types of ion selective electrode

1. Ion Selective
Electrodes
Guided by:- Prepared by:-
Dr. Falgun A Mehta Shweta Singh
(HOD pharmaceutical analysis) (Ist sem analysis)
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2. INDEX:
• The principle of the I.S.E.
• Advantages and limitations.
• Types of I.S.E.
• Application.
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3. Principle:-
• An ideal I.S.E. consists of a thin membrane across which only the
intended ion can be transported.
• The transport of ions from a high conc. to a low one through a
selective binding with some sites within the membrane creates a
potential difference. 3
Ref:-http://www.sfu.ca/chemistry/groups/Li/chem215/selective.PDF

4. •
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Ref:- http://www.sfu.ca/chemistry/groups/Li/chem215/selective.PDF

5. ADVANTAGE:-
 When compared to many other analytical techniques, Ion-
Selective Electrodes are relatively inexpensive and simple
to use and have an extremely wide range of applications
and wide concentration range.
 The most recent plastic-bodied, all-solid-state or gel-
filled models are very robust and durable and ideal for
use in either field or laboratory environments.
 Under the most favourable conditions, when measuring
ions in relatively dilute aqueous solutions and where
interfering ions are not a problem, they can be used very
rapidly and easily (e.g. simply dipping in lakes or
rivers, dangling from a bridge or dragging behind a boat).
 They are particularly useful in applications where only an
order of magnitude concentration is required.
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ref:-http://www.sfu.ca/chemistry/groups/Li/chem215/selective.PDF

6.  They are particularly useful in biological/medical
applications because they measure the activity of the ion
directly, rather than the concentration.
 They are unaffected by sample colour or turbidity.
 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.
 ISEs are one of the few techniques which can measure
both positive and negative ions.
 Non-destructive: no consumption of analyte.
 Non-contaminating.
 Short response time: in sec. or min. useful in industrial
applications. 6
ref:-http://www.sfu.ca/chemistry/groups/Li/chem215/selective.PDF

7. LIMITATION:-
• Precision is rarely better than 1%.
• Electrodes can be fouled by proteins or other organic
solutes.
• Interference by other ions.
• Electrodes are fragile and have limited shelf life.
• Electrodes respond to the activity of uncomplexed ion.
So ligands must be absent.
7
ref:-http://www.sfu.ca/chemistry/groups/Li/chem215/selective.PDF

8. Types of I.S.E :-
 Glass electrodes
 Liquid ion exchanger membrane electrodes
 Solid state membrane electrodes
 Neutral carrier membrane electrodes
 Coated wire electrodes
 Field effect transistor electrodes
 Gas sensing electrodes
 Air gap electrodes
 Biomembrane electrode
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9. Glass electrode:-
By altering the composition of the glass, it
is possible to make the electrode selective for
different ions.
 Usually the glasses contain 60 to 75 mole
% SiO2, 2 to 20 % Al2O3 or LaF3, 0 to 6 %
BaO and CaO, and a variable amount of a
group 1A oxide.
 The mixtures of the oxides is melted and
cooled to form the glass.
 Monovalent cations in the three
dimensional glass structures are relatively 9
mobile.

