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1. POTENTIOMETRY
Prasanta Deka
Pharmaceutical analysis
Krupanidhi college of pharmacy, Bangalore
2018-2019

2. INTRODUCTION
 Potentiometry is the field of Electroanalytical chemistry in which potential is measured under the
conditions of no current flow or zero current flow so as not to disturb the equilibrium at the sample
membrane interface.
 The measured potential can be used to determine the analytical quantity of interest, generally the
concentration of some component of the analyte solution.
 A typical cell for potentiometric analysis can be represented as
reference electrode | salt bridge | analyte solution | indicator electrode
Eref Ej Eind

3. Potentiometric methods consists of two types of analyses:
 First type involves the direct measurement of an electrode potential from which the activity (or
concentration) of an active ion may be derived (DIRECT POTENTIOMETRY).
 Second type involves measuring the variation in EMF of an electrolytic cell brought about by the
addition of a titrant to the sample. This is referred to as potentiometric titrations or INDIRECT
POTENTIOMETRY.

4. REFERENCE ELECTRODE
Reference electrode is a half cell having a known electrode potential that remains constant and is
independent of the composition of the analyte solution.
SALT BRIDGE
Salt bridge is used to prevent mixing of the contents of the two electrolyte solutions making up
electrochemical cells.
Usually, the two ends of the bridge are equipped with sintered glass disks or other devices to prevent
liquid from siphoning from one part of the cell to the other.

5. Fig : Salt Bridge

6. INDICATOR ELECTRODE
Indicator electrode is an electrode system having a potential that varies in a known way
with variations in the concentration of an analyte.
ELECTRODE POTENTIAL
An electrode potential is defined as the potential of a cell consisting of the electrode
acting as a cathode and the standard hydrogen electrode acting as a anode .
STANDARD ELECTRODE POTENTIAL
The standard electrode potential for a half reaction, E0, is defined as the electrode
potential when all reactants and products of a half reaction are at unit activity.

7. Consists of two conductors (called electrodes) each immersed in a suitable electrolyte solution.
For electricity to flow
The electrodes must be connected externally by means of a (metal) conductor.
The two electrolyte solutions are in contact to permit movement of ions from one to the other.
Cathode is electrode at which reduction occurs.
Anode is electrode at which oxidation occurs.
Indicator and Reference electrodes
Junction potential is small potential at the interface between two electrolytic solutions that differ in composition.
ELECTROCHEMICAL CELL

8. Fig: Electrochemical Cell

9. NERNST EQUATION
• The potential of a half cell can be varied with variables like temperature and
concentration of the solution and the number of electrons transferred.
• Mathematically, the relationship between the potential of a half cell consisting of a metal
in contact with its ions and the variables involved is represented by the Nernst
equation:

10. Where,
E = potential (emf) of the half cell
E0 = emf of half cell under standard conditions
R = constant (8.314 J/0C)
T = absolute temperature
n = number of electrons transferred during reaction (also
equal to the valence change of the metal)
F = Faraday number (96,495 C)
ln = log to base e

11. If the values of R, T (250C = 298K), and F are substituted into the equation and the natural log
is converted to log to the base 10, the equation reduces to
𝐸 = E0 +
8.314𝑋298𝑋2.303
𝑛𝑋96495
log (molar concentration of ions)
= E0 +
0.0591
𝑛
log ( molar concentration of ions)
= E0 +
0.0591
𝑛
log [ions]

12. More accurately, the potential developed is proportional to the logarithm of the activity of
the ion rather than to the logarithm of the molar concentration of ions.
The activity is the activity coefficient γ times the molar concentration of the ion.
The Nernst equation can therefore be written as
E= E0 +
𝑅𝑇
𝑛𝐹
ln(γ X molar concentration of ions)
The activity coefficient approaches unity when the solution is very dilute, which is accurate.
At higher concentrations, activity coefficient decreases. So, for accurate work, a correction should
be made for this decrease.

13. In the Nernst equation, the metal is in the reduced form and the ions are in the oxidized form.
If the equilibrium is written in the form of an oxidation – reduction reaction, then the Nernst
equation is written as
E = E0 + 𝑙𝑛
𝑅𝑇
𝑛𝐹
[𝑂𝑋]
[𝑟𝑒𝑑]
----------(1)
OR
E = E0
𝑅𝑇
𝑛𝐹
𝑙𝑛
[𝑟𝑒𝑑]
[𝑂𝑋]
(note the sign change) and inversion to
[𝑅𝑒𝑑]
[𝑂𝑋]
Where,
[ox] = concentration of oxidized form of metal ions
[red] = concentration of reduced form of metal ions

14. By inserting the values for the constants, equation (1) reduces to
E = E0 +
0.0591
𝑛
𝑙𝑜𝑔 +
[𝑟𝑒𝑑]
[𝑂𝑋]
This brings us to the very important relationship between E, the emf of the half
cell, and the concentration of the oxidized and reduced forms of the components of the
solution.

