The document discusses ion-selective electrodes (ISEs), highlighting their crucial role in measuring metal ions that affect cellular processes and metabolism. It describes various types of ISEs, including glass membrane, solid-state ion exchange, solid-state crystal, and liquid-membrane electrodes, and outlines their applications in diagnostics and chemical analysis. Furthermore, it addresses interferences and advantages of ISEs, emphasizing their wide response range and low cost.

1. Tapeshwar
Yadav
(Senior
Lecturer)
BMLT, DNHE,
M.Sc. Medical Biochemistry
Ion specific electrodes

2. Ion Selective Electrodes
 It is a well known fact that metal ions have a
profound effect on cellular processes
 The importance or the role that ions play in
cellular activity can be gauged by the fact that
most cells maintain a very critical Na+ & k+
balance between the extracellular and the
intracellular spaces.
 Any distribution in this critical balance is to the
cellular metabolism through a drastic change
in the osmotic pressure resulting in cellular
swelling.

3. ISE
 Another metal ions, Ca2+ is known to act as a
minatory intracellular message stimulating such
diverse process as
 Insulin secretion
 Chemotaxis
 Endocytosis
 Cellular proliferation

4. Basic principles
 An ISE operates an exactly the same principles
as a PH electrode
In fact, a PH electrode is a type of ion selective
electrode sensitive to hydrogen ion.
Just like a PH electrode, the electrode body
contains a reference solution and an metal
reference electrode

5. Contd…
On to this electrode body is sealed an ion selective
membrane which act as the ion sensor
Four different types of ion selective membranes are
in use
They are:-
1. Specially formulated glass
2. An ion exchanger dispersed in an innert matrix

6. Contd…
3. A crystal
4. A liquid ion exchanger
The external reference electrode is either a
calomel or a Ag/Agcl electrode
The potential developed a cross the ion selective
electrode can be measured on a milli volt scale
available in a PH meter.

7. Contd…
This is proportional to the activity of ion in the
sample
More sophisticated instruments employ specific
ion meters (high impedance mill volt meters)
which have readout scales directly calibrated in
concentration.

8. Types of ISE
1. Glass membrane electrodes
2. Solid-state ion exchanger electrodes
3. Solid-state crystal electrodes
4. Liquid-membrane electrodes

9. 1. Glass membrane electrodes
 Glass electrode consists of a stem of non-cation
responsive, high resistance glass on which is fused a thin
walled bulb of cation responsive glass.
 Selectivity of a glass electrode is a function of the comp.
of the glass.
 3 subtypes:
i. Type : pH
ii. Type : Cation sensitive
iii. Type : Sodium sensitive
Note: 2nd two subtypes are in general responsive to monovalent
cations and are more or less unresponsive to anions.

10. Contd…
 Appropriate adjustments of glass composition
change the degree of electrode selectivity.
 Thus, glass can be made more responsive to
cations by adding to it elements which have co-
ordination numbers greater than oxidation
numbers to alkali metal-silicate glasses (20%
Na2O – 10% CaO – 70% SiO2).
 It usually imparts an excessive negative change to
the glass making it suitable to attract cations
having a proper charge-size ratio.

11. Contd…
 Glasses with a composition of 27% Na2O – 15%
Al2O3 – 68% SiO2 show a general cation response.
 If the above composition is modified to 11% Na2O –
18% Al2O3 – 71% SiO2 , the glass becomes highly
sodium selective as compared to other alkali metal
ions.

13. Applications of Glass electrodes:
Measurement of sodium, lithium, or silver ions
because of their high specificity for these ions.
Due to its relative inertness it can be used in non-
aqueous media, organic solvents & also in the
presence of lipid soluble or surface active
molecules.

14. 2. Solid-State Ion Exchange
Electrodes
 In these electrodes, the glass membrane is
replaced with a solid-state ionically conducting
membrane.
 Ion responsive material is an insoluble or
sparingly soluble salt dispersed in an inert matrix.
 Often used inert matrices include silicon rubber,
polyvinyl chloride & other polymeric materials.

