Chapter 23- Potentiometry
A: Reference Electrodes
A reference is an electrode that has the half-cell potential known, constant, and
completely insensitive to the composition of the solution under study. In conjunction
with this reference is the indicator or working electrode, whose response depends upon
the analyte concentration.
Ideal Reference Electrode:
 Is reversible and obeys the Nernst equation
 Exhibits a potential that is constant with time
 Returns to its original potential after being subjected to small currents
 Exhibits little hysteresis with temperature cycling
Calomel Electrodes
Consist of mercury in contact with a solution that is saturated with mercury (I) chloride
(calomel) and that also contains a known concentration of potassium chloride. The
saturated calomel electrode (SCE) is widely used by analytical chemists because of the
ease with which it can be prepared. However, its temperature coefficient is greatly larger
than those of other calomel electrodes. Also, when the temperature is changed, its
potential comes to a new value only slowly because of the time required for solubility
equilibrium for the potassium chloride and for the calomel to be reestablished. The body
of the outer electrode consists of an outer glass or plastic tube that is 5 to 15cm in length
and 0.5 to 1.0cm in diameter. A mercury/mercury (I) chloride paste in saturated
potassium chloride is contained in an inner tube, which is connected to the saturated
potassium chloride solution in the outer tube through a small opening. A sleeve-type
electrode is particularly useful for measurements of nonaqueous solutions and samples in
the form of slurries, sludges, viscous solutions, and colloidal suspensions.
Silver/Silver Chloride Electrodes
The most widely marketed reference electrode system consists of a silver electrode
immersed in a solution of potassium chloride that has been saturated with silver chloride.
The electrode potential is determined by the half-reaction
AgCl(s) + e- = Ag(s) + Cl-
Silver/silver chloride electrodes have the advantage that they can be used at temperatures
greater than 60C, whereas calomel electrodes cannot. On the other hand, mercury (II)
ions react with fewer sample components than do silver ions; such reactions can lead to
plugging of the junction between the electrode and the analyte solution.
Precautions in the Use of Reference Electrodes
In using reference electrodes, make sure the level of the internal liquid should always be
kept above that of the sample solution to prevent:
 Contamination of the electrode solution.
 Plugging of the junction due to reaction of the analyte solution with silver or
mercury (I) ions from the internal solution.
The amount of contamination is so slight that it is of no concern. In determining ions
such as chloride, potassium, silver, and mercury, however, precaution must be taken to
avoid this source of error. A way to enforce this is to interpose a second salt bridge
between the analyte and the reference electrode; this bridge should contain a
noninterfering electrolyte, such as potassium nitrate or sodium sulfate.
B: Metallic Indicator Electrodes
An ideal indicator electrode responds rapidly and reproducibly to changes in activity of
the analyte anion. There are two types of indicator electrodes: metallic and membrane.
There are four types of metallic indicator electrodes:
1. Electrodes of the first kind.
2. Electrodes of the second kind.
3. Electrodes of the third kind.
4. Redox electrodes.
Electrodes of the First Kind
They are in direct equilibrium with the cation derived from the electrode metal. Here, a
single reaction is involved. For example,
Cu 2+ + 2e- = Cu(s)
They are not widely used for potentiometric analyses for several reasons:
 They are not very selective.
 Respond not only to their own cations, but also to other more easily
reduced cations.
 Many metal electrodes can be only used in neutral or basic solutions
because they dissolve in the presence of acids.
 Some metals are so easily oxidized that their use is restricted to
solutions that have been deaerated.
 Certain harder metals do not provide reproducible potentials.
 These electrodes have plots of pX versus activity yield slopes that
differ significantly and irregularly fro the theoretical.
