Background -- Ion Selective Electrodes
(ISE) are membrane electrodes that respond selectively to ions in the presence of others. These
include probes that measure specific ions and gases in solution. The most commonly used ISE is the pH probe. Other ions
that can be measured include fluoride, bromide, cadmium, and gases
in solution such as ammonia, carbon dioxide, and nitrogen oxide.
The use of Ion Selective Electrodes in environmental analysis offer several advantages over other methods of analysis. First, the cost of initial setup to make analysis is relatively low. The basic ISE setup includes a meter (capable of reading millivolts), a probe (selective for each analyte of interest), and various consumables used for pH or ionic strength adjustments.
Also see the newest ICE used with PDA.
The expense is considerably less than other methods, such as Atomic Adsorption
Spectrophotometry or Ion Chromatography. ISE determinations are not subject to interferences such as color in the sample. There are few matrix modifications needed to conduct these analyses. This makes them ideal for clinical use (blood gas analysis) where they are most popular; however, they have found practical application in the analysis of environmental samples, often where in-situ determinations are needed and not practical with other methods. A
large number of indicator electrodes with good selectivity for
specific ions are based on the measurement of the potential generated
across a membrane. Electrodes of this type are referred to as
ion-selective electrodes. The membrane is usually attached to
the end of a tube that contains an internal reference electrode.
This membrane electrode and an external reference electrode are
then immersed in the solution of interest. Since the potentials
of the two reference electrodes are constant, any change in cell
potential is due to change in potential across the membrane.
Different membrane materials have proved to give optimal responses
for certain species. For example, a glass membrane is unsurpassed
for measuring H+ activity, pH. This ISE may be referred
to as a glass or pH electrode.
Liquid membrane electrodes have non-glass, solid-state crystals
or pellets as the membrane component of the electrode. This approach
has proved effective for numerous cations and anions. The most
successful example is the excellent electrode for fluoride ion,
which is based on a crystal of LaF3 doped with Eu(II)
to create crystal defects to improve conductivity.
Gas-sensing electrodes or combination electrodes that respond
to the concentration of gases dissolved in aqueous solution. The
electrodes consist of an ion-selective electrode, usually pH,
in contact with a thin layer of solution that is held in place
with a membrane permeable to the desired gas such as NH3 or CO2. When the gas passes through the membrane, the
change of pH in the thin layer of solution is sensed by the glass
membrane pH electrode.
Other combination electrodes consist of an enzyme immobilized
on an ISE. The ISE is chosen to respond to a product of the enzyme-substrate
reaction and the selectivity is provided by the enzyme.
History -- Credit for the first glass sensing
pH electrode is given to Cremer, who first described it in his
1906 paper (Meyerhoff and Opdeycke). In
1949, George Perley published an article on the relationship of
glass composition to pH function (Frant).
In the interim there were numerous papers dealing with various
formulations of and several important contributions were made
The commercial development
of ISE began when an engineer by the name of John Riseman thought
he could develop a useful blood-gas analyzer. He teamed up with
Dr. James Ross, an electrochemist from MIT. Together they formed
Orion Research (Frant). By the mid 1960s,
the newly formed Orion Research Inc. was producing Calcium electrodes
for use in blood gas analyzers (Frant, 1994).
Since then numerous probes have been developed for the analysis
of samples containing many different ions.
How do they work or what is an Ion-Selective
Electrode? An Ion Selective Electrode measures the potential
of a specific ion in solution. (The pH electrode is an ISE for
the Hydrogen ion.) This potential is measured against a stable
reference electrode of constant potential. The potential difference
between the two electrodes will depend upon the activity of the
specific ion in solution. This activity is related to the concentration
of that specific ion, therefore allowing the end-user to make
an analytical measurement of that specific ion. Several ISE's
have been developed for a variety of different ions.
How Does the mV Reading Correspond to the Concentration? Standard solutions of known concentrations must be
accurately prepared. These solutions are then measured with the
pH/mV meter. The mV reading of each solution is noted and a graph
of concentration vs. mV reading must be plotted. Now the unknown
solution can be measured. The mV value of the unknown solution
is then located on the graph and the corresponding solution concentration
Ion Selective Electrodes (including the most common pH electrode)
work on the basic principal of the galvanic cell (Meyerhoff
and Opdycke). By measuring the electric potential generated
across a membrane by "selected" ions, and comparing
it to a reference electrode, a net charge is determined. The strength
of this charge is directly proportional to the concentration of
the selected ion. The basic formula is given for the galvanic
Ecell = Eise - Eref
the potential for the cell is equivalent to the potential of
the ISE minus the potential of the reference electrode.
Calibration -- Direct - The electric
potentials are determined for a series of standards and a standard
curve is developed. Additional analyses are fit to the standard
curve in order to determine concentration. Direct calibration
is the most common and easiest way to measure concentrations.