15. Contd…
 Membrane have good mechanical properties and
give reproducible potentials.
 The membrane so prepared is then cemented to a
glass or epoxy resin electrode body.
 The body holds an internal-reference solution and
a reference electrode .
 Sometimes, the back of an ion responsive
membrane is coated with mercury and platinum
wire is connected to it which works as the
reference electrode.

16. Contd…
Solid-state ion exchanger electrodes have 2
drawbacks:
1. Relatively short working life.
2. Concerns itself with the extremely high
resistance of silicone robber & other matrices.

18. 3. Solid-State Crystal Electrodes:
 Crystal of an insoluble salt can act as ion sensitive
elements operating in much the same way as the salt
dispersed in an inert matrix.
 Crystal should not have high resistance.
 Eg.
Lanthanum fluoride electrode:- measures fluoride
Silver chloride electrode:- measures chloride
These electrodes have a life of about 1-2 years.
However, if used at high temp., their life gets shortened
considerably 1-3 months

20. 4. Liquid-Membrane Electrodes:
 Sensing element of these electrodes is a layer of
organic solvent in which an ion exchanger is
dissolved.
 Most available electrodes use a porous diaphragm
(glass or ceramic disc) which separates the inner ion
exchanger sol. From the test sol.
 Ion exchange sol. Keeps the disc always saturated.
 This type of electrodes are used for the measurement
of calcium, nitrate, perchlorate & other ions.

23. Interferences
 2 main types of interferences are encountered by
ISEs. They are-
1. Method interference
2. Electrode interference

24. 1. Method interference:
It can occur when either or all of such factors at the pH,
ionic strength or temp. of the sample being tested are not
properly mixed.
Thus, while measuring fluoride ions, a low pH might
give very low values for the activity of this ion.
This is so since fluoride ions form complexes with the
hydrogen ions and consequently lose their measurable
activity.
The method interference can therefore be done away
with by fixing the measurement conditions diligently.

25. 2. Electrode interference:
These are mostly due to the passage of an interfering
ion into the membrane, being measured.
Thus, high barium ion conc. In a sample being
measured for calcium can be detrimental to the
experiment as barium ions can compete with calcium
ions for passage into the membrane.
Another type of electrode interference occurs chiefly
with crystal electrodes.
Crystal electrodes consequently loses its sensitivity to
the desired ion.

26. Applications of ISE
 Activity measurements are valuable because the
activities of ions determine rates of reactions and
chemical equilibria.
 Ion electrodes have been used for predicting
corrosion rates, extent of precipitation, degree of
acidity , formation of complexes, solution
conductivities and effectiveness of electroplating
bath solutions.

27. Contd…
 It can have many diagnostic application in
biology. For e.g., the chloride electrode is being
used to assay chloride ion activity in the sweat of
babies as a diagnostic test for cystic fibrosis.
 It is used to study the control of ion transport by
tissues and cells

28. Contd…
 Stimulating biological application of ion selective
electrodes is that they can be converted into
enzyme electrodes.
 These electrodes can be measure the activity of
an enzyme in a given sample
 For e.g., the ammonium ion electrode can be
converted into an enzyme electrode measuring
the activity of ureas.

29. Advanteages
- Exhibit wide response
- Exhibit wide linear range
- Low cost
- Color or turbidity of analyte does not affect results
- Come in different shapes and sizes
ION-SELECTIVE ELECTRODES (ISE)

30. - Made from an ion-conducting membrane
(ion-exchange material that allows ions of one electrical
sign to pass through)
- Reference electrode is inbuilt
- Internal solution (solution inside electrode) contains ion of
interest with constant activity
- Ion of interest is also mixed with membrane
- Membrane is nonporous and water insoluble
ION-SELECTIVE ELECTRODES (ISE)

31. - Selective (preferential) ion is C+
- Membrane is made of poly(vinyl chloride) (PVC)
- Membrane is impregnated with nonpolar liquid
- Membrane contains ligand L (ion-selective ionophore)
- Membrane contains the complex LC+
- Membrane contains hydrophobic anion R- (ion exchanger)
ION-SELECTIVE ELECTRODES (ISE)

32. - Responds preferentially to one species in solution
Internal reference
electrode
Ion-selective membrane
Internal (filling)
solution
ION-SELECTIVE ELECTRODES (ISE)