Electrodes of the Second Kind
A metal electrode can often be made responsive to the activity of an anion with which
its ion forms a precipitate or a stable complex ion. The electrode reaction can
then be written as
AgCl(s) + e- = Ag(s) + ClE0= 0.222V
An important electrode of the second kind for measuring the activity of EDTA anion
Y4- is based upon the response of a mercury electrode in the presence of a small
concentration of the stable EDTA complex of Hg(II). To employ this electrode
system, it is necessary to introduce a small concentration of HgY2- into the analyte
solution at the outset. The complex is so stable that its activity remains essentially
constant over a wide range of Y4- activities. This electrode is useful for establishing
the end points for EDTA titrations.
Electrodes of the Third Kind
A metal electrode can, under some circumstances, be made to respond to a different
cation.
Metallic Redox Indicators
Electrodes fashioned from platinum, gold, palladium, or other inert metals often serve
as indicator electrodes for oxidation/reduction systems. In these applications, the
inert electrode acts as a source or sink for electrons transferred from a redox system
in the solution. For example, the potential of a platinum solution containing Ce(III)
or CE(IV). Thus, a platinum electrode serves as the indicator electrode in a titration
in which Ce(IV) serves as the standard reagent. However, the electron-transfer
processes at inert electrodes are frequently no reversible.
C: Membrane Indicator Electrodes
A wide variety of membrane electrodes are available from commercial sources that
permit the rapid and selective determination of numerous cations and anions by direct
potentiometric measurements. Often, membrane electrodes are called ion-selective
electrodes because of the high selectivity of most of these devices.
Classification of Membranes
Properties of Ion-Selective Membranes
1. Minimal solubility. A necessary property of an ion-selective medium is that
its solubility in analyte solutions approaches zero.
2. Electrical conductivity. A membrane must exhibit some electrical
conductivity, albeit small. Generally, this conduction takes the form of
migration of singly charged ions within the membrane.
3. Selective reactivity with the analyte. A membrane or some species contained
within the membrane matrix must be capable of selectively binding the
analyte ion. Three types of binding:
 Ion-exchange


Crystallization
Complexation
The Glass Electrode for pH Measurements
The Composition and Structure of Glass Membranes
Corning 015 glass, which has been widely used for membranes, consists of
approximately 22% Na2O, 6% CaO, and 72% SiO2. The membrane is specific in its
response toward hydrogen ions up to a pH of about 9. The glass consists of an infinite
three-dimensional network of SiO44- groups in which each silicon is bonded to four
oxygens and each oxygen is shared by two silicons. Singly charged cations, such as
sodium and lithium, are mobile in the lattice and are responsible for electrical conduction
within the membrane.
The Hygroscopicity of Glass Membranes
The surface of a glass membrane must be hydrated before it will function as a pH
electrode. The amount of water involved is approximately 50mg per cubic centimeter of
glass. The hydration of a pH sensitive glass membrane involves an ion-exchange
reaction between singly charge cations in the glass lattice and protons from the solution.
In general, the ion-exchange reaction can be written as
H+ + Na+Gl- = Na+ + H+GlThe equilibrium constant for this process is so large that the surface of a hydrated glass
membrane ordinarily consists entirely of silica acid groups. An exception to this situation
exists in highly alkaline media, where the hydrogen ion concentration is extremely small
and the sodium ion concentration is large; here, a significant fraction of the sites are
occupied by sodium ions.
Electrical Conduction Across Glass Membranes
To serve as an indicator for cations, a glass membrane must conduct electricity.
Conduction within the hydrated gel layer involves the movement of hydrogen ions.
Sodium ions are the charge carriers in the dry interior of the membrane. Conduction
across the solution/gel interfaces occurs by the reactions. It is the potential difference
that serves as the analytical parameter in potentiometric pH measurements with a
membrane electrode.
Membrane Potentials
The Boundary Potential
The boundary potential is simply the difference between these potentials:
Eb = E1 – E2
Eb = E1 – E2 = 0.0592 log (a1/a2)
The boundary potential Eb depends only upon the hydrogen ion activities of the solutions
on either side of the membrane. For a glass pH electrode, the hydrogen ion activity of the
internal solution a2 is held constant so that it simplifies to
Eb = L’ + 0.0592 log a1 = L’ – 0.0592 pH
Where L’ = -0.0592 log a2.
