Download CONDUCTOMETRY

CONDUCTOMETRIC
METHODS OF ANALYSIS
Ashraf M. Mahmoud, Associate professor
Contents
Introduction
Ohm’s law.
Conductometric measurements.
Factor affecting conductivity.
Application of conductometry.
2.Conductometric titration-:
 Introduction.
Types of conductometric tiration.
Advantages of conductometric tiration.
3.Recent devlopement
4.References .
3
Introduction
Conductometry:
is the simplest of the electroanalytical
techniques; by Kolthoff in 1929.
Conductivity is:
“the ability of the medium to carry
electric current”.
Introduction
Conductors are:
either metallic (flow of electrons) or
electrolytic (movemenmt of ions).
Conductance of electricity:
migration of positively charged ions towards the
cathode and negatively charged ones towards the
anode
(i.e.) current is carried by all ions present in solution.
Conductance depends on the number of ions in solun.
Introduction
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Factors affecting conductance:
1- Temperature:
(1C increase in temperature causes 2 %
increase in conductance).
2- Nature of ions
Size, molecular weight and number of
charges.
3- Concentration of ions:
As the number of ions increases, the
conductance increases.
4- Size of electrodes
Conductance is directly proportional to
the cross sectional area (A).
Specific conductance (K)
Conductance is directly proportional to the
cross section area A
and is inversely proportional to the length of
a uniform conductor,
Thus,
G  A/l
so G =KA/L
where K is the specific conductance
(the conductance when A and L are
numerically equal).
When unit of A and L is centimeter, K is:
“the conductance of a cube of liquid one
centimeter on a side” its unit is Ohm-1 Cm-1
Equivalent conductance (o ):
“It is the conductance of a solution of
one-gram equivalent of solute (with no
respect of its volume) contained
between two electrodes placed 1 cm
apart.”
Due to the interionic effects,
the equivalent conductance (o ) is
concentration dependant.
The value of (o)
(equivalent conductance at infinite
dilution)
is used for comparison purposes.
The magnitude of (o) is determined
by the charge, size and degree of
hydration of the ion.
(o) is also known as
limiting ionic conductance
or
ionic mobility.
Table 6, shows limiting ionic
conductances or ionic mobilities
(o) in water at 25C for many ions.
Conductivity measurements
1.Electrodes
Two parallel platinized Pt. foil electrodes or Pt. black with
electrodeposited a porous Pt. film which increases the surface area of
the electrodes and further reduces faradaic polarization.
2.Primary standard solutions
Primary standard KCl solution ,at 25℃, 7.419g of KCl in 1000g of
solution has a specific conductivity of 0.01286Ω-1/cm.
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Conductivity Cell and Wheat stone Bridge
3. Conductivity Cell :
Avoid the change of temperature during determination
4.Wheat stone bridge :
1
2
Factors affecting conductivity
Factors affecting conductivity:
 Size of ions
 Temperature
 Number of ions
 Charge of ions
 Specific conductivity:-It is conductivity offered by a
substance of 1cm length and 1sq.cm surface area. units
are mhos/cm.
 Equivalent conductivity:-it is conductivity offered by a
solution containing equivalent weight of solute in it.
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Molar conductance of various ions at infinite dilution at 25℃
Cations
H+
Na+
K+
NH4+
Ag+
½Ba2+
Li+
Pb2+
Ca2+
Sr2+
Cu2+
Fe2+
Mg2+
Zn2+
(o)
350
50.1
74
73
62
64
38.7
73
59.5
59.46
54
54
35.06
52.8
Anions
OHClNO2CH3COOCH3CH2COO½SO42BrINO3ClO4ClO3BrO3IO4CH3COO-
(o)
198
76
71
41
36
80
78.1
76.8
71.44
67.32
64.58
55.78
54.4
40.9
Molar conductance of various ions at infinite dilution at 25℃
H+ and OH- ions
have by far
the largest equivalent conductances.
H2O has a very low conductivity,
So, acid-base titrations yield the most
clearly defined equivalence points by
conductometry.
Molar conductance of various ions at infinite dilution at 25℃
Solved Example:
Calculate the limiting equivalent
conductance (ionic mobility) of:
1- H2SO4
2- Propionic acid
3- Propionic acid from (Ao) of HCl, sodium
propionate and NaCl.
