pH and Potentiometry, Potentiometric titrations, Electrodes used in Potentiometry, Standard Hydrougen electrode, calamel electrode, silver silver chloride electrode, glass electrode

1. Potentiometry
Deokate U. A.
Copyright © by U. A. Deokate, all
rights reserved.

2. Points to be covered
• Electrochemical cell, construction and
• Working of reference (Standard hydrogen,
silver chloride electrode and
• calomel electrode) and indicator electrodes
(metal electrodes and glass electrode)
• methods to determine end point of
potentiometric titration and
• applications
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3. Electroanalytical methods
• involve the measurement of either the electrical
current flowing between a pair of electrodes immersed
in the solution tested (voltammetric and
amperometric methods) or
• An electrical potential developed between a pair of
electrodes immersed in the solution tested
(potentiometric methods).
• In either case, the measured parameter (current or
potential) is proportional to the concentration of
analyte
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Electro analytical chemistry
• Electro analytical chemistry encompasses a
group of quantitative analytical methods
that are based upon the electrical
properties of an analyte solution when it is
made part of an electrochemical cell.
• These methods make possible the
determination of a particular oxidation
state of an element.

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Advantages of electro analytical tech.
• Electrochemical measurements are specific for a
particular oxidation state of element, so determination
of con from a individual species from mixture is
possible.
• Instruments are relatively inexpensive as compare to
other
• They provide information about activates rather than
conc. of chemical specifies
• High degree of sensitivity, selectivity, and accuracy.
• Concentration less than 10-10 moles can be determine
using these methods
• Automation is possible for these methods.

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Electro Chemical Methods
• pH metry & Potentiometry
• Conductometry
• Voltametry
• Polarography
• Amparometery
• Coulombmetry

7. Potentiometry Principle:
• The principle involved
in the Potentiometry
is when the pair of
electrodes is placed in
the sample solution it
shows the potential
difference by the
addition of the titrant
or by the change in
the concentration of
the ions
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Electro chemical Cell
• A chemical cell is system in which chemical
energy is transferred in to electrical energy.
• Each cell is made up of 2 electrodes one
liberating e- is called oxidizing electrode
(anode) while the other absorb e- is called
reducing electrode (cathode)

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Types of electrochemical cells:
• There are two types of electrochemical cells:
1. Galvanic or Voltaic cell (ones that
spontaneously produce electrical energy) and
2. Electrolytic (ones that consume electrical
energy).

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Galvanic cell

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An electrolytic cell

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Electrodes used in Potentiometry
•Reference Electrodes: the electrode whose
potential is independent of nature and
composition of surrounding
•Indicator Electrodes: Electrode whose potential
depends on Con/ activity of one component in
solution.

13. • The reference electrode is the electrode which contains of
its own potential value and it is stable when dipped into
sample solution.
• The salt bridge is used to prevent the interference of the
analyte solution with that of reference solution.
• Here analyte solution is the solution whose potential is to
be measured.
• The indicator electrode is the electrode which responds to
change in the potential of analyte solution
• The electromotive force of the complete cell is given by the
following equation:
Ecell = Ereference + Eindicator + Ejunction
• where E reference is the electromotive force of the
reference electrode ,E indicator is electromotive force of
indicator electrode, E junction is the electromotive force at
the junction of the liquid.
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14. Theory:
• When the known potential electrode immersed in
the sample solution then the potential is given by
Nernst equation:
E= E0 +(0.592/n) log c
• Where E is the potential of the solution; E0 is the
standard electrode potential; n is the valency of
the ions; c is the concentration of the sample
solution;
• 0.592 is the value obtained from the RT/F; where
R is the gas constant, T is the temperature in
Kelvin, F is the faradays constant.
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15. Reference Electrodes
• The reference electrodes are classified into
two main classes they are as follows:
• Primary standard electrodes ex: Standard
hydrogen electrode
• Secondary standard electrodes ex: silver-silver
chloride electrode, saturated calomel
electrode
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Characteristics of Ideal Reference Electrode:
1. Reversible and follow Nernst equation
2. Potential should be constant with time
3. Should return to original potential after being subjected to
small currents
4. Little effect with temperature cycling
5. Should behave as ideal nonpolarized electrode
6. Made of some stable chemical species, is readily available
and usually simple to use.
7. Should be non-toxic if you are going to use it in a biological
system.
8. Should be rugged and portable if you are going to use it in the
field

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Standard Hydrogen reference electrode
• This is the reference half cell against which all others are
compared.
• It must be easy to construct, reversible and be highly
reproducible
• The standard hydrogen electrode (SHE) meets these
specifications
• It consists of a piece of platinum coated with finely
divided platinum black to increase its specific area.
• It is immersed in aq acid solution and hydrogen is
bubbled

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The Standard Hydrogen Electrode (SHE)
• The standard H2
electrode potential is
defined as the
potential that is
developed between
the H2 gas adsorbed on
the pt metal and H+ of
the solution when the
H2 gas at a pressure of
760 mm of Hg is in
equilibrium with H+ of
unit concentration

