Ion-selective electrodes (ISE) respond directly to specific ions in solution and convert their activity into measurable electrical potential. They consist of various types of membranes (glass, liquid, gas sensing) and are widely used in applications such as pollution monitoring, agriculture, and biomedical labs. While ISEs are relatively inexpensive and easy to use, they have limitations such as sensitivity to interfering ions and a fragile nature.

1. Prepared by:-
MANIKANT MANI

2. 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.

4. when a charged species diffuses from a region of activity , A1, to a region of
activity ,A2, there is a free energy change .
This decrease in free energy should equal the electrical work done to the
surroundings.
Hence,
Where Z is the charge of the species, including the sign.

5. 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

6. (1) Glass Membrane
Electrode
(i.e. H+ electrode)
(3) Liquid Membrane
Electrode
(e.g. Ca2+ electrode uses
a liquid chelator)
(4) Gas Sensing Electrode
(e.g. CO2 gas sensing
electrode)
(2) Solid State Electrode
(e.g. F- electrode uses a
Eu2+-doped LaF3
crystal)

7. The glass electrode Most of the metal cations (e.g. Na+ ) in the
hydrated gel layer diffuse out of the glass membrane and into the
solution, see Fig. Concomitantly, H+ from solution diffuse into the
membrane. Response of glass electrode (at 25 °C).

8. During two-point calibration, the y-intercept can be corrected by the “calibrate” knob.
The slope is corrected by the “slope” or “temperature” knob during pH meter calibration.

9. This is based on an inorganic crystal doped with a small amount of ions of a
different valency to create vacant sites.
For instance, in a F- ion electrode, a LaF₃ crystal is doped with EuF₂ so that
there are anion vacancies for the migration of F through the LaF3 crystal,
Response of F- ion electrode .
E = constant - (0.05916) log (AF-, outside)
= constant - (0.05916) log ᵞF- - (0.05916) log [F- ] , At high pH, there is
interference by OH- (Note kf-, OH- = 0.1) At low pH, F- is converted to HF
(pKa = 3.17) to which the electrode is insensitive.

10. The analyte solution is usually diluted in a high m buffer to keep the pH (at 5.5)
and m constant. E = constant´ - (0.05916) log [F- ]
Therefore, a plot of E versus log [F- ]
should yield a straight line with a negative slope,

11. 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.
• 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.

13. GAS SENSING ELECTRODE
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-
permeable membrane.
The change is detected by a combination pH sensor within the housing.
This type of electrode does not require an external reference electrode.

15. 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.

16. 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.

17. 1. Precision is rarely better than 1%.
2. Electrodes can be fouled by proteins or other organic solutes.
3. Interference by other ions.
4. Electrodes are fragile and have limited shelf life.
5. Electrodes respond to the activity of uncomplexed ion. So
ligands must be absent or masked. m must be kept constant.

18. 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

19. 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.

20. K in fruit juices and wine making.
Corrosive effect of NO3 in canned foods.
Detergent Manufacture: Ca, Ba, F for studying effects on waterquality.
Paper Manufacture: S and Cl in pulping and recovery-cycleliquors.
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 ofapplications.
Ca in dairy products and beer.

21. • 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 actsas
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

22. 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