Electrochemical Methods for Environmental Analysis - Fundamentals of Environmental Sampling and Analysis - page 295

a solution, where a potential is applied across the plates and the current is measured.
Using the voltage and current values, the resistance is determined by applying
Ohm's law:

R ¼ V = I ð11 : 10Þ

where R¼ resistance (ohms), V¼voltage (volts), and I¼current (amperes). Since
electrical conductance (G) is the reciprocal of electrical resistance (Eq. 10.14),
electrical conductivity of an electrolytic solution is actually the determination of the
electrical resistance of the solution under a known potential gradient using a sensor
of known geometry. Conductometry is directly used in water quality measurements,
such as conductivity and salinity. It is also the basis for all ion conductivity detectors
employed in ion chromatography.

11.1.3 Types of Electrodes and Notations
for Electrochemical Cells

Electrodes are the centerpieces of all electrochemical analyses. In the preceding
section, we introduced two major types of electrodes (reference electrode and
indicator electrode). Various electrodes are further described here along with some
shorthand notations for the electrodes.
Reference electrodes provide a constant potential without being affected by the
composition of a sample. An ideal reference electrode should also obey Nernst
equation. Two common reference electrodes, with their shorthand notations, are
given below:

Hg 2 Cl 2 ðsÞjHgðsÞ ðE 0 SHE at 25 CÞ

k KCl ðsaturatedÞ; ¼þ0 : 214 vs :

AgClðsÞjAgðsÞ ðE 0 SHE at 25 CÞ

k KCl ðsaturatedÞ; ¼þ0 : 197 vs :

The first reference electrode, called saturated calomel electrode (SCE), is mercury
in contact with a solution that is saturated with mercury chloride (calomel), which is
in turn saturated with KCl solution. The second one is a silver/sliver chloride
electrode in which a silver wire coated with a layer of silver chloride is immersed in
a saturated KCl solution. In the above notations, k denotes a salt bridge, and j
denotes an interface between a solid electrode and a solution. The half-cell standard
potential shown above is in reference to a standard hydrogen electrode (SHE), which
is defined to have zero volts at standard state conditions. If two half-cells are
combined to form a complete circuit, by convention, the half reaction involving
oxidation (anode) should be written on the left and the reaction involving reduction
(cathode) should be on the right.
Indicator electrodes respond directly to the analyte and ideally respond only
to it. Two types of indicator electrodes are available, namely, metallic electrodes
and membrane electrodes. Metallic electrodes, such as metal/metal ion and metal/
metal salt/anion, are no longer routinely used as indicator electrodes in env-
ironmental analysis. It is the membrane electrodes that are found the most in