10. Consequently, monovalent cations from a solution into
which the glass is dipped can penetrate into the surface of
the glass and be cation exchanged net negatively charged
sites in the glass.
 Because the concentration of the analyzed ion in the
sample solution differs from that in the internal reference
solution, a potential difference develops across the
membrane.
 Glass membranes are selective for monovalent cations
because polyvalent ions cannot easily penetrate the
surface of the membrane.
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11. Evidently the selectivity of glass electrodes is related both
to the ability of the various Monovalent cations to
penetrate into the glass membrane and to the degree of
attraction of the cations to the negative sites within the
glass.
 Glass electrodes which are selective for H+ (pH
electrode),Li+, Na+, K+, Cs+ ,Ag+, Ti+ and NH4+ are
commercially available
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12. Liquid ion exchanger membrane
electrodes:-
The inner compartment of the
electrode contains a reference
electrode and an aqueous reference
solution.
 The outer compartment contains
an organic liquid ion exchanger.
 The liquid ion exchanger is
insoluble in the solvent in which the
electrode is to be used (water) and is
nonvolatile at room temperature.
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13.  The ion exchanger is dissolved in a relatively high
molecular weight solvent such as dioctylphenyl
phosphonate.
 Liquid ion exchanger consists of polar ionic sites
attached to a relatively large non polar organic molecule.
 The ionic sites are negative in a cation exchanger and
positive in an anion exchanger.
 Typical liquid ion exchangers are (RO)2PO2- (for Ca2+
and Mg2+) and RSCH2COO- (for Cu2+ and Pb2+). R in the
ion exchangers can be any of several organic groups, for
example p-(1,1,3,3-tetramethylbutyl)phenyl, p-(n-
octyl)phenyl, and decyl. The S and O- in ion exchanger of
the RSCH2COO- type selectively form chelate rings with
a certain ions, e.g., Cu2+ or Pb2+.
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14.  The ion exchangers and reference solutions are held in
place inside the electrodes by a porous membrane.
 Although the membrane can be made from polyvinyl
chloride (PVC), it is usually constructed from some form
of cellulose, e.g., from cellulose acetate.
 The membrane is prepared to have a pore diameter of
about 100 nm.
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15. Chemical treatment makes the membrane hydrophobic.
 The membrane is in physical contact with the liquid ion
exchanger and becomes permeated with it.
 Because the membrane is hydrophobic, water from the
internal reference solution and from the sample solution
is prevented from mixing with the liquid ion exchanger.
 A second type of liquid ion exchanger membrane utilizes
a polymeric membrane to permanently hold the ion
exchanger solution in place within the membrane.
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16. That type of membrane does not need to be in direct
contact with a solution of liquid ion exchanger.
 In either case the shell of the electrode is made from an
inert material such as glass or an organic polymer.
 Liquid ion exchanger membrane electrodes owe their
selectivity to their ability to selectively exchange ions.
 Upon contacting the membrane, an ion from the aqueous
solution exchanges with an ion on a polar site in the ion
exchanger.
The newly created ion and ion exchanger combination
can freely diffuse throughout the membrane.
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17. The ionic conductivity of the membrane results from the
mobility of the ion within the membrane.
 The potential across the membrane is related to the ionic
conductivity within the membrane.
Liquid ion exchanger membrane electrode have been
used for the assay of :-
Ca2+, K+, Li+, Na+, Mg2+, Ni2+, Zn2+, Ti+, Ag+,
Hg2+, water hardness (Ca2+ + Mg2+), Cu2+,Pb2+, Cl-, BF4-,
NO3-, ClO4-, Cr2O72-, benzoates, SCN- and other ions.
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18. Solid state membrane electrode:-
A solid state membrane electrode
can be a single crystal, a pellet made
from a sparingly soluble salt, or a
sparingly soluble salt embedded in an
inert matrix, e.g., rubber.
Because the single crystal and pellet
membranes are homogenous,
electrodes containing them are referred
to as homogenous membrane
electrodes.
The membrane consisting of the
sparingly soluble salt in the inert
binding material is a heterogeneous
membrane electrode. 18

19.  The lanthanum fluoride (LaF3) membrane is the only
single crystal membrane that is widely used in ion
selective electrodes.
 In the process called as “doping”, the resistance of the
LaF3 crystal is decreased by replacing a relatively small
number of La3+ ions in the crystals with Eu2+ ions.
 Fluoride ions migrate from vacancy to vacancy in the
defective LaF3 crystal.
 As a fluoride ion abandons one position in the crystalline
structure, it leaves a hole into which another fluoride can
migrate.
 The result is a crystal which exhibits ionic conductivity.
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20. The conductance to the membrane, as well as the
potential across the membrane, can be related to the
analyte concentration for many solid state membrane
electrodes.
Vacancies in the crystalline structure have exactly the
proper size, charge, and shape to hold a fluoride ion.
 Because fluoride can selectively migrate to the crystal,
the lanthanum fluoride membrane is selective for
fluoride.
 If no fluoride is present in the sample solution, the LaF3
membrane electrode can be used to assay for La3+.
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21. A heterogeneous membrane consists of an active ingredient
dispersed throughout an inert binding material.
 The inert binder provides the physical properties that are
required of the membrane, and the active ingredient provides
the membrane selectivity.
 Wax, silicon rubber, polyvinyl chloride and several other
polymeric substances are used as inert binders in ion selective
electrode.
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22.  Silicon rubber and PVC are most popular binders.
 After mixing binder with active ingredient, the
membrane is formed into thin sheet of appropriate size
and attached to the end of the electrode body.
 The active ingredient is often a sparingly soluble
substance similar to the substance in homogenous
membranes.
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23. Neutral-carrier membrane
electrodes:-
 They have the same design as liquid-ion-exchanger
membrane electrodes.
 The liquid-ion-exchanger is replaced in neutral-carrier
membranes with a neutral complexing agent (a neutral
carrier) such as crown ether, which is dissolved in a highly
water insoluble organic solvent.
The neutral carrier complexes with the analyte at
membrane-sample interface to form a charged complex
which is extracted from the aqueous solution into the
organic solvent in the membrane.
 The selectivity of the membrane for a particular ion
depends upon the ability to extract the ion into the
membrane, which in turn depends upon the ability of the ion 23
to form a complex with the neutral carrier.

