Potentiometry and ion-selective electrodes
These systems involve galvanic cells and are based on measurement
of the potential (voltage) difference between two electrodes in solution when
no net current flows between them: no net electrochemical reaction occurs
and measurements are made under equilibrium conditions. These systems
include methods for measuring pH, ions, and gases such as CO2
typical potentiometric cell is shown in Fig. 34.2. It contains two electrodes:
- a 'sensing' electrode, the half-cell potential of which responds to changes
in the activity (concentration) of the substance to be measured; the most
common type of indicator electrodes are ion-selective electrodes (ISEs);
- a 'reference' electrode, the potential of which does not change, forming
the second half of the cell.
To assay a particular analyte, the potential difference between these
electrodes is measured by an mV meter (e.g. a standard pH meter).
Reference electrodes for potentiometry are of three main types:
- The standard hydrogen electrode, which is the reference half-cell
electrode, defined as 0.0 V at all temperatures, against which values of E0 are expressed. H2 gas at 1 atmosphere pressure is bubbled over a
platinum electrode immersed in an acid solution with an activity of
unity. This electrode is rarely used for analytical work, since it is
unstable and other reference electrodes are easier to construct and use.
- The calomel electrode (Fig. 34.3), which consists of a paste of mercury
covered by a coat of calomel (Hg2Cl2), immersed in a saturated solution
of KC!. The half-reaction Hg2Cl2 + 2e− → 2Hg + 2Cl− gives a stable
standard electrode potential of +0.24 V.
- The silver/silver chloride electrode. This is a silver wire coated with AgCl
and immersed in a solution of constant chloride concentration. The halfreaction
AgCl + e− → Ag + Cl− gives a stable, standard electrode
potential of +0.20 V.
|Fig. 34.2 Components of a potentiometric cell.
Ion-selective electrodes USEs) are based on measurement
of a potential across a membrane which is selective for a particular
An ISE consists of a membrane, an internal reference electrode, and an internal
reference electrolyte of fixed activity. The ISE is immersed in a sample solution
that contains the analyte of interest, along with a reference electrode. The
membrane is chosen to have a specific affinity for a particular ion, and if activity
of this ion in the sample differs from that in the reference electrolyte, a potential
develops across the membrane that is dependent on the ratio of these activities.
Since the potentials of the two reference electrodes (internal and external) are
fixed, and the internal electrolyte is of constant activity, the measured potential, E
, is dependent on the membrane potential and is given by the Nernst equation:
⇒ Equation [34.3]
||K + 2.303
represents a constant potential which is dependent on the reference
represents the net charge on the analyte, [a
] the activity of analyte
in the sample and all other symbols and constants have their usual meaning. For a series of standards of known activity, a plot of E
against log [a
should be linear over the working range of the electrode, with a slope of
(0.059 V at 25°C).
|Fig. 34.2 Components of a potentiometric cell.
Although ISEs strictly measure activity
, the potential differences can be approximated to concentration as long as (i) the
analyte is in dilute solution, (ii) the ionic strength of the calibration
standards matches that of the sample, e.g. by adding appropriate amounts of
a high ionic strength solution to the standards, and (iii) the effect of binding
to sample macromolecules (e.g. proteins, nucleic acids) is minimal.
Potentiometric measurements on undiluted biological fluids, e.g. K+
levels in plasma, tissue fluids or urine, are likely to give lower values than
flame emission spectrophotometry, since the latter procedure measures total
ion levels, rather than just those in aqueous solution.
All of the various types of membrane used in ISEs operate by incorporating
the ion to be analysed into the membrane, with the accompanying establishment
of a membrane potential. The scope of electrochemical analysis has been
extended to measuring gases and non-ionic compounds by combining ISEs
with gas-permeable membranes, enzymes, and even immobilized bacteria or
Glass membrane electrodes
The most widely used ISE is the glass membrane electrode for pH
measurement. The membrane is thin glass (50 µ
m wall thickness)
made of silica which contains some Na+
. When the membrane is soaked in
water, a thin hydrated layer is formed on the surface in which negative oxide
) in the glass act as ion-exchange sites. If the electrode is placed
in an acid solution, H+
exchanges with N+
in the hydrated layer, producing
an external surface potential: in alkaline solution, H+
moves out of the
membrane in exchange for N+
. Since the inner surface potential is kept
constant by exposure to a fixed activity of H+
, a consistent, accurate
potentiometric response is observed over a wide pH range. Glass electrodes
for other cations (e.g. N+
) have been developed by changing the
composition of the glass, so that it is predominantly sensitive to the
particular analyte, though the specificity of such electrodes is not absolute.
The operating principles and maintenance of such electrodes are broadly
similar to those for pH electrodes.
Gas-sensing glass electrodes
Here, an ISE in contact with a thin external layer of aqueous electrolyte (the
'filling solution') is kept close to the glass membrane by an additional, outer
membrane that is selectively permeable to the gas of interest. The
arrangement for a CO2
electrode is shown in Fig. 34.4: in this case the outer
membrane is made of CO2
-permeable silicone rubber. When CO2
gas in the
sample selectively diffuses across the membrane and dissolves in the filling
solution (in this case an aqueous NaHCO
/NaCl mixture), a change in pH
occurs owing to the shift in the equilibrium:
|Fig. 34.4 Underlying principles of a gassensing
|⇒ Equation [34.4]
||CO2 + H2O ↔ H2CO3 ↔ H+ + HCO3−
The pH change is 'sensed' by the internal ion-selective pH electrode, and its
response is proportional to the partial pressure of CO2
of the solution
). A similar principle operates in the NH3
electrode, where a Teflon®
membrane is used, and the filling solution is NH4
Liquid and polymer membrane electrodes
In these types of ISEs, the liquid is a water-insoluble viscous solvent
containing a soluble ionophore, i.e. an organic ion exchanger, or a neutral
carrier molecule, that is specific for the analyte of interest. When this liquid is soaked into a thin membrane such as cellulose acetate, it becomes effectively
immobilized. The arrangement of analyte (A+
) and ionophore in relation to
this membrane is shown in Fig. 34.5. The potential on the inner surface of
the membrane is kept constant by maintaining a constant activity of A+
the internal solution, so the potential change measured is that which results
in the sample interacting with the ionophore in the outer surface of
A relevant example of a suitable ionophore is the antibiotic valinomycin,
which specifically binds K+
. Other ionophores have been developed for
measurement of, for example, NH4+
|Fig. 34.5 Underlying principles of a liquid
membrane ion-selective electrode.
A+ = analyte; C = neutral carrier ionophore;Em = surface potential; membrane
potential = Emlinternal) - Em(external).
. In addition, electrodes have
been developed for organic species by using specific ion-pairing reagents in
the membrane that interact with ionic forms of the organic compound, e.g.
with drugs such as 5,5-diphenylhydantoin.
Solid-state membrane electrodes
These contain membranes made from single crystals or pressed pellets of salts
of the analyte. The membrane material must show some permeability to ions
and must be virtually insoluble in water. Examples include:
- the fluoride electrode, which uses LaF3 impregnated with Eu2+ (the latter
to increase permeability to F−). A membrane potential is set up when F− in
the sample solution enters spaces in the crystal lattice;
- the chloride electrode, which uses a pressed pellet membrane of Ag2S and