Potentiometry, working, electrode, ion selective electrods, modern pharmaceutical analytical techniques

1. POTENTIOMETRY
PREPARED BY:
Thakur Karanvir Singh)

2. Content
 Definition
 Principle
 Ion selective electrodes
 General method
 Application of potentiometry
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3. Definition
 Potentiometry is the field of electro analytical
chemistry in which potential is measured under the
condition of no current flow.
 It is a method of analysis in which we determine the
concentration of solute in solution, and the potential
difference between two electrode.
 Potentiometry is an instrument that used for the
determination of unknown concentration between the
reference solution and the sample.
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4. 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 titrant.
 Metal atoms may dissolve in the solution as
positive ions leaving electrons on the electrode.
 Metal ions may take up electrons from electrode
and get deposited as neutral atoms
In this way A POTENTIAL DIFFERENCE is
setup b/w electrode and solution
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5. REFERENCE ELECTRODE
 A reference electrode is a half-cell having a known potential.
 Has constant potential.
 Example: STANDARD HYDROGEN ELECTRODE (S.H.E),
SILVER-SILVER CHLORIDE ELECTRODE
INDICATOR ELECTRODE
 An indicator electrode is another half cell with unknown potential.
 Immersed in analyte solution.
 Its potential is sensitive to the concentration of analyte.
 Potential is directly proportional to the ion concentration.
 EXAMPLE: METAL ELECTRODE, ION SELECTIVE
ELECTRODE (I.S.E).
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6. The electromotive force of the complete cell is given
by the following equation:
Ecell = Ereference + Eindicator + Ejunction
Ereference = is the electromotive force of the reference
electrode
Eindicator = is electromotive force of indicator electrode,
Ejunction = is the electromotive force at the junction of
the liquid
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7. Salt bridge
 It is a laboratory device used to connect two half
cells of galvanic or voltaic cell.
 Salt should be inert.
 It should not react with anode/cathode/solution.
 Salt should not be precipitate.
FUNCTION
 To complete the circuit of cell.
 To maintained electrical neutrality.
SALT USED: NACL, KCL, KNO3,
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9. Theory:
The main theory involved in the potentiometry is, 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.
Electrodes: These are mainly used to measure the voltages. Mainly
two electrodes are used in the potentiometry .They are as follows:
•Reference electrode
•Indicator electrode

10.  Reference electrode: These are mainly used for the
determination of the analyte by maintaining the fixed
potential.
Eg:
• Standard hydrogen electrode
• Silver silver chloride electrode
• Saturated calomel electrode
The reference electrodes are classified into two main
classes they are as follows:
Primary standard electrodes
Eg: Standard hydrogen electrode
Secondary standard electrodes
Eg: silver-silver chloride electrode
saturated calomel electrode

11. Reference electrode: Standard
hydrogen electrode
• It consists of a platinum wire in an
inverted glass tube.
• Hydrogen gas is passed through the
tube at 1atm.
• A platinum foil is attached at the end
of the wire.
• The electrode is immersed in 1M H⁺
ion solution at 25°C.
• The electrode potential of SHE is
ZERO at all temperatures.

12. Reference electrode: Saturated Calomel
Electrode
• It is commonly used reference electrode.
• It consists of a glass tube that contains
Hg at bottom covered with solid Hg₂Cl₂
and above this, the tube is filled with KCl
solution.
• A platinum wire is in touch with Hg and
it is used for electrical contact.
• The KCl solution inside tube can have
ionic contact with solution outside and
act as a salt bridge.
• The electrode potential of SCE is
+0.2422V.

13. Indicator electrode: It is used to measure the potential
of the analyte solution comparing with that of reference
electrode . Its potential is directly proportional to ion
concentration.
Eg:
• Hydrogen electrode.
• Glass electrode.
• Antimony –antimony oxide electrode.

14. Antimony-Antimony oxide electrode
 It consist of a antimony rod dipped into a solution
whose potential or pH has to be determined. Antimony
oxide is formed on exposure to air.
 REACTION
Sb + H2O SbO+ + 2H+ + 3e
• ADVANTAGES
1. It can be used from pH 3 to 8.It can be used upto pH
12.
2. It is not easily poisoned or damaged.
3. It can be used even with viscous fluids.

15.  DISADVANTAGES
1. This electrode cannot be used in the presence of
dissolved oxygen, oxidising agent, highly acidic or
alkaline conditions.
GLASS ELECTRODE
Glass electrode is the most widely used
indicator electrode. It is selective and
responsible for change in concentration
of hydrogen ions.