The boundary potential is then a measure of the hydrogen ion activity of the external
solution.
The Potential of the Glass Electrode
The potential of a glass indicator electrode has three components:
1. The boundary potential.
2. The potential of the internal Ag/AgCl reference electrode.
3. A small asymmetry potential.
The Alkaline Error
Glass electrodes respond to the concentration of both hydrogen ion and alkali metal ions
in basic solution. The magnitude of this alkaline error for four different glass membranes
can be shown on a graph. The error is negative, which suggests that the electrode is
responding to sodium ions as well as to protons. This observation is confirmed by data
obtained for solutions containing different sodium ion concentrations. All singly charged
cations induce an alkaline error whose magnitude depends upon both the cation in
question and the composition of the glass membrane.
Selectivity Coefficients
The effect of an alkali metal ion on the potential across a membrane can be accounted for
by inserting an additional term in the previous equation to give
Eb = L’ + 0.0592 log (a1 + kb1)
Where k is the selectivity coefficient for the electrode and b1 is the activity of the alkali
metal ion. It applies not only to glass indicator electrodes for hydrogen ion but also to all
other types of membrane electrodes. Selectivity coefficients range from zero to values
greater than unity. A selectivity coefficient of unity means the electrode responds equally
to the analyte ion and the interfering ion. If an electrode for ion A responds 20 times
more strongly to ion B than to ion A, then k has the value of 20. If the response of the
electrode to ion C is 0.001 of its response to A, k is 0.001.
The Acid Error
A typical glass electrode exhibits an error, opposite in sign to the alkaline error, in
solutions of pH les than about 0.5; pH readings tend to be too high in this region. The
magnitude of the error depends upon a variety of factors and is generally not very
reproducible. The causes of acid error are not well understood.
Glass Electrodes for Other Cations
The alkaline error in early glass electrodes led to investigations concerning the effect of
glass composition upon the magnitude of this error. One consequence has been the
development of glasses for which the alkaline error is negligible below about pH 12.
Other studies have discovered glass compositions that permit the determination of cations
other than hydrogen. This application requires that the hydrogen ion activity a1 be
negligible relative to kb1; under these circumstances, the potential is independent of pH
and is a function of pB instead. Glass electrodes that permit the direct potentiometric
measurement of such singly charged species as Na+, K+, NH4+, and total concentration
of univalent cations are now available from commercial sources.
Crystalline Membrane Electrodes
The most important type of crystalline membranes is manufactured from an ionic
compound or a homogeneous mixture of ionic compounds. In some instances the
membrane is cut from a single crystal; in others, disks are formed from the finely ground
crystalline solid by high pressures or by casting from a melt. The typical membrane has a
diameter of about 10mm and a thickness of 1 or 2 mm. To form an electrode, the
membrane is sealed to the end of a tube made from a chemically inert plastic such as
Teflon or polyvinyl chloride.
Conductivity of Crystalline Membranes
Most ionic crystals are insulators and do not have sufficient electrical conductivity at
room temperature to serve as membrane electrodes. Those that are conductive are
characterized by having a small singly charged ion that is mobile in the solid phase.
Examples are fluoride ion in certain rare earth fluorides, silver ion in silver halides and
sulfides, and copper (I) ion in copper (I) sulfide.