Solution:
1. Limiting equivalent conductance (ionic
mobility) (Ao) of H2SO4 =
(2 x 350) + (2 x 80) = 860
Molar conductance of various ions at infinite dilution at 25℃
2.(Ao) for propionic acid =
= 350 + 36 = 386
Na-propionate + HCl = NaCl + propionic acid
(85.9)
(426) (126.4)
(?)
3. (Ao) for propionic acid =
[(Ao) for HCl + (Ao) for Na-propionate - (Ao)
for NaCl] =
= 426 + 85.9 – 126.4 = 385.5
No significant difference between
results in 2 & 3.
APPLICATIONS OF CONDUCTOMETRY
It can be used for the determination of: Solubility of sparingly soluble salts
 Ionic product of water
 Basicity of organic acids
 Salinity of sea water (oceanographic work)
 Chemical equilibrium in ionic reactions
 Conductometric titration
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CONDUCTOMETRIC TITRATIONS
 The determination of end point of a titration by
means of conductivity measurements are known
as conductometric titrations.
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Types of conductometric titrations





Acid-base titration
Precipitation titration
Replacement titration
Redox (oxidation-reduction) titration
Complexometric titration
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ACID-BASE TITRATIONS
Titration of strong acid
(a) with strong base e.g. HCl with NaOH
(b) with weak base e.g. HCl with NH4OH
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Titration of weak acid
(c) with strong base e.g. CH3COOH with NaOH
(d) with weak base e.g. CH3COOH with NH4OH
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PRECIPITATION TITRATIONS
[K+ + Cl-] + [Ag+ + No3_]
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REPLACEMENT TITRATIONS
A. Salt of strong acid and weak base vs. strong base
Ex: ammonium chloride vs. sodium hydroxide
NH4Cl+NaOH→NH4OH+NaCl
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REPLACEMENT TITRATIONS
B. Salt of strong base and weak acid vs. strong acid
eg. sodium acetate vs. hydrochloric acid
CH3COONa + HCl → CH3COOH + NaCl
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REDOX TITRATION
Titration of ferrous ions with dichromate ions:
6 Fe2+ + Cr2O72- + 14H+→ 6Fe3+ + 2Cr3+ +7H2O
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COMPLEXOMETRIC TITRATION
Eample:-
KCl vs. Hg(ClO4)2
 Non-aqueous titrations can also be measured using
conductometry.
Eample:a)titration of weak bases vs. perchloric acid in dioxanformic acid.
b)Titration of weak organic acids in methanol vs. tetra
methyl ammonium hydroxide in methanol-benzene.
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ADVANTAGES OF CONDUCTOMETRIC TITRATIONS
 No need of indicator
 Colored or dilute solutions or turbid suspensions can be
used for titrations.
 Temperature is maintained constant throughout the
titration.
 End point can be determined accurately and errors are
minimized as the end point is being determined
graphically.
DISADVANTAGES OF CONDUCTOMETRIC TITRATIONS
- non specificity
- interference of high conc. of other electrolytes.
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POLAROGRAPHY
Ashraf M. Mahmoud, Associate professor
Introduction
 The earliest voltammetric technique
 Heyrovsky invented the original polarographic
method in 1922, conventional direct current
polarography (DCP).
 It employs a dropping mercury electrode (DME) to
continuously renew the electrode surface.
 Diffusion is the mechanism of mass transport.
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Introduction
Theory of polarography
When an external potential is applied to a cell
containing a reducing substance such as CdCl2,
The following reaction will occur:
Cd2+
+ 2e + Hg = Cd(Hg)
The technique depends on increasing the applied
voltage at a steady rate and simultaneously
record photographically the current-voltage
curve (polarogram)
The apparatus used is called a polarograph .
A typical polarograph
Instrumentation – Three electrodes in solution containing analyte
Working electrode: microelectrode whose potential is varied with time
Reference electrode:
potential remains constant (Ag/AgCl electrode
or calomel)
Counter electrode: Hg or Pt that completes
circuit, conducts e- from signal source through
solution to the working electrode
Supporting electrolyte:excess of nonreactive
electrolyte (alkali metal) to conduct current
Inlet of inert gas
(H2 or N2 to expel
dissolved oxygen
Outlet of inert gas
A typical polarograph
Dropping Mercury Electrode
(Working electrode)
 Capillary tube about 10-15cm
 Int. diameter of 0.05mm
 A vertical distance being
maintained betwwen DME and
the solution
 Drop time of 1-5 seconds
 Drop diameter 0.5mm
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Supporting electrolyte
The supporting electrolyte
is a solution of (KNO3, NaCl, Na3PO4) in which the
sample (which must be electroactive) is dissolved.