19. Limitations SHE
• It is rather difficult to regulate the pressure of the H2
gas to be at exactly 1atm throughout the experiment.
• If the solution contains any oxidizing agent, the H2
electrode cannot be used.
• Excess of H2 bubbling out carries little HCl with it and
hence the H+ concentration decreases. In such a
system, it is difficult to maintain the concentration of
HCl at 1M.
• Platinum foil gets easily poisoned by the impurities
present in the gas and HCl. In fact, the attainment of
equilibrium is ensured by trial and error.
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Ag/AgCl
• Constructed of a silver wire, coated with silver chloride, in a
solution containing silver chloride
– Sometimes AgCl is in solution saturated with KCl
– Saturated KCl keeps the chloride activity constant and
helps keep the electrode stable
log!tenbasewithlnreplaceweIF
C25@0.05916
nF
RT
][Clln
nF
RT
EE
S.H.E)to(relativemV197KCl)(sat.E
S.H.E)to(relativemV222E
ClAgeAgCl
-o
o
(aq)
-
(s)
-
(s)





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Calomel (S.C.E)
• Calomel is mercury (I) chloride
• One of the most common reference
electrodes
2-0
o
(aq)
-
(l)
-
(s)22
][Clln
nF
RT
EE
S.H.E)to(relativemV241KCl)(sat.E
S.H.E)to(relativemV268E
Cl2Hg2e2ClHg





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Calomel electrode

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Indicator Electrodes
• Used for indicating the
potential caused by some
chemical species as
compared to the
reference electrode.
• Usually connected to the
+ (cathode) side of the
potentiometer
• Made of a variety of
often allegedly, but never
actually, inert materials.
• Metal Electrodes
– Electrode of first kind
– Electrode of Second kind
– Electrode of third kind
– Electrode of redox type
• Platinum
• Ion Selective (membrane)
– Glass membrane
– Crystalline
• Solid State
– Liquid Membrane
• Ecell=Eindicator-Ereference

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Metallic electrode
• 1st kind
– Used for determination of activity of cation
derived from electrode metal
– respond directly to changing activity of
electrode ion
– Direct equilibrium with solution
– Not very selective
– simple
• Eg: Ag, Hg, Cu, Zn, Cd, Bi, Tl, Pb

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2nd kind
• A metal which can often made responsive to
activity of ion to which it form ppt or stable
complex of ion
– Ag for halides
– Ag wire in AgCl saturated surface
• Complexes with organic ligands
– EDTA

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Indictor electrodes
• 3rd kind
– Electrode responds to different cation
– Competition with ligand complex
– Eg. Hg is used for determination of calcium
• Inert metals (Redox electrode)
– Pt, Au, Pd
• Electron source or sink
• Redox of metal ion evaluated
– May not be reversible

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Typical Metal
Electrode
Arrangement

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Membrane Indicator electrodes
– Non-crystalline membranes:
• Glass - silicate glasses for H+, Na+
• Liquid - liquid ion exchanger for Ca2+
• Immobilized liquid - liquid/PVC matrix for Ca2+ and NO3-
– Crystalline membranes:
• Single crystal - LaF3 for F Polycrystalline
• or mixed crystal - AgS for S2- and Ag+
• Properties
– Low solubility - solids, semi-solids and polymers
– Some electrical conductivity - often by doping
– Selectivity - part of membrane binds/reacts with analyte

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pH (Glass Membrane) Electrodes
• One of the simpler ion-selective electrodes (ISE)
• Hydrogen Ion imparts a charge across a
hydrated glass membrane
• Generally include an internal reference
electrode (Ag/AgCl) and a separate Ag/AgCl
electrode for sensing the charge imparted by
the hydrogen ions
• Not as simple to use as you think!

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Glass Membrane Electrode

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34. Advantages & Disadvantages
• ADVANTAGES :
– Response is very rapid
– Chemically resistant to oxidizing & reducing agents,
dissolved gases, salts etc.
– When Lithia -silica glasses are used, it can be used over the
entire pH range.
• DISADVANTAGES
– It is extremely fragile
– Minute abrasions on the surface of the tip, damages the
electrode
– It cannot be used with simple potentiometers, because of
the high resistance.
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35. Types of Potentiometric titrations:
• Acid-base titration
• Redox titration
• Complexometric titration
• Precipitation titration
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36. APPLICATIONS
• Clinical chemistry: Ion selective electrodes are present sensors for
clinical samples because of their selectivity for analyte in complex
matrices. The most common analytes are electrolytes such as Na, k
,Ca ,H, and Cl and dissolved gases such as CO2
• Environmental chemistry: For analysis of CN- ,NH3, NO3, F3 in
water and waste water.
• Potentiometric titrations: For determining the equivalence point of
an acid base titration.
• possible for redox, precipitation, acid-base, complexation as well as
for all titrations in aqueous n non aqueous solvents.
• Agriculture: NO3 ,NH4 ,I ,Ca, K ,CN, Cl in soils, plant materials, feed
stuffs, fertilizers.
• Detergent manufacturing: Ca, Ba, F for studying effects in water
quality.
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37. Applications
• Salt content of meat fish dairy products fruit juices
brewing solutions
• Ca in dairy products and beer
• K in fruit juice and wine making
• Corrosive effects of NO3 in canned foods
• F in drinking water and other drinks
• NO3 and NO2 in meat preservatives
• Assay of bisacodyl suppositories and assay of sulpha
drugs can be estimated potentiometrically.
• Food processing: Automatic potentiometer
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