The Fluoride Electrode
Lanthanum fluoride, LaF3, is a nearly ideal substance for the preparation of a crystalline
membrane electrode for the determination of fluoride ion. Although this compound is a
natural conductor, its conductivity can be enhanced by doping with europium fluoride,
EuF2. Membranes are prepared by cutting disks from a single crystal of the doped
compound. At the two interfaces, ionization creates a charge on the membrane surface as
shown by the equation
LaF3 = LaF2+ + FThe magnitude of the charge is dependent upon the fluoride ion concentration of the
solution. Thus, the side of the membrane encountering the lower fluoride ion
concentration becomes positive with respect tot eh other surface; it is this charge
difference that provides a measure of the difference in fluoride concentration of the two
solutions. The potential of a cell containing a lanthanum fluoride electrode is given by an
equation analogous to previous equations. That is,
E = L – 0.0592 log aF = L + 0.0592 pF
In most respects, the fluoride ion electrode approaches the ideal for selective electrodes.
Electrodes Based on Silver Salts
Membranes prepared from single crystals or pressed disks of various silver halides act
selectively toward silver and halide ions. Generally, their behavior is far from ideal,
however, owing to low conductivity, low mechanical strength, and a tendency to develop
high photoelectric potentials. It has been found, though, that these disadvantages are
minimized if the silver salts are mixed with crystalline silver sulfide in an approximately
1:1 molar ratio. The resulting disk exhibits good electrical conductivity owing to the
mobility of the silver ion in the sulfide matrix.
Liquid Membrane Electrodes
Liquid membranes are formed from immiscible liquids that selectively bond certain ions.
Membranes of this type are particularly important because they permit the direct
potentiometric determination of the activities of several polyvalent cations and of certain
singly charged anions and cations as well. The active substances in liquid membranes are
of three kinds:
1. Cation exchangers
2. Anion exchangers
3. Neutral macrocyclic compounds, which selectively complex certain cations
D: Ion-Selective Field-Effect Transistors (ISFETs)
The metal oxide semiconductor field-effect transistor (MOS-FET), which is widely used
in computers and other electronic circuits as a switch to control current flow in circuits.
One of the problems in employing this type of device in electronic circuits has been its
pronounced sensitivity to ionic surface impurities, and a great deal of money and effort
has been expended by the electronic industry in minimizing or eliminating this sensitivity
in order to produce stable transistors.
Mechanism of ISFET Ion-Selective Behavior
An ion-selective field-effect transistor is very similar in construction and function to an
n-channel enhancement mode MOSFET. The ISFET differs only in that variation in the
concentration of the ions of interest provides the variable gate voltage to control the
conductivity of the channel. The conductivity of the channel can be monitored
electronically to provide a signal that is proportional to the logarithm of the concentration
of hydronium ion in the solution. Note that the entire ISFET except the gate insulator is
coated with a polymeric encapsulant to insulate all electrical connections from the analyte
solution.
Application of ISFETs
The ion-sensitive surface of the ISFET is naturally sensitive to pH changes, but the
device may be rendered sensitive to other species by coating the silicon nitride gate
insulator with a polymer containing molecules that tend to form complexes with species
other than hydronium ion. Several ISFETs may be fabricated on the same substrate so
that multiple measurements may be made simultaneously. All of the ISFETs may detect
the same species to enhance accuracy and reliability, or each ISFET may be coated with a
different polymer so that measurements of several different species may be made
simultaneously. Their small size, rapid response time relative to glass electrodes, and
ruggedness suggest that ISFETs may be the ion detectors of the future for many
applications.
E: Molecular-Selective Electrode Systems
Two types of membrane electrode systems have been developed that act selectively
toward certain types of molecules. One of these is used for the determination of
dissolved gases, such as carbon dioxide and ammonia. The other permits the
determination of a variety of organic compounds, such as glucose and urea.
Gas-Sensing Probes
These devices are not, in fact, electrodes but instead are electrochemical cells made up of
a specific ion and a reference electrode immersed in an internal solution that is retained
by a this gas-permeable membrane. Thus, gas-sensing probes is a more suitable name for
these gas sensors. They are remarkably selective and sensitive devices for determining
dissolved gases or ions that can be converted to dissolved gases by pH adjustment.
