ION SELECTIVE ELECTRODE
Dept of biochemistry
In potentiometry, the potential of an electrochemical cell is
measured under static conditions because no current flows
while measuring a solution’s potential.
Use of electrodes to measure voltage that provide chemical
Concentration of ions in solution is calculated from the
measured potential difference between the two electrodes
immersed in solution under condition of zero current.
This type of system includes at least two electrodes,
identified as an indicator electrode (right half-cell) and a
reference electrode (left half-cell ) which act as the cathode
and anode respectively.
Each electrode is in contact with either the
sample (in the case of the “indicator
electrode”) or a reference solution ( in the case
of the “reference electrode”).
Electrodes + solution = electrochemical cell
Each electrode represent half cell reaction with
correspondance half cell potential.
• A device for measuring the potential of an
electrochemical cell without drawing a current or
altering the cell’s composition.
• The potential is measured under static conditions.
• Because no current, or only a negligible current flows
while measuring a solution’s potential, its composition
• For this reason, potentiometry is a useful quantitative
• Two electrodes connected to Potentiometer to measure
• Indicator electrode (Eind) – potential respond to change
according to conc. of ions
• Reference electrode (Eref) – half cell potential does not
Ecell = Eind ─ Eref + Elj
Schematic diagram of an electrochemical cell of potentiometric measurement
Indicator or measuring electrode
Readout device (Potentiometer)
Liquid junction – also known as a salt bridge are
required to complete the circuit between the
It allows electrical contact between the two solutions.
It prevents the mixing of the electrode solutions.
It maintains the electrical neutrality in each half cell as
ions flow into and out of the salt bridge.
Reference Electrode- is an electrochemical half-cell
that is used as a fixed reference for the measurement
of cell potentials.
A half-cell with an accurately known electrode
potential, Eref, that is independent of the
concentration of the analyte or any other ions in the
Always treated as the left-hand electrode
Normal hydrogen electrode
Saturated calomel electrode
Calomel electrode- composed of
mercury/mercurous chloride; It is dependable but
large, bulky, and affected by temperature.
Silver/silver chloride- Widely used because simple,
inexpensive, very stable and non-toxic.
o reference electrodes are more compact -- overall
better & faster
Normal Hydrogen Electrode- consists of a
platinized platinum electrode in HCl solution with
hydrogen at atmospheric pressure bubbled over the
-determination of pH of the solution.
Silver-silver chloride electrode
Standard hydrogen electrode (SHE)
Indicator or measuring electrode
• The potential of this electrode is proportional to the
concentration of analyte.
• Two classes of indicator electrodes are used in
o metallic electrodes
• Electrodes of the first kind
• Electrode of the second kind
• Redox electrode
o membrane electrodes (ion-selective electrodes)
• glass pH electrode
Electrodes of the first kind
• A metal in contact with a solution containing its cation.
• The potential is a function of concentration of Mn+ in a
Mn+ / M. The most common ones:
o Silver electrode (dipping in a solution of AgNO3)
• Ag+ + e ↔ Ag
o Copper electrode
• Cu+2 + 2e ↔ Cu
o Zn electrode
• Zn+2 + 2e ↔ Zn
Electrode of the second kind
• A metal wire that coated with one of its salts
• Respond to changes in ion activity through
formation of complex.
• A common example is silver electrode and
AgCl as its salt precipitate.
• This kind of electrode can be used to measure
the activity of chloride ion in a solution.
• An inert metal is in contact with a solution
containing the soluble oxidized and reduced
forms of the redox half-reaction.
• The inert metal is usually is platinum (Pt).
• The potential of such an inert electrode is
determined by the ratio of the reduced and
oxidized species in the half-reaction.
• A very important example of this type is the
Indicator Electrode- also called the measuring
It is immersed in a solution of the analyte,
develops a potential, Eind that depends on the
activity of the analyte.
Is selective in its response
It is the other electrochemical half-cell that
responds to changes in the activity of a
particular analyte species in a solution.
Ion selective electrode
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.
indicator electrode based on determination of cations
or anions by the selective absorption of these ions to a
TYPES OF ION SELECTIVE
•Glass Membrane Electrode
•Solid State Electrode
•Liquid Membrane Electrode
•Gas Sensing Electrode
GLASS MEMBRANE ELECTRODE
• Responsive to univalent cations ( H+ , Na+)
• Glass electrodes available for Na+, K+, NH4+, Li+,
Ag+(cations only) by varying glass composition
• The selectivity for this cation by varying the
composition of a thin ion sensitive glass membrane.
• 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
• Example: pH electrode
Glass pH electrode
• Advantages over other electrodes for pH
o Its potential is essentially not affected by the
presence of oxidizing or reducing agents.
o It operates over a wide pH range.
o It responds fast and functions well in physiological
o Selective for monovalent cations only because
polyvalent ions can not penetrate the surface of
Selective for the
detection of hydrogen
The measuring or
indicator electrode has
a “glass membrane”
pH is then determined
from potential between
the pH electrode and a
SOLID STATE ELECTRODE
• Solid state electrode are selective primarily to anions.
• It may be a
- homogenous membrane electrode
- heterogeneous membrane electrode.
• Homogenous membrane electrode: ion-selective
electrodes in which the membrane is a crystalline
• Heterogeneous membrane electrode: ion-selective
electrodes prepared of an active substance, or mixture
of active substances (silicone rubber or PVC)
• Example: Fluoride ion selective electrode
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
• Used for direct measurement of several polyvalent
cations (Ca ion) as well as a certain anions.
• Inner compartment of electrode contains reference
electrode & aqueous reference solution.
• Outer compartment – organic liquid ion exchanger
•The polymeric membrane made of PVC to separate the
test solution from its inner compartment which contains
standard solution of the target ion.
•The filling solution contains a chloride salt for
establishing the potential of the internal Ag/AgCl wire
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.
• The change is detected by a combination pH sensor.
• This type of electrode does not require an external
Measurement of PCO2 in
routine blood gases
A modified pH electrode
with a CO2 permeable
membrane covering the
glass membrane surface
A bicarbonate buffer
Change in pH is
proportional to the
dissolved CO2 in the
Application of Potentiometric
• Clinical Chemistry
o Ion-selective electrodes are important sensors
for clinical samples because of their selectivity
o The most common analytes are electrolytes,
such as Na+, K+, Ca2+,H+, and Cl-, and dissolved
gases such as CO2.
• Environmental Chemistry
o For the analysis of of CN-, F-, NH3, and NO3
water and wastewater.
• Potentiometric Titrations
o pH electrode used to monitor the change in pH
during the titration.
o For determining the equivalence point of an
o Possible for acid–base, redox, and precipitation
titrations, as well as for titrations in aqueous
and nonaqueous solvents.
o NO3, NH4, Cl, K, Ca, I, CN in soils, plant
• Detergent Manufacture
o Ca, Ba, F for studying effects on water quality
• Food Processing
o NO3, NO2 in meat preservatives
o Salt content of meat, fish, dairy products, fruit
juices, brewing solutions.
o F in drinking water and other drinks.
o Ca in dairy products and beer.
o K in fruit juices and wine making.
o Corrosive effect of NO3 in canned foods.
• Relatively inexpensive and simple to use and have an
extremely wide range of applications and wide
• 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.
• ISEs can measure both positive and negative ions.
• They are unaffected by sample colour or turbidity.
• Non-destructive: no consumption of analyte.
• Short response time: in sec. or min. useful in industrial
• Precision is rarely better than 1%.
• Electrodes can be affected by proteins or other organic
• Interference by other ions.
• Electrodes are fragile and have limited shelf life.