1. 1
Mrs. Prajakta B. Kothawade
Assistant Professor,
PES Modern College of Pharmacy, for ladies, Moshi, Pune
2. Introduction to Potentiometry
Different electrode systems are used in combination to measure
potential or pH of a solution. A pair of electrode is commonly
required in measurement. One electrode acts as an indicator
electrode while other serves as reference electrode.
DEFINATION OF pH
It is negative logarithm of hydrogen ion concentration.
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3. Introduction to Potentiometry
• When a metal is immersed in a solution containing its own ion,
then an electrode potential is established. the value of this electrode
potential is given by the Nearnst equation.
• The procedure of using a single measurement of electrode potential
to determine the concentration of ionic species in the solution is
called direct potentiometry.
• The electrode whose potential depends on concentration of the ion
to be determined is called as indicator electrode.
3
4. Introduction to Potentiometry
when the ion determined is directly involved in the electrode
reaction, the electrode is an electrode of First kind. This is case for
metal M immersed in a solution of Mn+ ions.
It is also possible in appropriate cases to measure by direct
potentiometry the concentration of ion which is not directly
concerned in the electrode reaction. This involves use of Second
kind electrode.
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5. Determination of PH
Method of PH indicator – Organic substances which shows different colours
below and above a small PH range. E.g. Phenolphthalein.
Potentiometric method –
i) Std Hydrogen Electrode –
Instrumentation
Advantages
Disadvantages
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6. Calibration of pH meter
Switch on and allow the instrument to warm up.
Adjust the temperature control
Insert the electrode assembly into the same beaker and if
available, set the selector switch of the instrument to read
pH.
Adjust the “set buffer” control until the meter reading
does not agree with known pH of the buffer solution.
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7. Calibration of pH meter
Remove the electrode assembly rinse in distilled water and place
into a small beaker containing a little of the second buffer
solution. If the meter reading does not agree exactly with the
known pH, adjust the slope control until required reading is
obtained.
Remove the electrode assembly, rinse in distilled water, place in
the first buffer solution and confirm that correct pH reading is
shown on the meter, if not repeat the calibration process.
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8. Calibration of pH meter
If the calibration is satisfactory rinse the electrode with
distilled water and introduce in the test solution. read off
the pH of the solution.
Remove the electrode rinse in distilled water and leave
standing in distilled water .
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10. Measurement of Potential
When a metal rod is immersed in a solution of its own ions a
potential is established between the metal rod and its ions in
solution for example zinc rod dipped in zinc sulphate
solution.
the potential is expressed by Nernst’s equation.
In order to determine potential of metal electrode it is
necessary to have another electrode whose potential is
accurately known.
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12. REFERENCE ELECTRODE
In most electroanalytic application, it is desirable that the
half-cell potential of one electrode be known, constant,
&completely insensitive to the composition of the solution
under study. An electrode that fit this description is called a
reference electrode.
1)Is reversible & obeys the Nernst equation.
2)Exhibit the potential that is constant with time.
3)Returns to its original potential after being subjected to
small currents. 12
13. CALOMEL ELECTRODE
Calomel reference electrodes consist of mercury in contact with a
solution that is saturated with mercury chloride (calomel) & that
also contains a known conc. Of KCL.
Calomel half cell can be represented as folllows
Hg/Hg2cl2 (satd),KCL(xM)//
X=molar conc. of KCL
The electrode potential for this half –cell is determined by the
reaction;
Hg2CL2(s)+2e- =2Hg(l)+2cl-
It also depend on the cl conc.
The potential of the saturated calomel electrode at 25 c is
0.2444V
The body of each electrode consist of an outer glass tube that is
5-15cm in length & 0.5-1cm in diameter.
It I
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15. What is happening?
As the hydrogen gas flows over the porous platinum, an
equilibrium is set up between hydrogen molecules and
hydrogen ions in solution. The reaction is catalyzed by the
platinum.
This is the equilibrium that we are going to compare all
the others with.
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17. 17
Cells and half cells
The whole of this set-up is described as a cell. It is a simple system
which generates a voltage. Each of the two beakers and their contents
are described as half cells.
The salt bridge
The salt bridge is included to complete the electrical circuit but without
introducing any more bits of metal into the system. It is just a glass tube
filled with an electrolyte like potassium nitrate solution. The ends are
"stoppered" by bits of cotton wool. This stops too much mixing of the
contents of the salt bridge with the contents of the two beakers.
The electrolyte in the salt bridge is chosen so that it doesn't react with
the contents of either beaker.
