2 potentiometry

2.Potentiometry
Sushil D.Patil
Bhujbal Knowledge City
MET Institute of Pharmacy,Nashik

Objectives
• understand the basic principles of potentiometry
• understand the measurement of electrode potential and
pH, pH meter& its calibration, potentiometric titrations
and applications
Syllabus:
• Introduction, theory & principle of potentiometry, types
of electrodes, Reference electrode(Normal hydrogen
electrode, calomel electrode, quinhydrone electrode,
silver- silver chloride electrode), indicator electrode
(Glass, ion sensitive – solid, liquid and gas membrane),
measurement of electrode potential and pH, pH meter&
its calibration, potentiometric titrations and applications

Principles
Potentiometric measurements are based upon
the determination of the voltage difference, at
zero current, i.e. under equilibrium conditions,
between two electrodes which are plunged
into a sample solution .Each of these
electrodes constitutes a half-cell.
The following concerning these two electrodes
should be noted:
• The external reference electrode (ERE), is the
electrochemical reference half-cell for which
the potential is constant with respect to that of
the sample solution.
• The Indicator electrode (IE), also known as the
working electrode.

• Electrode of interest is named working
electrode or the indicator electrode; the
second electrode is know as reference
electrode or auxiliary electrode(counter
electrode). In practice the potential of the
working electrode (WE)is measured with
respect to the reference electrode (RE).

Electrode potentials
The potential difference developed by the cell ΔE as the difference between
the potential of the working electrode Ework and the reference electrode
Eref
The cell potential is measured
as Ecell=Eind +Eref+Ej

• Reference Electrode
1.Primary Standard-Standard ( Normal) Hydrogen
Electrode
2.Secondary Standard- i) Calomel Electrode ii) Silver-
silver chloride Electrode
• Indicator Electrode
a. Metallic Indicator Electrode- i) First Kind,ii)
Second Kind,iii) Third Kind & iv) Metallic redox
indicator electrode
b. Membrane Electrodes-i) Ion selective
Electrode/membrane Electrode ,ii)Ion selective
field effect transistors,iii) Molecular selective
electrode system: Gas sensing probes,iv)
Quinhydrine Electrode
c. Biosensors

1.Primary Standard-Standard ( Normal) Hydrogen
Electrode

• The standard hydrogen electrode, or SHE, is
composed of an inert solid like platinum on
which hydrogen gas is adsorbed, immersed in
a solution containing hydrogen ions at unit
activity. The half-cell reaction for the SHE is
given by
• 2H+ (aq) + 2 e- H2 (g)
• and the half-cell potential arbitrarily assigned a
value of zero (E0 = 0.000 V).
• Pt(s), H2 (g,f H2 =1.00) | H+(aq,aH =1.00) ||

• Advantages:
1.Entire pH range
2.Free from salt error
3.Fundamental electrode ( all pH
measurements)
• Disadvantages:
1.Difficult to get pure H2
2. Difficult maintain pressure 1 atm.
3.Not useful in presence of oxygen

1. Saturated Calomel Electrode (SCE)
The SCE is a half cell composed of mercurous chloride
(Hg2Cl2, calomel) in contact with a mercury pool.
These components are either layered under a saturated
solution of potassium chloride (KCl) or within a
compartment surrounded by the saturated KCl solution
(called a double-junction arrangement).
A platinum wire is generally used to allow contact to the
external circuit. The half reaction is described by
Hg2Cl2 (s) + 2 e- 2 Hg (l ) + 2 Cl- (sat’d)
with an E0 value of +0.244 V. A common arrangement for
the SCE is shown below, left side.
In this arrangement, a paste is prepared of the calomel
and solution that is saturated with KCl.

What is the difference between Voltage and
EMF?
• 1. EMF is the voltage generated by a source
like battery or generator.
• 2. We can measure voltage between any two
points, but EMF exists only between the two
ends of a source.
• 3. Voltages in a circuit called ‘voltage drops’
are in the opposite direction of EMF and their
sum is equal to EMF according to Kirchhoff’s
second law.

Hg| Hg2Cl2 | (sat’d) ,KCl(x M)||.
Bell Jar Type:
• Mercury placed at the bottom wide mouth
bottle/jar
• Above this layer saturated KCl.
• Platinum wire introduced through cork
Side Arm Test Type:
• Side arm bearing stop cork
• Mercury placed at the bottom
• Above this layer calomel paste saturated KCl.

