This document provides information on ion selective electrodes (ISEs). It discusses the history of ISE development from early pH electrodes to modern commercial applications. The basic components and functioning of ISEs are described, including the electrochemical cell setup with an ion selective membrane and reference electrode. Different types of membranes - glass, crystal, gas-sensing, and polymer - are outlined. Examples are given of specific ions that can be measured with different ISE configurations. The document concludes by discussing the mechanisms of ion exchange and selectivity in polymer membrane electrodes.

1. Ion Selective Electrode
Dr. Ritu Bashyal
MD Biochemistry

2. History
• Credit for the first glass sensing pH electrode is given to Cremer,
who first described it in his 1906 paper
• In 1949, George Perley published an article on the relationship of
glass composition to pH function
• The commercial development of ISE began when John Riseman
(an engineer) 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
• By the mid 1960s, the newly formed Orion Research was producing
Calcium electrodes for use in blood gas analyzers
• Since then numerous probes have been developed for the analysis
of samples containing many different ions.

3. Introduction to ion analysis
1. Electrochemical Analysis
Is the group of chemical analytical methods in which a
signal is generated by an electrochemical reaction and is
used for quantitative determination of a substance
2. Electrodes
• Is an Electronic Conductor in contact with an Ionic
Conductor, the Electrolyte
3. Electrode Reaction - in which charge transfer takes place
at the interface between the electrode and the
electrolyte.

4. • An Indicator Electrode shows by means of its potential
the Activity of an ion in a solution
3. Electrochemical Cells
• An electrode in contact with an electrolyte represents a Half-
Cell and two combined half-cells form an Electrochemical
Cell - i.e. one solution in contact with an ISE and a reference
electrode
• The Electrochemical Potential (E) can be measured across the
two terminals of the cell.

5. • Electrochemical and optical sensors are established in
clinical analysis system for measurement of blood gases,
electrolytes, and metabolites
• When integrated into chromatographic systems , they
provide a very sensitive and selective means to detect a
variety of other analytes, such as therapeutic drugs,
neurotransmitters, glutathione, and homocysteine
• Also, electrochemical detection has been applied
successfully for monitoring coagulation reactions,
detecting toxic lead in blood samples, and developing
novel ultrasensitive immunoassay schemes.

6. BASIC CONCEPTS
• Potentiometry is the measurement of an electrical
potential difference between two electrodes (half-
cells) in an electrochemical cell when the cell current
is zero (galvanic cell)
• Such a cell consists of two electrodes that are
connected by an electrolyte solution (ion conductor)
• An electrode, or half-cell, consists of a single metallic
conductor that is in contact with an electrolyte
solution

7. • The ion conductors can be composed of one or more
phases that are either in direct contact with each other or
separated by membranes permeable only to specific
cations or anions
• One of the electrolyte solutions is the unknown or test
solution; this solution may be replaced by an appropriate
reference solution for calibration purposes
• By convention, the cell notation is - left electrode is the
reference electrode; the right electrode is the indicator
(measuring) electrode

8. TYPES OF ELECTRODES
• Many different types of electrodes are used for
potentiometric applications
• They include
1. Redox electrode
2. Ion-selective membrane (glass and polymer)
3. PCO2 electrodes

9. Ion Selective Electrodes (ISE)
Aka a specific ion electrode (SIE), is a transducer (or sensor) that
converts the activity of a specific ion dissolved in a solution into an
electrical potential, which can be measured by a voltmeter or pH
meter
How ISE work??
• Works on the theory that an electrode develops a potential due to
ion-exchange occurring between the sample and the inorganic
membrane
• 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

10. • ISEs all work on the basic principle of the Galvanic Cell.
• The Galvanic cell consists of two half cells that are
made up of two different metals dipped into two
separate solutions of salt of the corresponding metal.
For e.g. Copper electrode in Copper Sulfate solution or
Zinc electrode in Zinc Sulfate solution.
• One of these metals is able to reduce the cation of the
other metal while the other cation will oxidize the first
metal.

