many membranes is highly
selective in that the membrane potential is a function of the activity of only one ion or a small number
of ions.
In essence, the cell comprises two reference electrodes, whose potentials are constant, separated by the
membrane whose potential governs the overall cell potential. Ideally, the response will be Nernstian,
and at 298.15 K the cell potential is given by
where k is a constant including the external and internal reference electrode potentials, and a 1 and a 2 are
the activities of the ion to be measured in the external and internal solutions respectively. As a 2 is also
constant, then
where k' includes log 10 a 2
Electrode Response and Selectivity
The term 'ion-selective' is to be preferred to 'ion-specific' in discussing membrane electrodes as most if
not all are subject to the influence of ions other than the one to which they nominally respond.
Interference from such