Deep in test tube Type:
• Two test tubes, outer tube is 15 cm long & 2
cm diameter
• With opening for filling KCl & porous fiber
plug
• Inner tube is filled with mercury mercurous
chloride & having metal wire connected
through mercury.

• Advantages:
1.Entire pH range
2.Mercury ion reacts with fewer samples as
compared to silver- silver electrode.
3.Sturdy electrode
• Disadvantages:
1.Temperature coefficient is large
2. Take longer time to established
3.Chloride ion solution show incompatibility.

2. Silver/Silver Chloride (Ag/AgCl)
• The silver/silver chloride reference electrode is
composed of a silver wire, sometimes coated
with a layer of solid silver chloride, immersed
in a solution that is saturated with potassium
chloride and silver chloride. The pertinent half
reaction is
• AgCl (s) + e - Ag (s) + Cl- (sat’d)

• Ag| AgCl , (sat’d) ,KCl(x M)||.
• Advantages:
1.Easy to manufacture
2.Showes potential rapidly & it attains
equilibrium between -300C & 1350 C & is
reproducible.
3.Stable & accurate even wide temperature
fluctuation
• Disadvantages:
1. It react with samples

Indicator Electrode
• Metallic Indicator Electrode.-
• i) Electrode of First Kind:
An indicator electrode of first kind is pure metal
electrode that is in direct equilibrium with its cation
in the solution.
Eg. Some metals such as silver,
mercury,copper,cadmium,Zinc & lead act as
indicator electrodes with respect to their own ions.
A single electrode potential for silver wire dipping in
to a solution of silver salts varies with the activity of
silver ions according Nernst equation.
Ag ++ e - Ag
Ecell=E0−(0.05916/n)log(1/aAg
+)

• ii) Electrode of Second Kind:
• It as indicator electrode also respond activites
of anion that form sparingly soluble precipates
or soluble stable complexes with such cations.
• Eg.Silver-silver chloride
• Half cell reaction-
• AgCl + e - Ag(s) + Cl-
• Ecell=E0Agcl/Ag−(0.0592/n)logaCl
-

• ii) Electrode of Third Kind:
• These are metal electrode which are used to
measure activites of ions for which no
electrode of the first kind exist.
• Eg.mercury serves as an indicator electrode of
second kind for EDTA anion Y 4-
• When a small amount of Hg Y 2- mercury
chelate is added to solution containing Y 4- the
half reaction at a mercury electrode is
• Hg Y 2- +2 e - Hg(l) + Y 4-

• Inert or Redox Electrodes:-
• These electrodes such as gold,platinum,or
carbon, immersed in a solution containing
both the oxidized & reduced states of
homogenous & reversible
• Eg.pt electrodes in the solution of iron(III) &
iron(ii) ions.
• Half cell reaction-
• Fe+3+ e - Fe+2
• Ecell=E0 Fe+3 / Fe+2 −(0.0592/n)log[Fe+2 /Fe+3 ]

Ion selective Electrode:-
• Ion selective electrode have several advantages
over conventional methods of determining
ionic concentration:-
• They do not affect the solution being analyzed.
• They are portable, not too expensive.
• They are suitable for direct determination of
ion activites & can be used as sensor in
titrations.

Ion Exchange Process
In the following ion exchange process, a lithium
cation displaces a potassium cation from the
organic anion, R-:
KR + Li+ ⇋ LiR + K+
We can imbed the lipophilic R- in a membrane, as
shown in Figure, and place it in a solution of Li+
KR(mem) + Li+(aq) ⇋ LiR(mem) + K+(aq)

Ion Selective Electrode are:-
• Glass Membrane Electrode
• Solid-State Electrode
• Precipitate Electrode
• Liquid-liquid Electrode

Glass Electrode(pH electrode)
• This electrode most widely used for
determination of hydrogen. Also H+, Na+,
Ag+&Li
• Three sub type of glass electrode-
1.pH type.
2.Cation Sensitive type
3.Sodium sensitive type

• Glass Membrane Structure:SiO4-
framework with charge balancing cations-
SiO2-72 %, Na2O- 22 %, CaO-6 %