11. • When one of the metals from the half cell is oxidized, anions
are transferred into that half cell. This is done to keep a
balance of electrical charge from the cation that is produced.
• The solutions cannot mix for this to work. To accomplish this,
the two solutions are connected
via a salt bridge that is porous
This allows the solutions to be in
separate beakers while still
allowing the charge to flow
throughout the entire system.

12. • As the current flows, it is attached to a volt meter that
record and displays the charge
• This is accomplished by measuring the electric potential
that will be generated by the selected ions
• From here it is compaired to the reference electrode where
the volt meter displays the difference in the two
• It is extremely important that the reference electrode
remains constant.
The general equation for the Galvanic cell is
E cell = E ISE - E ref
• Where E ISE = the potential of the ISE
• E ref = The potential of the reference electrode

13. How Does the mV Reading Correspond to
the Concentration?
• Calibration cuve helps in this situation
• 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 is measured; The mV value of
the unknown solution is then located on the graph and the
corresponding solution concentration is determined.

14. Components of ISE
1. Ion selective electrode with membrane at the end –
allows ions of interest to pass, but excludes the passage
of the other ions
2. Internal reference electrode – present within the ion
selective electrode which
is made of silver wire coated
with solid silver chloride,
embedded in concentrated
potassium chloride solution
(filling solution) saturated with
silver chloride.This solution
also contains the same ions as that to be measured

15. • Reference electrode similar to ion selective electrode, but
there is no to-be-measured ion in the internal electrolyte
• Commonly used – calomel electrode
• The lower end of the reference
electrode is sealed with a
porous ceramic frit which allows
the slow passage of the
internal filling solution and forms the liquid junction with the
external test solution
• Dipping into the filling solution is a silver wire coated with a
layer of silver chloride (it is chloridised) which is joined to a
low-noise cable which connects to the measuring system.

16. • Reference Electrode is needed as the potential between an
electrode and a solution cannot be directly measured
• It is necessary to include in the circuit a stable reference
voltage which acts as a half-cell from which to measure the
relative deviations.
• The reference electrode should have a known, or at least, a
constant Potential value under the prevailing experimental
conditions.
• One problem with reference electrodes is that, in order to
ensure a stable voltage, it is necessary to maintain a steady
flow of electrolyte through the porous frit

17. • Thus there is a gradual contamination of the test solution
with electrolyte ions
• This can cause problems when trying to measure low levels
of K, Cl, or Ag, or when using other ISEs with which these
elements may cause interference problems
• In order to overcome this difficulty the double junction
reference electrode was developed
• In this case the silver / silver chloride cell described above
forms the inner element and this is inserted into an outer
tube containing a different electrolyte which is then in
contact with the outer test solution through a second
porous frit

18. • The outer filling solution is said to form a "salt bridge"
between the inner reference system and the test solution and
is chosen so that it does not contaminate the test solution
with any ions which would effect the analysis.
Commonly used outer filling solutions are:
• potassium nitrate - for Br, Cd, Cl, Cu, CN, I, Pb, Hg, Ag, S, SCN.
• sodium chloride - for K,
• ammonium sulphate - for N03,
• magnesium sulphate - for NH4,

19. • One disadvantage with double junction reference
electrodes is that they introduce an extra interface
between two electrolytes and thus give the opportunity for
an extra liquid junction potential to develop.
• It must be noted that the E0 factor in the Nernst equation is
the sum of all the liquid junction potentials present in the
system and any variation in this during analyses can be a
major source of potential drift and error in measurements.
• The two solutions in a double junction reference electrode
are connected by a so called Liquid Junction
• A liquid junction is a semi-permeable membrane separating
two solutions, which prevents wholesale mixing but allows
the passage of ions by diffusion.