16.  ADVANTAGES
1. It gives rapid response.
2. It is chemically resist to oxidising and reducing
agents.
• DISADVANTAGES
1. It can be easily broken or damaged.
2. Minute abrasions on the surface of the tip, damage
the electrode.

17. Metal indicator electrodes: These develop electric potential in response to
redox reaction on the metal surface. Platinum or Au are used as metal indicator
electrodes. These are mainly classified into three types of electrodes used in the
potentiometry. They are as follows.
First kind electrodes: They are composed of the metal rod immersed in its metal
solution. These electrodes respond to the ionic activity of the electrode.
Ex: silver electrode dipped in the silver nitrate solution.
copper electrode dipped in the copper sulphate solution.
Second kind electrode: These are composed of the metal wires coated with the
salt precipitates. These electrodes respond to the changes in the ionic activity
through the formation of the complex.
Ex: Ag/ AgCl/ KCl
Hg/ Hg2Cl2/ KCl
Third kind electrodes: These electrodes are also known as inert electrodes and
redox electrodes. They are composed of inert metal electrode immersed in the redox
solution.
Ex: Pt-H2 electrode

18.  Ion selective indicators: An ion-selective
electrode (ISE), also known as a specific ion
electrode (SIE) converts the activity of a
specific ion dissolved in a solution into an
electrical potential.
An ideal ISE consists of a thin membrane
across which only the ion can be
transported.
Ex: Glass membrane electrode

19. Ion Selective Electrodes: Glass
membrane electrode
A glass electrode is a type of ion-selective
electrode.
Glass membrane manufactured from SiO2
with negatively charged oxygen atoms. The
most common application of ion-selective
glass electrodes is for the measurement
of pH. The pH electrode is an example of a
glass electrode that is sensitive to hydrogen
ions.

20. Ion Selective Electrodes: Fluoride
selective electrode
A fluoride selective electrode is a
type of ion selective electrode sensitive
to the concentration of the fluoride ion.
A common example is the lanthanum
fluoride electrode.
In the lanthanum fluoride electrode,
the sensing element is a crystal
of lanthanum fluoride (LaF3 )

21.  ADVANTAGES
1. When compared to many other inexpensive and simple to
use.
2. Measure both positive and negative ions.
3. Unaffected by sample color and turbidity.
4. Short response time.
• LIMITATIONS
1. Interference by other ions.
2. Electrodes are fragile.
• APPLICATIONS
1. Used to determine concentration of various ions.
2. Education and research.

22. General method for potentiometry:
•Switch on the instrument
•Connect the saturated calomel electrode to the positive terminal or socket and
the glass electrode to the negative. Arrange the calomel electrode and the paddle
of the stirrer in such a way that it must be at the lower level of the glass electrode
which is placed between them.
•Adjust the meter to zero and standardize the electrode system as above. as
subsequent readings are to be taken with the stirrer motorrunning.

23. •Rinse the beaker and electrodes thoroughly with water, then with the solution
to be titrated and place a measured volume in the beaker .add sufficient water
to cover the bulb of the glass electrode adequately
•Switch on the stirrer and measure the pH of the solution.
•Add about 2ml of titrant ,allow sufficient time to mix and measure the pH of
the solution.
•Add further quantities of titrant and as the end point is approached reduce to
0.1ml increments and measure the corresponding values of the pH.
•Obtain further readings of pH for about 5ml beyond the equivalence point.
Switch the meter to standby and turn off the stirrer. wash the beaker and
electrodes thoroughly with the distilled water.
•Plot a graph of pH (vs) titrant added. Read off the equivalence point from the
graph and calculate any required data from this value. if the titration is of a
weak acid against a base, calculate the pKa value of the acid from the
Henderson equation, which is valid over the range of PH4 topH10

24. 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.

25. Food processing:
•Salt content of meat fish dairy products fruit juices brewingsolutions
•Ca in dairy products and beer
•K in fruit juice and wine making
•Corrosive effects of NO3 in cannedfoods
• 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.
Automatic potentiometer

26. REFERENCE
1. Chatwal, G. R. (2009). Spectroscopy. Himalaya
Publishing House.
2. https://en.wikipedia.org/wiki/Potentiometric_titration
3. https://www.slideshare.net/ravishankar05/potentiom
etry-45654797
4. https://www.chemicool.com/definition/potentiometry.
html