15. To perform potentiometry, the following is needed:
 Reference Electrode
 Indicator Electrode
 Potential Measuring Device
POTENTIOMETRIC METHOD
Fig: cell of potentiometric measurements

16. Reference electrode is a half cell having a known electrode potential that remains constant
and is independent of the composition of the analyte solution.
1. Hydrogen electrode
2. Saturated Calomel electrode
3. Silver – silver chloride electrode
It consists of three parts:
1. An internal element.
2. Some filling solution which constitutes the salt bridge electrolyte.
3. An area in the tip of electrode that permits a controlled flow of filling solution to escape the
electrode (called the fluid junction) into the sample.
Reference electrode

17.  Follow Nernst equation.
 Used for half of the cell to determine the potential of the analyte of interest.
 Maintains a fixed potential (i.e. reference, stable over time) – in contrast, the indicator
electrode responds to the analyte activity.
 Potential should be constant with time.
 Should return to original potential after being subjected to small currents.
 Little hysteresis with temperature cycling.
 Should behave as ideal nonpolarized electrode.
Characteristics of ideal reference electrode

18. 1. It is a primary reference standard for pH measurements.
2. It consists of a platinum electrode surrounded by an outer tube along which H2 passes,
entering through a side inlet and escaping at the bottom through the test solution.
3. The hydrogen ions of the solution are brought into equilibrium with the gaseous
hydrogen by means of platinum black.
4. The platinum black adsorb H2 and acts catalytically.
5. Since potential of the electrode is very sensitive to traces of O2, air must be removed
from the electrode compartment.
6. The half-cell reaction responsible for the transmission of current across the interface is
H2(g) H2(Pt) 2H+ + 2e-
Hydrogen electrode

19. Fig: Hydrogen Electrode

20. Advantages:
a) It is a fundamental standard electrode of pH measurement.
b) It can be used over the entire range of pH.
c) It checks the accuracy of other pH electrodes.
d) It has low internal resistance and has negligible electric leakage errors.
Disadvantages
a) It can not be used in presence of strong oxidising or reducing agents.
b) It can not be used in a solution containing ions of metals that are below hydrogen in the electrochemical series.
Interaction with hydrogen will occur and the metal will be deposited on the electrode surface.
c) Although hydrogen electrode is the primary reference electrode, yet it is rarely used. This is because the platinum
black coating of the electrode is easily poisoned by substances such as mercury and hydrogen sulphide.

21. A calomel electrode can be represented schematically as
Hg Hg2Cl2 (sat’d).KCl (xM)
Where x represents the molar concentration of potassium chloride in the solution.
Three concentrations of potassium chloride are common 0.1M, 1M and saturated.
The electrode potential of the saturated calomel electrode is 0.2444 V at 250C.
The electrode reaction in calomel half cell is
Hg2Cl2 (s) + 2e- 2Cl- + 2Hg(l)
Ecell = +0.24 V vs.SHE
SATURATED CALOMEL ELECTRODE

22. 1) It consists of a 5 to 15 cm long tube that is 0.5 to 1.0 cm in diameter.
2) A mercury/mercury (I) chloride paste in saturated potassium chloride is
contained in an inner tube and connected to the saturated potassium chloride
solution in the outer tube through a small opening.
3) An inert metal electrode is immersed in the paste.
4) Contact with the analyte solution is made through a fritted disk, a porous fiber, or
a piece of porous Vycor (thirsty glass) sealed in the end of the outer tubing.

23. Fig: saturated calomel electrode

24. It consists of a silver electrode immersed in a solution that is saturated in both potassium
chloride and silver chloride.
Ag AgCl (sat’d),KCl (sat’d)
The electrode reaction in silver/silver chloride half cell is
AgCl (s) + e- Cl- + Ag(s)
Ecell = +0.20 V vs. SHE
In this electrode, the internal tube is replaced by a silver wire that is coated with a layer of
silver chloride; this wire is immersed in a potassium chloride solution that is saturated with
silver chloride.
This electrode have the advantage that they can be used at temperatures greater than 600C,
whereas calomel electrodes cannot.
Silver/ silver chloride electrode

25. Disadvantage of silver-silver chloride electrode
1. It is more difficult to prepare than SCE.
2. AgCI in the electrode has large solubility in saturated KCl
Advantage of Ag-AgCI electrodes over SCE.
1. It has better thermal stability.
2. Less toxicity and environmental problems with consequent cleanup
and disposal difficulties.