20. 4. Circuit
• Other end of both the electrode is fitted with a low
noise cable or gold plated pin for connection to the
millivolt measuring device.
• Since the potentials of the two reference electrodes
are constant, any change in cell potential is due to
change in potential across the membrane.

21. Ions that can be measured
• The most commonly used ISE is the pH probe.
• CATIONS: Ammonium (NH4
+), Barium (Ba++), Calcium (Ca++),
Cadmium (Cd++), Copper (Cu++), Lead (Pb++), Mercury (Hg++),
Potassium (K+), Sodium (Na+), Silver (Ag+)
• ANIONS: Bromide (Br-), Chloride (Cl-), Cyanide (CN-), Fluoride
(F-), Iodide (I-), Nitrate (NO3
-), Nitrite (NO2
-), Perchlorate (ClO4
-)
Sulphide (S-), Thiocyanate (SCN-).

22. The pH Electrode
• The pH electrode is the most well known and simplest one
and can be used to illustrate the basic principles of ISE
• This is a device for measuring the concentration of
hydrogen ions and hence the degree of acidity of a solution
- since pH is defined as the negative logarithm of the
hydrogen ion concentration;
• i.e. pH=7 means a concentration of 1x10-7 moles per litre.

23. • The most essential component of a pH electrode is a
special, sensitive glass membrane which permits the
passage of hydrogen ions, but no other ionic species
• When the electrode is immersed in a test solution
containing hydrogen ions the external ions diffuse
through the membrane until an equilibrium is reached
between the external and internal concentrations
• Thus there is a build up of charge on the inside of the
membrane which is proportional to the number of
hydrogen ions in the external solution.

24. • Because of the need for equilibrium conditions there is
very little current flow and so this potential difference
can only be measured relative to a separate and stable
reference system which is also in contact with the test
solution, but is unaffected by it
• A sensitive, high impedance millivolt meter or digital
measuring system must be used to measure this
potential difference accurately
• The potential difference developed across the
membrane is in fact directly proportional to the
Logarithm of the ionic concentration in the external
solution

25. • The relationship between the ionic concentration (activity) and the
electrode potential is given by the Nernst equation:
E = E0 + (2.303RT/ nF) x Log(A)
Where E = the total potential (in mV) developed between the sensing
and reference electrodes.
E0 = is a constant which is characteristic of the particular ISE/reference
pair.(It is the sum of all the liquid junction potentials in the
electrochemical cell)
2.303 = the conversion factor from natural to base10 logarithm.
R = the Gas Constant (8.314 joules/degree/mole).
T = the Absolute Temperature.
n = the charge on the ion (with sign).
F = the Faraday Constant (96,500 coulombs).
Log(A) = the logarithm of the activity of the measured ion.

26. Differences Between pH and Other Ion-
Selective Electrodes
i) In contrast to the pH membrane, other ion-selective
membranes are not entirely ion-specific and can permit the
passage of some of the other ions which may be present in
the test solution, thus causing the problem of ionic
interference
• The influence of the presence of interfering species in a
sample solution on the measured potential difference is taken
into consideration in the Nikolski-Eisenman equation
z charge including the sign
a the activity
i the ion of interest
j the interfering ions
kij is the selectivity coefficient
• The smaller the selectivity coefficient, the less is the interference by j.

27. ii) The calculation of ionic concentration is far more
dependent on a precise measurement of the
potential difference than is the pH, because the pH
depends on the order of magnitude of the
concentration rather than the precise value
• For example it would take an error of more than 5
millivolts to cause a change of 0.1 pH units, but only
a 1 millivolt error will cause at least a 4% error in the
calculated concentration of a mono-valent ion and
more than 8% for a di-valent ion.

28. Types of membrane
• A membrane is considered to be any material that
separates two solutions
• It is across this membrane that the charge develops.
• 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 ion.

