2. Ion selective electrodes(ISE)
• - Also known as indicator electrodes
• - Respond directly to the analyte
• - Used for direct potentiometric measurements
• - Selectively binds and measures the activity of one ion
(no redox chemistry)
Examples
• pH electrode
• Calcium (Ca2+) electrode
• Chloride (Cl-) electrode
3. • An ion-selective electrode (ISE), also known as 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.
• 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.
6. • Ion Selective Electrodes (including the most common pH electrode) work on
the basic principal of the galvanic cell .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 cell:
• Ecell = EISE - ERef
7. TYPES OF ION SELECTIVE ELECTRODE
(ISE)
• Glass Membrane Electrode
• Solid State Electrode
• Liquid Membrane Electrode
• Gas Sensing Electrode
8. GLASS MEMBRANE ELECTRODE
• Glass electrode are responsive to univalent cations
( H+ , Na+)
• The selectivity for this cation by varying the composition of a thin ion
sensitive glass membrane.
• Example: pH electrode
- used for pH measurement
- used as a transducer in various gas and biocatalytic sensor,
involving proton generating or consuming reaction.
9. • 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.
• The electrode is immersed in the solution and pH is measured.
10. SOLID STATE ELECTRODE
• Solid state electrode are selective primarily to anions.
• It may be a homogenous membrane electrode or heterogeneous
membrane electrode.
• Homogenous membrane electrode: ion-selective electrodes in which
the membrane is a crystalline material (AgI/Ag2S).
11. • Solid state electrode are selective primarily to anions.
• It may be a homogenous membrane electrode or heterogeneous
membrane electrode.
• Homogenous membrane electrode: ion-selective electrodes in which
the membrane is a crystalline material (AgI/Ag2S).
12. LIQUID MEMBRANE ELECTRODE
• Liquid membrane is a type of ISE based on water-immiscible liquid
substances produced in a polymeric membrane used for direct
potentiometric measurement.
• Used for direct measurement of several polyvalent cations (Ca ion) as
well as a certain anions.
13. • The polymeric membrane made of PVC to separate the test solution
from its inner compartment.
• Contains standard solution of the target ion.
• The filling solution contains a chloride salt for establishing the potential
of the internal Ag/AgCl wire electrode.
14. GAS SENSING ELECTRODE
• Available for the measurement of ammonia, carbon dioxide and
nitrogen oxide.
• This type of electrode consist of permeable membrane and an internal
buffer solution.
• The pH of the buffer changes as the gas react with it.
15. • The change is detected by a combination pH sensor within the
housing.
• This type of electrode does not require an external reference
electrode.
16. Advantages of Ion Selective Electrode
(ISE) Technique
• 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.
• Under the most favorable 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.
• They are particularly useful in applications where only an order of
magnitude concentration is required, or it is only necessary to know
that a particular ion is below a certain concentration level.
17. • They are invaluable for the continuous monitoring of changes in
concentration for example in potentiometric titrations or monitoring the
uptake of nutrients, or the consumption of reagents.
• They are particularly useful in biological/medical applications because
they measure the activity of the ion directly, rather than the
concentration.
• ISEs are one of the few techniques which can measure both positive
and negative ions.
• 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.
18. Non-destructive: no consumption of analyte.
Non-contaminating.
Short response time: in sec. or min. useful in industrial applications
19. 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.
20. Empirical Calibration Plot
Potential (mV)
Slope = 59/zi mV
zi = charge of ion
Called Nernstian slope
- Used to determine the unknown concentration of analytes
- Departure from linearity is observed at low concentrations
21. 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.
22. • 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 andCl 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.
23. Determination of Fluoride Ion
• In the lanthanum fluoride electrode, the sensing element is a crystal
of lanthanum fluoride LaF3, doped with europium fluoride EuF2 to
create lattice vacancies. Such a crystal is an ionic conductor by virtue of the
mobility of fluoride ions which jump between lattice vacancies.
• An electrochemical cell may be constructed using such a crystal as a
membrane separating two fluoride solutions.This cell acts as
a concentration cell with transference where the fluoride transport number
is 1. As transference of charge through the crystal is almost exclusively due
to fluoride, the electrode is highly specific to fluoride
24. • The only ion which significantly interferes is hydroxide (OH-). Generally such
"alkaline error" can be avoided by buffering the sample to a pH below 7
• The cell diagram of a typical experimental arrangement is:
• Cu' | Ag,AgCl | KCl || solution | LaF3 | KF,KCl | AgCl,Ag | Cu
• where:
• Cu' | Ag, AgCl | KCl is an external reference electrode
• KF,KCl/AgCl,Ag/Cu is an internal reference inside the fluoride selective electrode
25. REFERENCE
1.Environmental chemical analysis –lain L Marr, Malcolm s cresser
2.Instrumental analysis –skoog,heller,crouch,2007
3.Standard methods-21st edition 2005
4.www,doiserbia.b.rs/img/doi/0352-5139/2013/0352-5139-1300098r.pdf