Standard Additions - The use of standard additions (the
addition of known amounts of a standard) allows the use of the
electrode in very complex matrices, without the need for direct
calibration prior to measurement (Covington).
Titration's - ISEs have also been used as detectors for
titration's (Orion). Titration methods use
a titrant (such as EDTA) which will complex or react with the
ion to be analyzed. The concentration of the ion in the sample
is back calculated from the volume of the titrant used in the
Membranes -- The nature of the membrane
determines the selectivity of the electrode. A membrane is considered
to be any material that separates two solutions. It is across
this membrane that the charge develops. The term "membrane"
is often confuse as implying permeability. While this is true
in many cases, the term here is used denote any material which
the charge can develop across (Covington).
Several types of sensing electrodes are commercially available.
They are classified by the nature of the membrane material used
to construct the electrode. It is this difference in membrane
construction that makes an electrode selective for a particular
1. Polymer Membrane Electrodes (Organic Ion
Exchangers and Chelating Agents) -- Polymer membrane electrodes
consist of various ion-exchange materials incorporated into an
inert matrix such as PVC, polyethylene or silicone rubber. After
the membrane is formed, it is sealed to the end of a PVC tube.
The potential developed at the membrane surface is related to
the concentration of the species of interest. Electrodes of this
type include potassium, calcium, chloride, fluoroborate, nitrate,
perchlorate, potassium, and water hardness.
2. Solid State Electrodes (Insoluble Conductive
Inorganic Salts) -- Solid state electrodes utilize relatively
insoluble inorganic salts in a membrane. Solid state electrodes
exist in homogeneous or heterogeneous forms. In both types, potentials
are developed at the membrane surface due to the ion-exchange
process. Examples include silver/sulphide, lead, copper(II), cyanide,
thiocynate, chloride, and fluoride.
3. Gas Sensing Electrodes -- Gas sensing
electrodes are available for the measurement of dissolved gas
such as ammonia, carbon dioxide, nitrogen oxide, and sulfur dioxide.
These electrodes have a gas permeable membrane and an internal
buffer solution. Gas molecules diffuse across the membrane and
react with a buffer solution, changing the pH of the buffer. The
pH of the buffer solution changes as the gas reacts with it. The
change is detected by a combination pH sensor within the housing.
Due to their construction, gas sensing electrodes do not require
an external reference electrode.
4. Glass Membrane Electrodes -- Glass
membrane electrodes are formed by the doping of the silicon dioxide
glass matrix with various chemicals. The most common of the glass
membrane electrodes is the pH electrode. Glass membrane electrodes
are also available for the measurement of sodium ions.
Sources of Error -- Diffusion - Orion Research
points out that differences in the rates of diffusion of ions
based on size can lead to some error. In the example of Sodium
Iodide, sodium diffuses across the junction at a given rate. Iodide
moves much slower due to its larger size. This difference creates
an additional potential resulting in error. To compensate for
this type of error it is important that a positive flow of filling
solution move through the junction and that the junction not become
clogged or fouled.
Sample Ionic Strength - Covington points out that the
total ionic strength of a sample affects the activity coefficient
and that it is important that this factor stay constant. In order
accomplish this, the addition of an ionic strength adjuster is
used. This adjustment is large, compared to the ionic strength
of the sample, such that variation between samples becomes small
and the potential for error is reduced.
Temperature - It is important that temperature be controlled
as variation in this parameter can lead to significant measurement
errors. A single degree (C) change in sample temperature can lead
to measurement errors greater than 4%.
pH - Some samples may require conversion of the analyte
to one form by adjusting the pH of the solution (e.g. ammonia).
Failure to adjust the pH in these instances can lead to significant
Interferences - The background matrix can effect the
accuracy of measurements taken using ISE's.
Covington points out that some interferences may be eliminated
by reacting the interfering ions prior to analysis.
Approved Methods for using ISE -- Standard methods
for sample analysis using Ion Selective Electrodes are published
by several agencies. These include the American Society of Testing
and Material (ASTM), United States Environmental Protection Agency
(EPA) , American Public Health Association (APHA), Association
of Analytical Chemists (AOAC), and the United States Geological
Survey (USGS). Approved Methods for using Ion Selective Electrodes
are (Species Measured): Acidity, Alkalinity, Ammonia, Bromide,
Carbon Dioxide, Chlorine, Chloride-Leachable, Chloride by Titration,
Chloride Total in Coal, Chlorine-Residual, Chlorine in Organics,
Conductivity, Cyanate, Cyanide, Fluoride, Fluoride in Air, Fluorine
in Coal, Iodide, Kjeldahl Nitrogen, Nitrate, ORP (Oxidation-Reduction
Potential), Oxygen, Potassium, Salinity, Sodium, Sulfide, Sulfur
in Coal, pH, and pH Titration's.
