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Reference Electrodes
Types of Ion selective electrodes
Potentiometric titrations
When a metallic rod is dipped in its salt solution some potential is developed
on the surface of the metallic rod. This system is called single electrode and
the potential developed is called single electrode potential.
The difference in the Single Electrode
Potentials of the galvanic cell is
called Electromotive force, which is
the driving force for the flow of
electricity through the external
circuit. This happens only when EMF
is negative.
• A standard electrode is used and EMF is
measured by using a test electrode, whose
single electrode potential is calculated.
• Standard Hydrogen electrode or Calomel
electrode is used as Reference electrode.
• The ultimate reference electrode is Standard
Hydrogen Electrode.
Standard Hydrogen Electrode
Calomel Electrode
The combination of the Reference electrode and the
indicator electrode measures the electro motive force of the
Ion Selective Electrodes
• An electrode that responds to a particular ion’s activity is
called ion-selective (or) ion-sensitive electrode (ISE).
• Principle: When ever two solutions of different concentrations
are separated by a membrane, a potential difference is set up
across the membrane due to the unequal distribution of ions
in the solutions. This potential difference is known as
membrane potentials. This membrane potential difference
will be measured by ion-selectometers with the help of ion
selective electrodes.
Types of Ion Selective Electrodes
There are six types of ISE’s based on the
membranes used in the electrode:
1. Glass membrane electrode
2. Liquid membrane electrode
3. Solid membrane electrode
4. Gas – sensing electrode
5. Enzyme – based electrode
6. Bio catalytic membrane electrode
Glass Membrane Electrode
• The electrode has a thin glass membrane
which is selective to various univalent cations,
by suitable changes in he glass composition.
• The best and the oldest glass membrane
electrode is Glass Electrode which is
responsive towards the activity of hydrogen
Glass Electrode (Combined)
• The nature of the glass used for construction of the glass
electrode is very important. If a lime soda glass of
approximate composition of SiO2 72%, Na2O 22% and CaO 6%
used for manufacturing a glass electrode, such electrodes are
satisfactorily work over the pH range 1-9.
• By replacing most of all the sodium content of glass by lithium
content, found suitable to measure in the pH range upto 14.
Solid membrane Electrode
• In solid membrane electrodes, the glass
membrane is replaced by an ionically
conducting membrane.
• The LaF3 electrode for fluoride determination
is an example for the solid membrane
Fluoride Electrode:
In the fluoride electrode, the active membrane is a single crystal of LaF3
doped with europium(II) (to lower its electrical resistance and facilitate ionic
charge transport).
The LaF3 crystal, seated into the end of a rigid plastic tube, is in contact with
the internal and external solutions. Typically the internal solution is 0.1M
each NaF and NaCl; the fluoride ion activity controls the potential of the inner
surface of the LaF3 membrane, and the chloride ion activity fixes the potential
of the internal Ag/AgCl reference electrode the electro chemical cell for this
E = Constant + (RT/F) ln
𝐹 − 𝑖𝑛𝑡
𝐹 − 𝑒𝑥𝑡
Since [𝐹 − ] is contant, E = Constant + 0.0591 pF
Ag/AgCl(s), Cl-(0.1M), F-(0.1M)/LaF3 crystal/test solution //Reference electrode
The fluoride electrode also responds to hydroxide ion
concentration. Hence, the hydroxide ion concentration
is kept constant with buffer solutions. It contains of
0.25M acetic acid, 0.75M sodium acetate, 1M sodium
chloride and 1M sodium citrate. Sodium citrate masks
Al3+ and Fe3+ which interfere by complexing Fluoride.
The buffer controls the overall ionic strength as well as
the pH. This buffer is also known as Total Ionic Strength
Adjusting Buffer (TISAB) solution.
Liquid Membrane Electrode
• Liquid membrane or ion exchange electrodes are prepared
using an organic liquid ion-exchanger which is immiscible with
water, or with ion sensing material is dissolved in an organic
solvent which is immiscible with water.
• The solvent is placed in a tube sealed at the lower end by a
thin hydrophobic membrane such as cellulose acetate paper,
aqueous solutions will not penetrate this film.
• Example for Liquid membrane Electrode is Calcium
Responsive Electrode.
It contains calcium salts of bis (2 ethyl 1
hexyl) phosphoric acid (d2EHP)
dissolved in straight chain alcohols (or)
di decyl hydrogen phosphate dissolved
in di-n-octyl phenyl phosphate.
The neutral undissociated molecules of
Ca(d2EHP) diffuse easily in the solvent
saturated pores of the membrane, but
are insoluble in water.
Gas Sensing Electrode
Dissolved ammonia from the sample diffuses
through a gas permeable fluoro carbon membrane
until a reversible equilibrium is established between
the ammonia level of the sample and internal filling
solution. Hydroxide ions are formed in the internal
filling solution by the reaction of ammonium water.
NH3 + H2O
NH4+ +OHThe hydroxide level of the internal filling solution
measured by the internal sensing element and is
directly proportion to the level of ammonia in the
Enzyme Based Electrode
• These electrodes make use of an enzyme to
convert the substance to be determined into an
ionic product the which can itself be detected
by a known ion selective electrode.
• A typical example is the urea electrode in which
the enzyme urease is employed to hydrolyse
urea and the progress of the
CO(NH2)2 + H2O+H+
2NH4+ +CO2
• Reaction can be followed by means of glass
electrode which is sensitive to ammonium ions.
• The Urease is incorporated in to a polyacrylamide
gel which is allowed to set on the bulb of the
glass electrode and may be held in position by
nylon gauze.
• Then the electrode is inserted in to a solution
containing urea, ammonium ions are produced,
diffuse through the gel and cause a response by
the ammonium ion probe.
Bio-Catalytic Membrane Electrode
In bio catalytic membrane
electrodes, a biocatalyst is
immobilized at the surface of
an electro chemical sensor
(membrane electrode) the
membrane electrode may be
ion-selective (glass, Solid (or)
polymer) or gas sensing (NH3,
CO2 (or) H2S) electrode. The
biocatalysts may be enzyme,
tissue, bacteria etc.
Bio-catalytic electrode life times are dependent
on stability of the bio-catalyst, which in turn
depends on
(1) Methods of the biocatalyst immobilization
(2) Solution pH,
(3) Storage conditions and
(4) Presence of activators (or) inhibitors.
In potentiometric titrations,
the change in the electrode
potential upon the addition
of the titrant is noted against
the volume of titrant added.
At the end point the rate of
change of potential
maximum. (The end point is
determined by plotting a
curve of potential versus the
volume of titrant and
recording the inflation
EMF in mVolts
Potentiometric Titrations
Volume of the Titrant