The pH Electrode
• Alkaline error – sensitive to alkali metal ions at
pH greater than 12.
• Acid error – at pH less than 0.5, values
obtained with the pH electrode are high.
• Dehydration – must keep membrane moist.
• Errors in low ionic strength – varying junction
potentials.
• Errors in pH of standard buffer solutions

1.Thick walled glass tubing, providing very high
electrical resistance, is attached at its bottom to
glass bulb.
2.A glass bulb, used to respond to changes in H+ ion
concentration. It is made up of a thin wall or a
membrane from oxide of sodium,calcium&silicon.
The glass bulb is filled with 0.1M HCl.
3.Asilver wire, coated with silver chloride, is dipped
in 0.1M HCl inside the bulb so as to make an
electrical contact. It also act as a reference
electrode. The potential of this reference electrode
is not affected by changes in the pH of a solution.

Applications
• Glass electrode is commonly used to find the
pH of solutions.
• It is also used as an indicator electrode for
titrations involving neutralization.

Advantages:-
• Response slight change in the pH of the test solution
is very fast.
• It is unaffected by oxidizable & reducible substances
also colored liquids, gases or the presence of
moderate salt except sodium salts
• Lithium & silica glass membrane can be used wider
range of pH.
• A combination of glass electrode & ref.electrode in
one single unit has increased the ease of handling
the system.

Disadvantages
• Glass electrode is fragile & hence, needs
careful handling.
• Scratches on the glass bulb alter the response
from the glass electrode.
• It is not used for dehydrating agents &
colloidal substances.
• Glass electrode provides a very high internal
resistance & hence, is not useful in simple
potentiometers

• The cation-exchanger is an aliphatic diester of
phosphoric acid, (RO)2PO2-, where each R group
is an aliphatic hydrocarbon chain containing
between 8 and 16 carbons. The phosphate group
can be protonated, but has a strong affinity for
Ca2+. The cation exchanger is dissolved in an
organic solvent and held in a porous compartment
between the analyte solution and internal
reference calcium chloride solution. The ion-
exchanger uptakes Ca2+ into the membrane by the
following mechanism, forming a complex with
the structure shown in Figure
Liquid Membrane Electrodes

Liquid Membrane Electrodes
Ca2+ (aqueous) + 2(RO)2PO2- (organic) ⇋
[(RO)2PO2]2Ca (organic)
used for calcium determination

Gas-sensing electrodes
• Combined electrodes have been
developed to measure dissolved gases
such as
• carbon dioxide CO2, ammonia NH3 and
sulfur dioxide SO2. These chemical
• sensors contain an internal solution that is
isolated from the sample solution
• by an hydrophobic membrane which is
permeable to dissolved gas molecules

• For the three gases given above, the selective
electrodes include
• a glass electrode in contact with a bicarbonate
solution of low concentration(0.01 M).
• The bicarbonate solution is separated from
the sample solution by a polymer membrane
that allows diffusion of the gas analyte
towards the inner electrode compartment.

• This diffusion in the bicarbonate brings
about a change in pH in the proximity
of the membrane. For other gases, such as
hydrogen cyanide (HCN), fluorhydric
• acid (HF) or hydrogen sulfide H2S, the
signal is generated by the modification
• of the concentration of anions close to the
internal wall of the membrane.

Calibrating the pH electrode:
Obtain two buffer solutions, one each of buffer pH 4
and buffer pH 7.
Place the pH probe into the pH 4 buffer solution. Go
to the menu and under the Experiment open
Calibrate.
Click Calibrate now. The electrode will be reading a
voltage– when this value has stabilized, type in the
pH of your buffer (4.0) into the “Enter Value” box
under Reading 1 and then click Keep.

Rinse your electrode and place it into the pH 7 buffer
solution. When the voltage has stabilized again, enter
that pH value (7.0) in the box under Reading 2 and click
keep. Click done to return to the graph
and data table screen. Rinse the electrode and place the
probe back into the pH 4 buffer to make
sure it is stable. If the electrode is properly calibrated,
the pH of the solution should read “4.00”
± 0.05 units.

• Submerse the electrode into the pH 7 buffer
Press the calibrate button.

Wait until the pH icon stops flashing and press the
calibrate button again.