Membrane Probe Design
The heart of the probe is a thin, porous membrane, which is easily replaceable. This
membrane separates the analyte solution from an internal solution containing sodium
bicarbonate and sodium chloride. A pH-sensitive glass electrode having a flat membrane
is fixed in position so that a very thin film of the internal solution is sandwiched between
it and the gas-permeable membrane. A silver/silver chloride reference electrode is also
located in the internal solution. It is the pH of the film of liquid adjacent to the glass
electrode that provides a measure of the carbon dioxide content of the analyte solution on
the other side of the membrane.
Gas-Permeable Membranes
Two types of membrane:
 Microporous materials- manufactured from hydrophobic polymers that have a
porosity of about 70% and a pore size of less than 1m, and are about 0.1mm
thick.
 Homogeneous films- solid polymeric substances through which the analyte gas
passes by dissolving in the membrane, diffusing, and then desolvating into the
internal solution. They are usually thinner than microporous in order to hasten the
transfer of gas and thus the rate of response of the system.
Mechanism of Response
When a solution containing dissolved carbon dioxide is brought into contact with the
microporous membrane, the gas effuses through the membrane, as described by the
reactions
CO2(aq) = CO2(aq) = CO2(aq)
The potential of the cell consisting of the internal reference and indicator electrode is
determined by the CO2 concentration of the external solution. Note that no electrode
comes directly in contact with the analyte. Note also that the only species that will
interfere with measurement are dissolved gases that can pass through the membrane
and can additionally affect the pH of the internal solution.
Biocatalytic Membrane Electrodes
In these devices the sample is brought into contact with an immobilized enzyme
where the analyte undergoes a catalytic reaction to yield a species such as ammonia,
carbon dioxide, hydrogen ions, or hydrogen peroxide. The concentration of this
product, which is proportional to the analyte concentration, is then determined by the
transducer. The most common transducers in these devices are membrane electrodes,
gas-sensing probes, and voltammetric devices. Biosensors based upon membrane
electrodes are attractive from several standpoints. First, complex organic molecules
can be determined with the convenience, speed, and ease that characterize ionselective measurements of inorganic species. Second, biocatalysts permit reactions to
occur under mild conditions of temperature and pH and at minimal substrate
concentrations. Third, combining the selectivities of the enzymatic reaction and the
electrode response yields procedures that are free from most interferences. The main
limitation to enzymatic procedures is the high cost of enzymes, particularly when
used for routine or continuous measurements. Despite the considerable effort, no
commercial enzyme electrodes based upon potential measurements are available, due
at least in part to limitations. Enzymatic electrodes based upon voltammetric
measurements are, however, offered by a commercial source.
Disposable Multilayer pIon Systems
Disposable electrochemical cells, based on pIon electrodes, have become available.
F: Instruments for Measuring Cell Potentials
A prime consideration in the design of an instrument for measuring cell potentials is
that its resistance must be large with respect to the cell. If it is not, significant error
results as a consequence of the IR drop in the cell. It is important to appreciate that
an error in potential would have an enormous effect on the accuracy of a
concentration measurement based upon that potential. Two types of instruments have
been employed in potentiometry- the potentiometer and the direct-reading electronic
voltmeter. Both instruments are referred to as pH meters when their internal
resistances are sufficiently high to be used with glass and other membrane electrodes;
with the advent of the many new specific ion electrodes, pIon or ion meters would
perhaps be a more descriptive name. Modern ion meters are generally of the directreading type; thus, they are the only ones that will be described here.
Direct-Reading Instruments
Numerous direct-reading pH meters are available commercially. These are solid-state
devices employing a field-effect transistor or a voltage follower as the first amplifier
stage in order to provide the needed high internal resistance.
Commercial Instruments
A wide variety of ion meters are available from several instrument manufacturers.