18. Advantages & disadvantages
Advantages
1)Measurement of PH
2)It can be used over a entire
pH range.
3)It gives no salt error.
Disadvantages
1)It cannot be used in
presence of air,
dissolved oxidizing or
reducing agent.
2)Platinum black coating
deteriorates.(poisoned)
3)It is difficult to
maintain pressure.
4)It is not easy to get pure
H2.
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19. SILVER/ SILVER CHLORIDE ELECTRODE
The most widely marketed reference electrode system consist of a
silver electrode immersed in a solution of KCL that has been
saturated with silver chloride.
Ag/Ag2cl2(satd),KCL(Xm)//
The electrode potential for this half –cell is determined by the
reaction;
AgCl (s)+ e- =Ag(S) + Cl-
Model is similar to that of calomel electrode.
Advantage ; used at temp. greater than 60 c.(calomel cannot)
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20. GLASS ELECTRODE
If a glass electrode has two solutions of different hydrogen ion
concentration.
A potential difference will be set up across the membrane.
Under certain conditions this P.D. is determined by the difference in pH of the
two solutions so that if the pH of one is known, that of the other can be
calculated. This method of measuring pH is especially well adapted to many
biological purposes.
..The glass electrode is as accurate as the hydrogen electrode (within the PH
range of biological significance), as rapidly operated as the quinhydrone
electrode, and more widely applicable than either of the other two.
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21. 1. The Electrode.-The electrode is made of ordinary soft soda
glass. The glass we used was supplied by the Kimball Glass
Company of Vineland, New Jersey. It has the following analysis:
per cent Silica. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 69.7
Alumina, iron oxide, etc.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.0
Zinc oxide (with small amount of manganous oxide). . . . . . . . . 1.5
Calcium oxide.............................................. 6.0
Magnesia . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . 3.8
Boric anhydride.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 0.8
Sodium oxide (including small amount of potassium oxide).. 15.7
A borosilicate glass, such as Pyrex, cannot be used. Hughes (3)has recently
tested a variety of soda glasses and finds that those
of low alumina content are most suitable.
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22. The hydrogen ion concentration of the glass phase remains
constant.
Recently Hughes (3) has pointed out that the hydrogen ion
concentration in the glass phase may be held relatively constant
by the buffer action of the glass which is a mixture of the salt of a
weak acid (Na2Sio3) with the anhydride of that acid (excess
SiO2).
At about pH 9 the divergence in values of pH between the glass
and hydrogen electrodes becomes noticeable and it is here that
the buffer action of the glass would be less effective since the
apparent first dissociation constant of H2Si03 is about 10mg.
it is at pH 13 that the divergence between the glass and hydrogen
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24. Advantages & disadvantages
Advantages
1) It may be used in
presence of strong
oxidizing or reducing
agent in viscous media&
in presence of protein.
2) It can be used for the
solution having pH range
2-10.
3)It is simple to operate.
4)The equilibrium is reached
quickly.
Disadvantages
1)The bulb is very fragile & therefore
has to be used with care.
2)As the glass membrane has a very
high electric resistance therefore
electric potentiometer is used.
3)Standardization has to be carried out
frequently.
4)It can’t be employed in ethanol
,acetic acid & gelatin.
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25. DIFFERENT ELECTRODES USED IN POTENTIOMETRIC
TITRATION
WHAT IS ELECTRODE
A solid electric conductor through which an electric current enters or leaves an
electrolytic cell or other medium.
OR
A collector or emitter of electric charge or of electric-charge carriers, as in a
semiconducting device.
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26. METALLIC INDICATOR ELECTRODES .
Types
1)Electrodes of first kind ; they are in direct equilibrium with the
cation derived from the electrode metal.
Cu2+ + 2e- =Cu(s )
2)Electrodes of second kind; a metal electrode can often be
made responsive to the activity of an anion with its ion forms
a precipitate or stable complex ion.
AgCl( s) + e- = Ag( s) +Cl- .
E*=0.222V.
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27. 3)Electrodes of third kind
A metal electrode can ,under some circumstances, be
made to respond a different cation.
e.g. ;a mercury electrode has been used for the determination
of the pCa of calcium-containing solutions.
4)Metallic redox electrode; electrodes fashioned from
platinum, palladium, or other than metals often serve as
indicator electrodes for oxidation / reduction system.
The inert electrode act as a source.
