Electrochemical Detection - Waterborne Pathogens: Detection Methods and Applications - page 142

membranes can be made of either a solid-state material (e.g. a crystalline
or polycrystalline inorganic salt, like Ag 2 S or LaF 3 , or noncrystalline like
glass or polymer), or a liquid material (e.g. an aqueous solution of Ca 2+ ) .
The glass-membrane electrode used in a pH meter belongs to this family of
electrodes. It was discovered by Cremer in 1906. It measures the difference
in activity of hydronium ions (H 3 O + ).
The working range for most ISEs is from 0.1-1 M to 10 −5 -10 −11 M. This
broad range is significantly greater than many other analytical techniques.
There are several limitations to the determination of an analyte's activity
by potentiometry. One problem is that standard-state potentials ( E °) are
temperature dependent, and the values in reference tables are usually for
a temperature of 25 °C. A second problem is that standard-state potential
depends on the chemical composition of the solution. A third problem is
the fact that a small electrical current may pass through the cell. A final
limitation is the appearance of junction potentials at the interface of two
ionic solutions (i.e. at the interfaces between the sample and the salt bridge).
Due to all of these uncertainties, accuracies of 1%-10% are usually observed
depending on the type of ion. The precision is typically lower than ±0.8%,
again depending on the type of analyte.
Perhaps the most frequent use of potentiometry is the determination
of a solution's pH. It consists of determining the activity of hydrogen ions
(H + ). When a glass surface is immersed in an aqueous solution, then a thin
solvated layer (gel layer) is formed on the glass surface in which the glass
structure is softer. This applies to both the outside and inside of the glass
membrane. The concentration of protons inside the membrane is constant
(pH=7), and the concentration outside is determined by the concen-
tration, or activity, of the protons in the electrolyte. This concentration
difference produces the potential difference that we measure with a pH
meter. Measurement of pH is not trivial and requires careful calibration
and procedure. In a similar way, for clinical applications, most common
analytes' activity, such as Na + , K + , C a 2+ , and Cl − , can be measured by
potentiometry.
Another application of potentiometric sensor is gas sensing. Indeed, a
number of membrane electrodes respond to the concentration of a dis-
solved gas. The basic arrangement consists of a thin membrane separating
the sample from an inner solution containing an ISE. The membrane is
permeable to the gaseous analyte. The gas molecules pass through the mem-
brane where they react with the inner solution, producing a species whose
concentration is monitored by the ISE. For example, in a CO 2 electrode,