26. Indicator electrode is an electrode system having a potential that varies in a known way with
variations in the concentration of an analyte.
Two Broad Classes of Indicator Electrodes
Metal Electrodes
Develop an electric potential in response to a redox reaction at the metal surface
Ion-selective Electrodes
Selectively bind one type of ion to a membrane to generate an electric potential
INDICATOR ELECTRODE

27. Electrodes of the first kind:
An electrode of this type is a metal in contact with a solution containing its cation. The most common ones:
a) Silver electrode (dipping in a solution of AgNO3) Ag+ + e ↔ Ag
b) Copper electrode: Cu+2 + 2e ↔ Cu
c) Zn electrode: Zn+2 + 2e ↔ Zn
Electrode of the second kind:
Electrode of this kind is a metal wire that coated with one of its salts precipitate. A common example is
silver electrode and AgCl as its salt precipitate.
Redox electrode:
An inert metal is in contact with a solution containing the soluble oxidized and reduced forms of the redox
half-reaction. The inert metal is usually is platinum (Pt).
Metallic indicator electrode

28. Selectively bind one type of ion to a membrane to generate an electric potential
Glass electrode
Other indicator electrodes are:
1. Quinhydrone electrode
2. Antimony electrode
ION SELECTIVE ELECTRODE

29. I. It is the most sensitive hydrogen ion responsive indicator electrode.
II. The glass bulb is immersed in the test solution and the electrical circuit is completed by filling
the bulb with a solution of 0.1 M HCl and inserting a Ag-AgCl electrode or SCE.
III. The internal HCl solution is maintained at constant concentration, the potential of the Ag-AgCl
electrode inserted into it will be constant and so too will be the potential between HCl and the
inner surface of the glass bulb.
IV. Hence the only potential which can vary is the potential between the outer surface of the glass
bulb and the test solution in which it is immersed.
V. Thus the overall potential of the electrode is governed by H+ ion concentration of the test
solution.
Glass electrode

30.  Glass electrode is most common ion-selective
electrode
 Combination electrode incorporates both glass and
reference electrode in one body

31. Advantages:
1. It can be used in solutions with pH values ranging from 0 to 12. Lithium glass electrode can be
employed to measure pH upto 14.
2. It is simple to operate.
3. It can be used in oxidising, reducing solutions as well as in colored, turbid or colloidal solutions, in
viscous media, in presence of proteins and similar substances which seriously interfere with other
electrodes.
4. Equilibrium is attained immediately permitting rapid measurements especially in continuously changing
solutions.
5. It does not affect the experimental solution. Thus the pH of poorly buffered solutions and those having
volatile or suspended matter can be measured.
6. It is most suited for continuous automatic recording.

32. Disadvantages:
1) The bulb is very fragile and has to be used with great care.
2) It can not be employed in pure ethyl alcohol, acetic acid and gelatin.
3) Standardization has to be carried out frequently.
4) In solutions of colloids that tend to adhere to the sensitive membrane, glass electrode may
give errorneous results.
5) It can not be used
1) In acid fluoride solution
2) Strongly alkaline solution
3) At high temperatures (373K) for a prolonged period.

33.  Potentiometric titration involves measurement of the potential of a suitable indicator electrode
as a function of the volume of titrant added.
[OR]
 Potentiometric titration involves changes in the cell emf brought about by the addition of a
titrant of an accurately known concentration to the test solution.
 The equivalence point of the titration is detected by rapid changes in cell emf.
 The equivalence point may be calculated or it can be detected by the inflection point of the
titration curve.
 Inflection point is the point that corresponds to the maximum rate of change of cell emf per unit
volume of titrant added.
POTENTIOMETRIC TITRATION

36. When a curve is obtained by plotting EMF Vs Volume of titrant. From this curve, the equivalence
point is determined by the following 3 methods.
Method of bisection: can be applied when the curve shows reasonably good straight lines before & after
the steep part of the curve.
Method of parallel tangents: This method is made used when the portions of the curve on either side of
the steep portion shows a marked curvature.
Method of circle fitting: portion on either steep part of the curve is circular in nature.
LOCATION OF EQUIVALANCE
POINT

37. Methods of end point location:
1. Graphical method
2. Analytical or Derivative method
3. Pinkhof-Treadwell method
4. Direct titration to the equivalence point
5. Gran’s procedure
Graphical method
 The simplest approach of determining the end point is the visual
estimation of the mid point in the steeply rising portion of the
titration curve.

38. Analytical or Derivative method
 It consists of plotting the first derivative curve (ΔE/ΔV against V) or the second
derivative curve (Δ2E/ΔV2 against V) .
 The first derivative curve gives a maximum at the point of inflextion of the titration
curve, that is at the end point.
 The second derivative curve is zero at the point where the slope of ΔE/ΔV curve is
a maximum.

39. Pinkhof-Treadwell method
 The titration cell consists of two electrodes.
 The indicator electrode is placed in the solution which is to be
titrated.
 The reference electrode is placed in a solution whose composition or
potential is equal to the indicator electrode potential at the end point.
 The two solutions are brought in contact through salt bridge.
 Such type of electrode combination has zero potential at the end point
which is signified by null deflection in a galvanometer.