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28. ION-SELECTIVE MEMBRANE ELECTRODES
Crystalline
1 .)Single Crystal
e.g. ; LaF3
for F-
2)Polycrystalline
or mixed
e.g.Ag2S
for S2- & Ag+
NonCrystalline
1)Glass
e.g. silicate glasses for
Na+ & H+
2)Liquid
e.g. Liquid ion
exchangers for Ca+ &
neural carriers for K+
3) Immobilized liquid in a
rigid polymer
e.g. PVC matrix for Ca+ &
No3-
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29. 29
Compound or multiple membrane (multilayer) ion-selective electrodes
1. Gas sensing electrode is a sensor composed of an ion-selective electrode
(indicator
electrode) and a reference electrode in contact with a thin film of solution which
is separated from the bulk of the sample solution by a gas-permeable membrane
or an air gap. This intermediate solution interacts with the gaseous species
under test (penetrated through the membrane or an air gap) which results in a
change of a measured constituent (e.g., the H+ activity) of the intermediate
solution.
This change is then sensed by the ion-selective electrode and is related to the
partial pressure of the gaseous species in the sample. (Note: In electrochemical
literature the term gas electrode is used for the classical, redox-equilibrium-based
gas electrodes as well, such as the hydrogen or the chlorine gas electrodes [Pt(s)
H2(g) H+(aq)] or [Pt(s) Cl2(g) Cl-(aq)] (See section 8.3.1.). These electrodes
respond both to the partial pressure of the gas (H2 or Cl2) and to the ionic
activities (H+ or Cl-). The Clark oxygen electrode fits under this classification
although, in contrast to other gas sensors, it is an amperometric and not a
potentiometric sensor device.
30. 30
2. Enzyme substrate electrode is a sensors in which an ion-
selective electrode is
covered with a coating containing an enzyme which reacts
with an organic
substance (substrate) resulting in a species to which the
ion-selective electrode
responds. Alternatively, the sensor could be covered with a
layer of substrate
which reacts with the enzyme, co-factor, or inhibitor to be
assayed.
31. ANTIMONY-ANTIMONY OXIDE
ELECTRODE
This electrode also operates as an reversible to
hydrogen ion conc. It consists of a rod of
antimony placed in a solution having hydrogen
ions. The antimony surface is cleaned & polished
with fine emery paper. Antimony takes up a thin
adhering layer of Sb2O3 almost instantaneously
when exposed to air. This oxide is not dissolved by
moderate acid conc.
Sb2O3 + H2O = 2Sb3+ + 6OH-
The electrode reaction for antimony can be written
as
Sb3+ + 3e- = Sb0
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32. Advantages & disadvantages
Advantages
1)It is used for viscous
solution.
2) It can be used for
determining pH in
range of 4-12.
3)It has a low
resistance.
4)It does not
contaminate the
solution to be tested.
Disadvantages
1) It can’t be used in
presence of
dissolved oxygen,
oxidizing agent, H2S,
heavy metal ions ,
highly acidic &
alkaline solution.
2)It is sensitive to
temperature
changes.
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33. QUINHYDRONE ELECTRODE
The use of this electrode a rapid & easy determination of
pH is possible.
QUINHYDRONE is a 1:1 molar compound of quinone &
hydroquinone.
C6H4O2.C6H4( OH)2 C6H4O2 + C6H4( OH)2
quinhydrone quinone hydroquinone
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34. Advantages & disadvantages
Advantages
1)It has a low
resistance.
2)Equilibrium is
reached quickly.
3)Its use is not
affected by oxygen.
4) ) It can be used for
micro
determination.
Disadvantages
1)It can be used for determining pH
values less than 8 only.
2)The solution to be tested get
contaminated.
3) ) It can’t be used in presence of
oxidizing & reducing agent.
4)It is not stable for long time ,
particularly above 30 *C.
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35. Application of Potentiometer
Applications Potentiometric Titration
Acid Base Titration
Complexometric Titration
Redox Titration
Precipitation Titration
Non-aqueous Titration
Types of Potentiometric Titration curve
Plot of EMF vs. ml of NaOH
Plot of ΔE / ΔV vs. ml of NaOH
Plot of Δ2E / Δ2V vs. ml of NaOH
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36. 36
References
Fundamentals of Analytical Chemistry by Skoog, West, Holler, Harvest,
8/Ed., Thomson Brookslcole, page no: 591-618.
A Textbook of Pharmaceutical Analysis by Connors KA, 4/ed., John Wiley &
Sons, page no: 111-119.
Pharmaceutical Analysis Vol. I & K. R. Mahadik, S.G. Wadodkar, H. N, I.
More, Nirali Prakashan, page no: 197-213.
Instrumental methods of analysis, G.R. Chatwal, page no: 2.504-2.522.
