INSTRUMENTATION: WATER AND WASTEWATER ANALYSIS 563
We shall make a simplifying, practical, and slightly
erroneous substitution replacing activity with concentration
[Moles per liter] as follows:E E^0 (0.0591/n)Log[red]/[oxid]. (13)A convenient expression for the log of the concentration of
some species if the p function. By definition it ispF Log 1/[F]. (14)The most common p function is pH, defined by Log 1/[H^ ^ ]
as measured by a glass electrode (a membrane electrode).
However this function applies to other species, e.g., pOH,
pCa, pNO 3 etc. The p functions can replace the log of the
reciprocal of the concentration terms.
The specificity of the indicating electrode is determined
by the materials of construction and the structure. There are
several types of indicating electrodes, metallic electrodes of
the first, second, and third kinds and membrane (selective)
electrodes including glass, solid-state, liquid membrane, gas
sensing, and enzyme and microbial electrodes. The latter
two electrodes can be of the potentiometric and amperomet-
ric types. For organizational convenience and simplicity all
these electrodes shall be discussed in this section entitled,
potentiometric instrumentation.(1) Metallic electrodes^12
An electrode of the first kind of a metal wire, rod or
plate in equilibrium with its metallic ion in solution. For
example, a silver indicating electrode, a silver wire, in con-
tact with silver ions, Ag^ ^ , gives the potential mathematically
described by the Nernst equation,E E^0 (0.0591/n)Log 1/[Ag^ ^ ] (15a)e E^0 (0.0591/n) pAg (15b)Metallic electrodes of inert metals such as platinum, gold,
palladium, etc. and non-metals such as carbon and boron
carbide may be used as indicating electrodes when the
redox couples are soluble species as Fe^2 ^ /Fe^3 ^. However the
electrode response is not always reversible for some com-
binations of electrodes and redox couples leading to non-
reproducible potentials.
An electrode of the second kind consists of a metal elec-
trode whose surface is coated with a slightly soluble salt of
that metal or a metal electrode in contact with a solution con-
taining a low concentration of a complex of that metal. An
example of the former is a mercury electrode coated with
slightly soluble mercurous chloride making the electrode
potential sensitive to chloride ion concentration in the solu-
tion. Most reference electrodes are of the former type. Two
examples are the calomel (mercury/mercurous chloride) and
the silver/silver chloride electrodes. Figure 19 is a schematic
of a calomel reference electrode.^38 A solution of known con-
centration of chloride ion bathes the electrode allowing thepotential to remain constant. Potassium chloride concentra-
tions at several different levels, 0.1 or 1.0 M or saturated are
often used providing several different reference electrodes.
As predicted by the equilibrium and Nernst equations for
the calomel reference electrode (see equations 16a and 16b),
different chloride ion concentrations will result in different
potentials.(16a)E E^0 (0.0591/n)Log [Cl^ ^ ]^2 (16b)Since Hg 2 Cl 2 (s) and Hg(1) are a solid (s) and liquid (1),
respectively, insoluble and not in ionic form in the electrode
electrolyte (see Figure 19), they are not represented in the
Nernst equation. (2e represents the number of electrons
taking part in the reaction.) Similar equations can be written
for the silver/silver chloride reference electrode.
A metal electrode made sensitive to a second metal is an
electrode of the third kind. The metal electrode is immersed
in the analyte solution containing a small concentration of
the metal complex and of a similar complex of the second
metal. The electrode potential is dependent on the concen-
tration of the ion of the second metal.^39(a) Oxidation–reduction potential, ORP^40
The measurement of the ORP of a system can provide
valuable information about its oxidative state. It must be
understood that the ORP value is the overall potential of the
system being measured, not necessarily at equilibrium, and
very seldom relates to one species. With caution in interpreta-
tion in mind, the ORP value can be used essentially in two
area: biological and chemical inprocessing measurements and
control.
The ORP measurement is made in a cell using a non-
polarizing indicating electrode, a reference electrode and a
suitable potential readout device—an electronic voltmeter.
The indicating electrode, an electrode of the third kind, is
usually a noble metal such as gold or platinum in the form
of a wire or button. The redox system equilibrates electroni-
cally with the indicating electrode and the reference elec-
trode. For the following system,(17)one can write the reduction potential, E Mb /Ma , using the
Nernst equation, where b and a are the number of charges
and b a, and E^0 is the standard reduction potential of the
system.EMb Ma EMb^0 Ma 0.0591/ b( a)logaaMaMb (18)If one assembles a cell to measure the ORP of this system
using an indicating electrode and a standard hydrogen elec-
trode, the cell potential value measured is the potential of theC009_005_r03.indd 563C009_005_r03.indd 563 11/23/2005 11:12:23 AM11/23/2005 11:12:23