24. After complexation and extraction, the species in the
neutral-carrier membrane has the same charge as the
extracted ion.
The solvent in which the neutral carrier is dissolved is
usually a high boiling organic compound such as
nitrobenzene (used in Ba2+ selective electrodes),
dibutylsebacate (used in K+ selective electrode) and o-
nitrophenyl-n-octylether (used in a Ca2+ selective
electrode).
The physical support for the neutral carrier and solvent is
usually a cellulose membrane, more commonly, a PVC
membrane. In addition to K+, Ca2+, Ba2+, neutral-carrier
membrane electrodes are also selective for Li+, H+, Mg2+,
NH4+ , Sr2+.
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25. Coated wire electrodes:-
They are considerably smaller than
other forms of ion selective electrodes
because the internal filling solution is
eliminated and the ion selective
membrane is coated directly on the
internal electrode wire.
 The ion selective membranes
utilized in coated wire electrodes
consists of either an ion exchanger or
neutral carrier immobilized in a
polymeric material that is coated on
the electrode.
 They are more sturdy than other
ISEs and can be constructed with
small tips.
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26. Method of preparation of the electrode:-
 First the metal on the interior of the electrode is sealed
into a glass or some other suitable material so that several
mm or less of the wire is exposed.
 The exposed wire is successively dipped into a solution
of the polymeric material and then into a solution of the
ion exchanger or neutral carrier.
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27. After the electrode has air-dried, the dipping procedure is
repeated, if necessary, until the membrane coating on the
wire is the desired thickness.
 Alternatively, the wire can be dipped into a single
solution containing both the membrane material and the
polymerizer.
 The polymeric matrix can be any of materials including
PVC, polymethyl acrylate(PMM) or epoxy.
 The internal electrode can be constructed from metals
like platinum, copper, silver wire and graphite rods.
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28. Ion selective field effect
transistors (ISFETs):-
The electrode consists of an ion
selective membrane deposited or
coated on the gate of a field effect
transistor (FET). The membrane can
be a sparingly soluble compound such
as silver bromide (solid state
membrane) or some other type of
membrane such as an ion exchanger or
neutral carrier in a PVC matrix.
Often membranes in a PVC matrix
are used. Membranes in a PVC matrix
can be forced to adhere to the gate of
the FET by placing a polyimide mesh
over the gate prior to coating it with 28
the membrane.

29. The potential at the membrane is partially determined by
the activity of the analyte in solution.
 That potential determines the flow of current through the
drain of the FET.
 The drain current consequently varies with the activity
of the analyte and is the monitored factor.
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30. Gas-sensing electrodes:-
They are used to assay the gases
dissolved in aqueous solutions.
 It is constructed by enclosing the
glass pH membrane in a second, gas-
permeable hydrophobic membrane.
 A thin layer of an electrolyte
solution is held between the two
membranes.
 They also have a small reference
electrode enclosed within the gas- 30
permeable membrane.

31.  The potential between the internal ISE and the reference electrode
within the outer membrane is monitored.
 The gas permeable membrane holds a constant volume of
solution around the internal ISE into which the gaseous analyte
can diffuse.
 The hydrophobic gas-permeable membrane can be composed of
substance which allows passage of dissolved gas but prevents the
solution within the membrane from escaping.
 The materials used are silicon rubber, Teflon polypropylene,
fluorinated ethylene propylene, polyvinylidene fluoride etc…
 Gas from the sample solution passes through the submerged gas-
permeable membrane and equilibrates in the electrolyte solution
between the two membranes.
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32.  The gas reacts reversibly with the electrolyte solution to form an ion
to which the ion selective electrode responds.
 Because the activity of the ion that is formed between the two
membranes is proportional to amount of gas dissolved in sample, the
electrode response is directly related to the activity of the gas in the
sample.
 The gases (primarily NH3, SO2 and CO2) which are detected by gas
sensing electrodes based on the pH electrode equilibrate with the
electrolyte solution to alter its pH:
• NH3 + H2O = NH4+ + OH-
• SO2 + H2O = HSO3- + H+
• CO2 + H2O = HCO3- + H+
 H2S, HCN, HF and chloride can be assayed by using internal
homogenous membrane electrode containing the appropriate silver
salt.
 Disadvantage possesses relatively long response time i.e; require 1-7
minutes after insertion in to a sample solution to reach equilibrium. 32