Four categories of meters are based on price and readability are described. These
include utility meters, which are portable, usually battery-operated instruments that
currently range from $100 to $500 and are readable to 0.1 pH unit or better. Generalpurpose meters are line-operated instruments, which are readable to 0.05 pH unit or
better. Prices for these range from $300 to $900. Expanded-scale instruments are
generally readable to 0.01 pH unit or better and cost from $700 to $1500. Research
meters are readable to 0.001 pH unit or better and cost between $1500 to $2200. It
should be pointed out that the readability of these instruments is usually significantly
better than the sensitivity of most ion-selective electrodes.
G: Direct Potentiometric Measurements
The determination of an ion or molecule by direct potentiometric measurement is
rapid and simple, requiring only a comparison of the potential developed by the
indicator electrode in the test solution with its potential when immersed in one or
more standard solutions of the analyte.
The Sign Convention and Equations for Direct Potentiometry
The sign convention for potentiometry is consistent with the convention for standard
electrode potentials. The indicator electrode is treated as the cathode and the
reference electrode as the anode. For direct potentiometric measurements, the
potential of a cell can then be expressed as a sum of an indicator electrode potential, a
reference electrode potential, and a junction potential:
Ecell = Eind – Eref + Ej
After rewriting the equation so it follows the Nernst equation, and recognizing that all
direct potentiometric methods are based upon these equations, Ecell can be found for
the cations to be
Ecell = K – (0.0592/n)pX
And for anions to be
Ecell = K + (0.0592/n)pA
The electrodes are attached in a certain way so that the cation increases with increases
in pX and the anion increases with pA to yield larger readings.
The Electrode Calibration Method
In the electrode-calibration method, K is determined by measuring Ecell for one or
more standard solution of known pX or pA. The assumption is then made that K is
unchanged when the standard is replaced with analyte. The calibration is then
ordinarily performed at the time pX or pA for the unknown is determined. With
membrane electrodes recalibration may be necessary if measurements extend over
several hours because of the slowly changing asymmetry potential.
Inherent Error in the Electrode
A serious disadvantage of the electrode calibration method is the existence of an
inherent uncertainty that results from the assumption that K remains constant between
calibration and analyte determination.
% rel error = (ax/ax)(100%) = 3.9E3 nK%
It is important to appreciate that this uncertainty is characteristic of all measurements
involving cells that contain a salt bridge and that this uncertainty cannot be eliminated
by even the most careful measurements of cell potentials or the most sensitive and
precise measuring devices; nor does it appear possible to devise a method for
completely eliminating the uncertainty in K that is the source of the problem.
Activity Versus Concentration
Electrode response is related to activity rather than to analyte concentration. The
scientist is interested in concentration, and the determination of this quantity from a
potentiometric measurement requires activity coefficient data. However, activity
coefficients will be unavailable because the ionic strength of the solution is either
unknown or so high that the Debye-Huckel equation is not applicable. Unfortunately,
the assumption that activity and concentration are identical may lead to serious errors,
particularly when the analyte is polyvalent. In potentiometric pH measurements, the
pH of the standard buffer employed for calibration is generally based on the activity
of the hydrogen atoms. Thus, the resulting hydrogen ion results are also on an
activity scale. If the unknown sample has a high ionic strength, the hydrogen ion
concentration will differ appreciably from the activity measured.
Calibration Curves for Concentration Measurement
A way of correcting potentiometric measurements to give results in terms of
concentration is to make use of an empirical calibration curve. It is essential that the
ionic composition of the standards closely approximate that of the analyte- a
condition that is difficult to realize experimentally for complex samples. Calibration
curves are also useful for electrodes that do not respond to pA.
Standard Addition Method
The standard addition method is equally applicable to potentiometric determinations.
The potential of the electrode system is measured before and after addition of a small
volume of a standard to a known volume of the sample. The assumption is made that
this addition does not alter the ionic strength and thus the activity coefficient of the
analyte. It is also assumed that the added standard does not significantly alter the
junction of potential. This method had been applied to the determination of chloride
and fluoride in samples of commercial phosphors.