• Quininydrone Electrode:-
• This electrode was introduced by E.Billman in
1921.
• By this electrode a rapid & easy determination
of pH is possible
• Quinhydrone is a 1:1 molar compound of
quinone & hydroquinone & in solution it
provides equimolecular quantities of these
two substances.
• C6H4O2.C6H4(OH)2 C6H4O2 +C6H4(OH)2
Quinhydrone quinone hydroquinone

• C6H4O2+ 2H++ 2e- C6H4(OH)2
• Quinone(Q) hydroquinone(QH2)
• E =E0-2.303RT log[QH2] (:.n=2)
2F [Q] [H+] 2
=E0-2.303RT log[QH2]+ E0- 2.303RT log[H+] 2
2F [Q] 2F
[QH2]=[Q] in aqueous solution of quinhydrone
log[QH2]
[Q] =log1=0 = E0-2.303RT x log[H+]
2F
=0.591pH at 250 C

• Advantages-
• It has a low resistance
• Equilibrium reached quickly
• Its use is not affected by dissolved oxygen
• It can be used for micro-determinations
• Disadvantages-
• It can be used for determining pH values less than 8
only
• The solution to be tested gets contaminated
• Salt error defect
• Can not be used in presence of oxidising &reducing
agents

Potentiometric measurements
• In Potentiometric methods two methods of
measurements exists.
• A) Measurement of electrode potential from
which the concentration of an active ion may be
found.
• B) Measurement of changes in E.M.F.(electro
motive force) brought about by the addition of
the titrant could be made.
• Thus the E.M.F. of the cell can be calculated as
follows.
• E cell = E reference + E indicator + E function

• In Potentiometric titrations, the change in the
electrode potential upon the addition of the
titrant are noted against the volume of the
titrant & added.
• At the end point the rate of the change of the
potential would be the maximum.
• The end point is found by plotting a curve of
potential versus the volume of the titrant.

Types of Potentiometric titrations
• Acid base titrations,
• Redox titrations and
• Precipitation titrations.
• Acid base titrations:-
• Neutralization of acids & bases is accompanied by
the changes in the concentration of H+ & OH- ions.
• The electrodes used are hydrogen electrode & N-
calomel electrode (Reference electrode).
• The glass electrode is usually used in conjunction
with a calomel electrode that makes contact with the
test solution through a salt bridge.

• A known volume of the acid to be titrated is
kept in a beaker fitted with a stirrer and a
standard hydrogen electrode.
• It is connected to normal calomel electrode
through a salt bridge.
• Both the electrodes are connected to a
potentiometer which records the E.M.F. of the
solution.
• After each addition of base from the burette,
the E.M.F is measured & a graph is plotted
(fig.)

• The potential of any hydrogen electrode is given by
E = E0 + 0.0591 pH.
• It is clear that the change in electrode potential or E.M.F. of the cell is
proportional to the change in pH during titration.
• The point where the E.M.F. increases rapidly gives the end point.
• A more satisfactory method to find the end point is to plot ÄE/ÄV
against V.
• The maximum value gives the end point of the titration (fig.)

• Redox Titrations
• Redox reactions can be followed by an inert
indicator electrode. The electrode
• assumes a potential proportional to the
concentration of the reactant or the titrant.
• They involve the transfer of electron from the
substances being oxidized to the substance being
reduced.
• Ce+4+ Fe+2 Ce+3+ Fe+3
• Such titrations may be used for monitoring of
cyanide wastes from metal plating industries, in
water pollution, sewage treatment, agricultural
and in biochemical studies.

• Precipitation titrations
• Any such titration that involves insoluble salts
of metals such as mercury, silver lead and
copper may be followed potentiometrically.
The indicator electrode may be made of the
metal involved in the reaction.
• The magnitude of potential change at the end
point depends on the solubility of the
substance being precipitated. The titration of
Cl- ions with a stand and solution of AgNo3
using a silver metal indicator electrode is an
example of a precipitation titration.

Advantages of Potentiometric titrations
1. The method is suitable for the analysis of
dilute solutions too.
2. It could be applied for colored solutions also.
3. The interpretation of titration curves is easier.
4. The apparatus used is inexpensive, reliable
and readily available.
5. Different components of different
characteristic colour could be titrated at a same
time.

Applications of Potentiometric titrations:-
• Assay of Carbidopa
• Assay of Clonidine Hydrochloride
• Assay of -
Bendrofluazide,cimetidin,Ethinyloestradiol,
Lomustine etc.
• Compexometric titrations.

2 potentiometry

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