Standard procedures may be found in the following references
or on their respective home web page.
(1) "Annual Book of ASTM Standards, Water and Environmental
Technology," American Society for Testing and Materials,
(2) "Methods for Chemical Analysis of Water and Wastes,"
Environmental Protection Agency, Environmental Monitoring Systems
Laboratory, EPA-600/4-79-020, 1983.
(3) "Standard Methods for Water and Wastewater Analysis,"
18th Edition, 1994.
(4) Official Methods of Analysis of the Association of Official
Analytical Chemists, Methods Manual, 15th edition (1990).
(5) Methods for Analysis of Inorganic Substances in Water and
Fluvial Sediments, U.S. Dept. of the Interior, Techniques of Water
Resource Investigations of the U.S. Geological Survey, Denver,
CO, Revised 1989.
(6) "Test Methods for Evaluating Solid Waste, Physical/Chemical
Methods", SW-846, Update III.
Considerations -- This discussion is designed to apply,
in general, to all specific ion electrodes. Typically, the ISE
will come with its own instruction manual that pertains to that
particular type of electrode. It is best to read all instructions
thoroughly before using the electrode. When the ISE is received,
open up the package immediately and check all the parts of the
electrode. Most ISE's have a pre-treatment procedure that should
be followed prior to operation. Before beginning measurements
the following are a few basic facts that will aid in designing
What Type of Equipment is Needed for an
ISE Measurement? A pH meter that also
measures millivolts can be used to interface with an ISE. Most
ISE's are combination electrodes that have the reference electrode
built into the body of the ISE, however, some ISE's require a
separate reference electrode. If this is the case, the pH/mV meter
must have a pin-connector to connect the reference electrode.
Agitation -- When carrying out selective
ion measurements, it is important to have good agitation. This
allows a fresh supply of ions to be exposed to the sensing portion
of the ISE. It is best to select a speed that keeps a constant,
smooth motion. A turbulent rate should be avoided.
pH Adjustment -- In many cases pH control
is necessary for accurate, repeatable measurements. Certain ions
exhibit different activity when different concentrations of hydrogen
ions are present in solution. This occurrence will not only alter
the potential due to the specific ion that is measured, it may
also allow other ions in solution to become active that otherwise
were not. This increased activity from the other ions will interfere
with the ability to evaluate the ion of interest.
Response Time -- ISE's require a much
longer time for the readings to stabilize. At least fifteen minutes
should be allowed for equilibrium to be established when measuring
Establishing a Calibration Curve -- It
is recommended to use three standard solutions when establishing
a calibration curve. To choose the concentrations of the standard
solutions it is helpful to know the approximate values of the
unknown solutions. For example, if the unknown solutions are in
the 100 ppm range, the choice of standards may include a 10 ppm,
a 100 ppm, and a 1000 ppm solution.
Rinsing -- It is necessary to rinse
the ISE between measurements to insure accurate readings. Use
a steady stream of deionized or distilled water. Take care not
to rub the electrode with a cloth to dry the probe. It is usually
best to "shake off" any excess water. Take care not
to hit the probe against anything while shaking the electrode.
Conditioning -- The ISE needs to remain
moist at all times even when not in use. Consult the operator's
manual that accompanies the electrode for details on cleaning,
conditioning, and storing the ISE.
General Comments on Ion-Selective Electrodes:
- 1. Electrodes with a polymer membrane must not come in contact
with organic solvents
- 2. Do not store in water for extended periodsdry before
3. Store Combined Ion Selective Electrodes in dilute ISA (ionic
strength adjuster) solutionfor long term storage, remove
reference solution and store dry.
- 4. Clean crystal membranes with a mild abrasive, then rinse
with water. Toothpaste is an excellent cleaning agent, for fluoride
electrodes use a fluoride toothpaste
What do some ISE look like?
Various ion selective electrodes
by one manufacturer are shown at the left. The pHoenix
Electrode Company. All of their ISE's are now available in
both glass and plastic body combination electrodes. pHoenix Electrode
Company is a manufacturer of electrochemical analytical sensors as well as Vernier and hand held data capture devices.
They specialize in manufacturing pH, ORP, Conductivity, Oxygen
and Ion Selective Electrodes designed for Laboratory, Industrial,
Biotechnology and Medical applications.
Covington, AK. "Introduction: Basic Electrode Types, Classifications,
and Selectivity Considerations." In. Covington, AK (ed.), Ion Selective Electrode Methodology. Volume 1. CRC Press.
Boca Raton. 1-20.
Frant, MS. 1994. "History of the Early Commercialization
of Ion-Selective Electrodes." Analyst 199, 2293-2301.
Meyerhoff, ME and WN Opdycke. 1986. "Ion Selective Electrodes." Advances in Clinical Chemistry 25, 1-47.
Orion Research, Inc. 1997. Web Site Index. http://www.orionres.com/.