Potentiometric pH Measurements with a Glass Electrodes
The glass electrode is unquestionably the most important indicator electrode for
hydrogen ion. It is convenient to use and is subject to few of the interferences that
affect other pH-sensing electrodes. Glass electrodes are available at relatively low
cost and come in many shapes and sizes. The reference electrode is usually a
silver/silver chloride electrode. The glass electrode is a remarkably versatile tool for
the measurement of pH under many conditions.
Summary of Errors Affecting pH Measurements with the Glass Electrode
1. The alkaline error. Modern glass electrodes become somewhat sensitive to
alkali-metal ions at pH values greater than 11 to 12.
2. The acid error. At a pH less than 0.5, values obtained with a glass electrode
tend to be somewhat high.
3. Dehydration. Dehydration of the electrode may cause unstable performance
and errors.
4. Errors in low ionic strength solutions. It has been found that significant errors
may occur when the pH of low ionic strength samples, such as lake or stream
samples, are measured with a glass/calomel electrode system. The prime
source of such errors has been shown to be nonreproducible junction
potentials, which apparently result form partial clogging of the fritted plug or
porous fiver that is used to restrict the flow of liquid from the salt bridge into
the analyte solution. In order to overcome this problem, free diffusion
junctions (FDJ) of various types have been designed, and one is produced
commercially. In the latter, an electrolyte solution is dispensed from a syringe
cartridge through a capillary tube, the tip of which is in contact with the
sample solution. Before each measurement, 6L of electrolyte is dispensed so
that a fresh portion of electrolyte is in contact with the analyte solution.
5. Variation in junction potential. It should be reemphasized that variation in the
junction potential between standard and sample leads to a fundamental
uncertainty in the measurement of pH for which a correction cannot be
applied. Absolute values more reliable than 0.01 pH unit are generally
unobtainable. Even reliability to 0.03 pH unit requires considerable care. On
the other hand, it is often possible to detect pH difference between similar
solutions or pH changes in a single solution that are as small as 0.001 unit.
For this reason, many pH meters are designed to permit readings to less than
0.01 pH unit.
6. Error in the pH of the standard buffer. Any inaccuracies in the preparation of
the buffer used for calibration, or changes in its composition during storage,
will be propagated as errors in pH measurements. A common cause of
deterioration is the action of bacteria on organic components of buffers.
The Operational Definition of pH
The utility of pH as a measure of the acidity or alkalinity of aqueous media, the wide
availability of commercial glass electrodes, and the relatively recent proliferation of
inexpensive solid-state pH meters have made the potentiometric measurement of pH one
of the most common analytical techniques in all of science. It is thus extremely
important that pH be defined in a manner that is easily duplicated at various times and
various laboratories throughout the world. To meet this requirement, it is necessary to
define pH in operational terms- that is, by the way the measurement is made. Only then
will the pH measured by one worker be the same as that measured by another. For
general use, the buffers can be prepared from relatively inexpensive laboratory reagents.
For careful work, certified buffers can be purchased from the NIST.
H: Potentiometric Titrations
The potential of a suitable indicator electrode in conveniently employed to establish the
equivalence point for a titration. A potentiometric titration provides different information
that noise a direct potentiometric measurement. The potentiometric end point is widely
applicable and provides inherently more accurate data than the corresponding method
employing indicators.
Schematic representation of an automatic potentiometric titrator devised by Lingane in
1948.
The Beckman Automatic Titrator, Beckman Instruments, Inc., Fullerton, Calif., U.S.A.
References
http://ull.chemistry.uakron.edu/analytical/Potentiometry/
http://chem.ch.huji.ac.il/~eugeniik/instruments/electrochemical/potentiometric_titrators.h
tm
http://www.fz-juelich.de/isg/sensorik/bcs-isfet-e.html
http://www.chemistry.msu.edu/courses/cem333/Chapter23,potentiometry.PDF