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conditions of near-zero current.Amperometric sensors are usually operated by impos-
ing an external cell voltage sufficiently high to maintain a zero oxygen concentration
at the cathodic surface; therefore, the sensor current response is diffusion controlled
(Tan and Tan, 1996). According to Tan and Tan (1996), sensitivity of amperometric
sensors is better than potentiometric sensors. In addition, U.S. DOE (1994, p. A-1)
reports that amperometric methods are used in high-performance liquid chromatog-
raphy because of its enhanced sensitivity. Additional details of potentiometric and
amperometric sensors can be found in Janata (1990).
A common application for potentiometric and amperometric sensors is for water
analysis. The most common is the pH sensor system. The basic principal of these
devices is that they require two separated, carefully controlled liquid reservoirs with
two different chemically unstable electrodes (called reference electrodes), for exam-
ple a silver wire with a coating of silver chloride. The pH is measured by the voltage
difference between the two reference electrodes, so the unknown sample must be in
electrochemical connection with both solutions through a glass membrane. However,
these thin porous membranes can break, the solutions can leach out or dry out, or
the chemistry of the reference electrode itself can change giving a slightly different
voltage. Small changes in the chemistry can result in large changes in output voltage.
Consequently these systems require constant attention and calibration against known
pH solutions.
Many so-called ion selective electrodes for particular ions are sold using basically
the same system described above but with special membranes taking the place of
the pH-sensitive glass that give potential differences for different ions. The same
maintenance and calibration problems exist, as well as interference problems from
other ions. Some gases that can be detected using potentiometric methods include
carbon dioxide, oxygen, carbon monoxide, hydrogen, chlorine, arsenic oxides, and
oxidizable pollutants.
Commercial potentiometric cells for VOCs are less common; most are used for
toxic gases and oxygen. There are several research papers describing how to mea-
sure different VOCs in liquid electrochemical cells (Sawyer et al., 1995). The same
corrosion and drift problems exist for these experiments as described above. In
addition, a membrane or porous plug must be used to provide the diffusion of
the vapor phase VOC molecules from the gas phase into the electrolyte and elec-
trode surface. To speed up the process, the working electrode is placed virtually
on top of the gas-permeable membrane. A recent research example of trace detec-
tion of explosive molecules is given in Berger (2000) with some discussion of
the difficulties (interference of electroactive O 2 is important) and virtues (under
controlled conditions, very low vapor-phase concentrations (ppb) of TNT can be
detected).
Pros: These devices can be specific for a particular gas or vapor and are typically
very accurate. They do not get poisoned and can monitor at ppm levels.
Cons: Primary sensitivity is for toxic gases and oxygen, not VOCs. Not amenable for
in-situ applications. Membranes are sensitive and may degrade with time. Devices