29. 1. Glass membrane
• Glass membranes are made from an ion-exchange type of
glass (silicate or chalcogenide)
• This type of ISE has good selectivity, but only for several
single-charged cations; mainly H+, Na+, and Ag+
• Chalcogenide glass also has selectivity for double-charged
metal ions, such as Pb2+, and Cd2+
• The glass membrane has excellent chemical durability and
can work in very aggressive media
• A very common example of this type of electrode is
the pH glass electrode

30. pH electrode
• Glass membrane manufactured from SiO2 with
negatively charged oxygen atom
• Inside the glass bulb, a dilute HCl solution and silver
wire coated with a layer of silver chloride is kept
• The electrode is immersed in the solution and pH is
measured.

31. 2. Crystal-Membrane Electrodes
• Made from mono- or polycrystallites of a single substance
• They have good selectivity, because only ions which can
introduce themselves into the crystal structure can
interfere with the electrode response
• Selectivity of crystalline membranes can be for
both cation and anion of the membrane-forming substance
• An example is the fluoride selective electrode based
on Lanthanum fluoride crystals

32. • The membrane consists of a single lanthanum fluoride
crystal which has been doped with europium fluoride to
reduce the bulk resistivity of the crystal
• It is 100% selective for F- ions and is only interfered with by
OH-which reacts with the lanthanum to form lanthanum
hydroxide, with the consequent release of extra F- ions
• This interference can be eliminated by adding a pH buffer
to the samples to keep the pH in the range 4 to 8 and hence
ensure a low OH- concentration in the solutions.

33. 3. 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 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.

34. 4. Ion-exchange resin or polymer membrane electrode
Are based on special organic polymer membranes which
contain a specific ion-exchange substance (resin)
• Are employed for monitoring pH and for measuring
electrolytes) including K+, Na+, CI-, Ca2+, Li+, Mg‘+
• They are the predominant class of potentiometric
electrodes used in modern clinical analysis instruments
• The mechanism of response of these ISEs falls into three
categories:
(1) charged, dissociated ion-exchanger
(2) charged associated carrier
(3) the neutral ion carrier (ionophore)

35. • An early charged associated ion-exchanger type ISE
for Ca'+ was developed and commercialized for
clinical application in the 1960s
• This electrode was based on the Ca2+-selective ion
exchange/complexation properties of 2-ethylhexyl
phosphoric acid dissolved in dioctyl phenyl
phosphonate (charged associated carrier)
• A porous membrane was impregnated -with this
cocktail and mounted at the end of an electrode body
• This type sensor was referred to as the «liquid
membrane" ISE

36. • Later a method was devised where these ingredients could
be cast into a plasticized poly(vinyl chloride) (PVC)
membrane, that was more rugged and convenient to use
than its wet liquid predecessor.
• This same approach is still used today to formulate PVC-
based ISEs for clinical use."
• A major breakthrough in the development and routine
application of PVC type ISEs was the discovery by Simon
and co-workers that the neutral antibiotic valinomycin
could be incorporated into organic liquid membranes (and
later plasticized PVC membranes), resulting in a sensor
with high selectivity for K+ over Na+

37. • The membrane is usually in the form of a thin disc of PVC
impregnated with the macrocyclic antibiotic valinomycin
• This compound has a hexagonal ring structure with an
internal cavity which is almost exactly the same size as the
diameter of the K+ ion. Thus it can form complexes with this
ion and preferentially conducts it across the membrane
• Unfortunately it is not 100% selective and can also conduct
small numbers of sodium and ammonium ions. Thus these
can cause errors in the potassium determination if they are
present in high concentrations

38. 5. Enzyme electrodes
• Enzyme electrodes definitely are not true ion-selective electrodes
but usually are considered within the ion-specific electrode topic
• Such an electrode has a "double reaction" mechanism - an enzyme
reacts with a specific substance, and the product of this reaction
(usually H+ or OH-) is detected by a true ion-selective electrode,
such as a pH-selective electrodes
• All these reactions occur inside a special membrane which covers
the true ion-selective electrode, which is why enzyme electrodes
sometimes are considered as ion-selective
• An example is glucose selective electrodes.