40. Direct titration to the equivalence point
 This method is applicable only when the equivalence point is known by theoretical
considerations or by a previous titration.
 The additional advantage of the method is that no titration data need to be recorded.
 Titrant is slowly introduced near the equivalence point until the potential of the
indicator electrode, as measured with an appropriate instrument, attains the desired
value.

41. Gran’s procedure
 It is a simple method for fixing an end point.
 If a series of additions of reagent are made in a potentiometric titration and the emf (E) is
noted after each addition and then if antilog (E X nF / 2.303 RT) is plotted against the
volume of reagent added, a straight line is obtained.
 When extrapolated, this line cuts the volume of axis at a point corresponding to the
equivalence point volume of the reagent.
 Plotting is simplified if the special semi – antilog Gran’s plot paper is used.
 The advantage of this method is that the titration need not be pursued to the actual end
point.
 It is only necessary to have the requisite number of observations before the end point.

42. Potentiometric titrations may be applied to a variety of systems including those
involving oxidation-reduction, neutralization, precipitation and complexation equilibria
reactions.
1. Redox titrations
2. Neutralization titrations
3. Precipitation titrations
4. Complexometric titrations
5. Potentiometric titrations in Non-aqueous solvents
6. Acid Base titrations
7. Differential titrations
8. Automatic titrations
TYPES OF POTENTIOMETRIC
TITRATION

43. It is done in aqueous as well as non-aqueous medium. Indicator electrode used is glass
electrode & Reference electrode is saturated calomel electrode.
All acid base combinations of any strength, mixture of acids or bases Vs bases or acids
respectively, polybasic acids Vs bases can be analysed.
In non-aqueous medium same electrode used – weak acids Vs methoxides & weak bases Vs
perchloric acid.
ACID BASE TITRATION
 Indicator electrode - silver electrode or mercury electrorode
 Reference electrode - saturated Calomel or any ref electrode
Example: Several divalent ions, trivalent ions, cyanide ions are assayed against EDTA.
COMPLEXOMETRIC TITRATION

44.  Indicator electrode- platinum wire or foil
 Reference electrode- saturated calomel or silver-sliver chloride electrode.
Example
 Ferric Ammonium Sulphate Vs pot permangante or
Pot dichromate
 Sodium arsenite Vs pot bromate,
 FeSO4 Vs CAS
REDOX TITRATION

45.  Indicator electrode – Glass electrode
 Reference electrode – saturated calomel
Alkaloids, amines, sulpha drugs, drugs containing free primary aromatic amines
can be assayed using 0.1N NaNO2 in dil HCl.
DIAZOTIZATION TITRATION
 Indicator electrode - silver wire electrode
 Reference electrode - saturated calomel electrode, hydrogen electrode, silver-
silver chloride electrode
Example determination of mercury, silver, lead, copper & several other ions
using precipitants.
PRECIPITATION TITRATION

46.  Determination of moisture or water by Karl Fischer reagent.
 It consists of two platinum electrodes, between which a small potential (1-100 mV) is
applied.
 Current flows as long as electrodes remains depolarised.
 No current flows till the solution is free from polarising substances.
 Current flow only when both electrodes are polarised (at the end point).
 Normally the titrant is added through an automatic burette & at the end point the
stopper gets automatically closed & titrant stops flowing.
DEAD STOP END POINT TECHNIQUE OR BIAMPEROMETRY

47. Magnetic
stirrer
Titrating
Vessel
Buret
te
Pt
electrodes

48.  Potentiometric titrations is employed for acid-base, redox, neutralization, precipitation
and complexometric reactions.
 It is applicable to chemical systems where the end point obtained by the indicator is
masked, e.g. if the analyte solution is colored, turbid or fluorescent.
 There is no need of external indicator for redox titrations.
 It is employed for the analysis of dilute solutions with high degree of accuracy.
 The technique is applicable even in non-aqueous media.
 Beside locating the equivalence point, these titrations can be successfully employed to
gather thermodynamic informations including dissociation constants for weak acids and
stability constants for complex ions.
ADVANTAGES OF POTENTIOMETRIC TITRATION

49.  It is widely used electro-analytical method.
 Potentiometric principle is useful in checking the pH of the official buffers & different
test solutions.
 It is useful to locate the equivalence point in acid-base, redox, precipitation &
complexometric titrations in aqueous & non-aqueous media.
 A no. of drugs official in pharmacopoeia are assayed by this procedure, some of them
are Caffeine, Phenobarbitol, Tetrahydrazine HCl, Amoxyllin sodium, Disulfuram,
Hydrallazine HCl, Metaclopramide HCl, Propranolol HCl, Nalixidic acid, Lomustine
etc.
APPLICATIONS