33. Air-Gap electrodes:-
 They are another form of gas
sensing electrodes invented by
Ruzicka and Hansen.
 A very thin layer of an
appropriate electrolyte solution is
adsorbed on the surface of the
membrane of the glass electrode.
 The electrolyte solution is
adsorbed on glass membrane when
membrane comes in contact with
the sponge containing the
electrolyte solution and a wetting
agent.
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34. The reference electrode makes contact with the adsorbed
electrolyte layer through a small, porous, ceramic salt
bridge.
 The air gap electrode is used to assay ionic species which
can be chemically converted to gases,
e.g. HCO3-
 The HCO3- solution is placed in the sample holder and an
acid is added to convert HCO3-(aq) to CO2 (g).
The sample holder is placed in position under the electrode
and stirred with a magnetic stirrer and stirrer bar.
 Carbon dioxide which is emitted during the chemical
reaction equilibrates with the electrolyte solution on the
glass membrane and alters the pH of the solution. 34

35. The glass electrode measures the pH of the resulting solution.
 The electrolyte solutions used with air gap electrode are the
same as those used with other gas-sensing electrodes.
The air-gap electrode has a faster response time due to the
thinner layer of electrolyte solution and a longer lifetime than
most of the other types of sensing electrodes.
 A typical response time for an air-gap electrode is less than a
minute.
 Air-gap electrode is primarily used for analysis of NH4+,
HSO3-. As an example they can be used for the determination
of urea in blood.
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36. Biomembrane electrodes:-
It is an ion selective electrode which is coated with an
enzyme-containing acrylamide gel.
 The gel and enzyme are held in place on the surface of the
ion selective electrode by an inert physical support.
 The design is same as gas-sensing electrode.
 The support is a sheet of cellophane or a piece of gauze
made from dacron or nylon.
 The physical support is wrapped around the electrode
membrane and tied in place. 36

37.  The acrylamide gel containing the enzyme is
coagulated on the support-electrode combination.
 Enzymes are highly selective biochemical catalysts.
The selectivity of Biomembrane electrode is due to
the selectivity of the enzymes that are used in
electrodes.
 Here the enzyme-catalyzed reaction of the analyte
yields an ionic reaction product which is monitored
by the internal ion-selective electrode.
The operation of the urea-selective electrode will
serve to illustrate the operation of Biomembrane
electrodes.
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38. The glass membrane of an ammonium-sensitive glass
electrode is coated with an acrylamide gel layer
containing the enzyme urease.
 When the electrode is dipped into a solution containing
urea, the following reaction occurs to yield NH4+ :
CO(NH2)2 + H2O 2NH4+ + CO2
 The NH4+ formed during the reaction is measured at the
ammonium-selective electrode.
A working curve is prepared by plotting the potential of
the electrode in standard urea solutions as a function of
the logarithm of urea concentration.
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39.  The urea concentration in the sample is obtained from
the working curve.
 Unfortunately the enzymes used in Biomembrane
electrodes gradually decay and the enzyme containing gel
must be periodically replaced.
The Biomembrane of urea electrode lasts about 2 weeks.
 Biomembrane electrodes have long response time of 5 or
more minutes.
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40. APPLICATION:-
• Ion-selective electrodes are used in a wide variety of
applications for determining the concentrations of various
ions in aqueous solutions. The following is a list of some
of the main areas in which ISEs have been used.
• Pollution Monitoring: CN, F, S, Cl, NO3 etc., in effluents,
and natural waters.
• 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.
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ref:- http://www.cma-science.nl

41. • 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.
• Biomedical Laboratories: Ca, K, Cl in body fluids (blood,
plasma, serum, sweat).
• F in skeletal and dental studies.
• Education and Research: Wide range of applications.
• Ca in dairy products and beer.
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42. References:-
• http://www.cma-science.nl
• www2.vernier.com/booklets/ise.pdf
• http://www.sfu.ca/chemistry/groups/Li/chem215/selective
.PDF
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