39. Sources of Error
1. Diffusion - 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.
2. Sample Ionic Strength - 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.

40. • 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
measurement errors.
• Interferences - The background matrix can effect the accuracy of
measurements taken using ISE's
• There may interference from other, undesired, ions; All are
sensitive to other ions having similar physical properties, to an
extent which depends on the degree of similarity
• Covington points out that some interferences may be eliminated by
reacting the interfering ions prior to analysis.

41. Advantages of (ISE) Technique
• Relatively inexpensive (as compaired to Atomic
Adsorption Spectrophotometry or Ion
Chromatography), simple, have an extremely wide
range of applications and wide concentration range.
• It is a real-time measurement, which means it can
monitor the change of activity of ion with time.
• As it measure the activity, instead of concentration, it is
particularly useful in biological/medical application.
• It can determine both positively and negatively
charged ions.

42. • Not interfered by color or turbidity in the sample
• ISEs can be used in aqueous solutions over a wide
temperature range. Crystal membranes can operate in the
range 0 C to 80 C and plastic membranes from 0 C to 50 C.
• Ideal for monitoring environmental pollution or water
quality etc. - where the operator simply wants to be sure
that a certain ion is below a particular threshold value, or
where only the order of magnitude may be required

43. Other applications of ISE
• Pollution Monitoring: Cyanide, Flourine, Sulphur, Chlorine, Nitrate
• Agriculture: NO3, Cl, NH4, K, Ca, I, CN in soils, plant material, fertilisers and
feedstuffs.
• Food Processing: NO3, NO2 in meat preservatives.
• Salt content of meat, fish, dairy products, fruit juices, brewing solutions.
• F in drinking water and other drinks.
• Ca in dairy products and beer.
• K in fruit juices and wine making.
• Corrosive effect of NO3 in canned foods.
• Detergent Manufacture: Ca, Ba, F for studying effects on water quality.
• Paper Manufacture: S and Cl in pulping and recovery-cycle liquors.
• Explosives: F, Cl, NO3 in explosive materials and combustion products.
• Electroplating: F and Cl in etching baths; S in anodising baths.
• Biomedical Laboratories: Ca, K, Cl in body fluids (blood, plasma, serum,
sweat).
• F in skeletal and dental studies.
• Education and Research

44. Limit of ISE
• Though the membrane is ion selective, the truth is there is no such
membrane that only permits the passage of one ion, and so as a
result, the measured potential are affected by the passage of the
“unwanted” ions
• Also, because of its dependence on ion selective membrane, one
ISE is only suitable for one ion and this may be inconvenient
sometimes
• Another problem worth noticing is that ion selective measures the
concentration of ions in equilibrium at the surface of the membrane
surface
• This does matter much if the solution is dilute but at higher
concentrations, the inter-ionic interactions between the ions in the
solution tend to decrease the mobility of ions and thus the
concentration near the membrane would be lower than that in the
bulk.
• To better analyze the results of ISE, we have to be aware of these
inherent limitations of it.

45. Conditioning
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.

46. • The ISE needs to remain moist at all times even when not in
use.
• Electrodes with a polymer membrane must not come in
contact with organic solvents
• Do not store in water for extended periods—dry before
storing
• Store Combined Ion Selective Electrodes in dilute ISA (ionic
strength adjuster) solution—for long term storage, remove
reference solution and store dry
• Clean crystal membranes with a mild abrasive, then rinse
with water. Toothpaste is an excellent cleaning agent, for
fluoride electrodes use a fluoride toothpaste

47. THANK YOU

48. References
• Tietz, Clinical chemistry and molecular diagnostic
• Harvey, Modern analytical Chemistry
• Clinical chemistry, Techniques, Principles and Correlations
• Beginners Guide to ISE Measurement, Chris C Rundle