Function of the supporting electrolyte
It raises the conductivity of the solution.
It carries the bulk of the current so prevent the
migration of electroactive materials to working
electrode.
It may control pH
It may associate with the electroactive solute as
in the complexing of the metal ions by ligands.
Polarographic measurements
Polarography measurement is governed by
ilkovic equation:
id= 708 nD1/2m2/3t1/6C
n= no. of electrons
t= droptime(second/drop)
D= diffusion coefficient of analyte (cm2/s)
m= rate of flow of Hg through capillary (mg/s)
C= analyte’s concentration in mM
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Polarogram
0.001 M Cd2+ in 0.1 M KNO3 supporting electrolyte
Current i (A)
Electrode become more and more
reducing and capable of reducing Cd2+
Cd2+ + 2e-
Cd
Current starts to be registered at the
electrode (decomposition
potential)
residual current
flow through the
electrode
Base line
of residual
current
Half –wave potential
E½
-0.4
id
Current at the working
electrode continue to rise as the
electrode become more
reducing and more Cd2+ around
the electrode are being reduced.
Diffusion current of Cd2+
is not limited
Working electrode is no
yet capable of reducing
Cd2+  only small
-0.2
All Cd2+ around the electrode has
already been reduced. Limiting
Current at the electrode is reached
by limiting the diffusion rate of Cd2+
from the solution to the electrode.
Thus, current stops rising and levels
off at a plateau
-0.6
-0.8
-1.0
Applied potential, V vs SCE
-1.2
-1.4
Polarogram
The Polarogram is characterized by
the following parameters:
• Residual current
• Limiting current
• Diffusion current
• Half wave potential (E1/2)
A typical Polarogram
Factors affecting electrode reaction rate and current
A- Mass transfer
The movement of sample from one location in solution to
another, it arises from either:
1. Migration (under the influence of electric field difference),
2. Diffusion (under influence of concentration difference),
3. Convection (under the influence of stirring),
B. Diffusion
C. Chemical reaction at
the electrode surface
Factors affecting electrode reaction rate and current
Concentration gradient created between
the surrounding of the electrode and the
bulk solution
-1.0 V vs SCE
Pb2+ + 2e-
Pb
K+
Pb2+
K+
K+
K+
Pb2+
K+
Pb2+
K+
Pb2+
K+
Pb2+
K+
K+
Pb2+
K+
Pb2+
K+
2+
Pb
K+2+ migrate to
Pb
the electrode via
Pb2+
diffusion
Pb2+
K+
Pb2+
Pb2+
K+
K+
K+
K+
K+
Pb2+
Pb2+
K+
Pb2+
K+
Pb2+
Pb2+
K+
K+
Pb2+
K+
K+
Pb2+
K+
Pb2+
K+
Pb2+
Layers of K+ build up around the electrode stop the
migration of Pb2+ via coulombic attraction
Advantages and disadvantages of DME
Advantages of DME
Its surface is reproducible, smooth and continuously
renewed, this eliminates the poisoning effect.
Mercury forms amalgams (solid solution) with many metals.
The diffusion current assumed a steady value immediately
after each change of applied potential and is reproducible.
The large hydrogen over-potential of mercury renders
possible deposition of substance that difficult to reduce.
The surface area can be calculated from the weight of drop.
Disadvantages of DME
At potential more positive than + 0.4 V vs SCE, mercury
dissolves producing anodic polarographic wave which
masks other waves, therefore DME can be used only for the
analysis of reducible or easily oxidizable substances.
The capillary is very small so easy to be blocked→ malfunction
of the electrode
Mercury is very toxic and easily oxidized
Application of Polarography
A. Qualitative: by using the half wave potential which is
characteristic to each substance
B. Qualitative
INORGANIC ANALYSIS
ORGANIC ANALYSIS
 Analysis of metals
 Analysis of
 Zn
 Cd
 Analysis of anions as
dromate, iodate, etc.
carbonyl,peroxide,
nitro, azo group, etc.
 Biochemical analysis
Application of Polarography
Advantages of using polarography in pharmaceutical
analysis
•1- Only small volume of sample is required.
•2- Turbid and coloured solutions can be analyzed.
•3- It can be used for the determination of
substances, which are not electrochemicall active
(indirect).
•4- Prior separation of excepients is not required.
•5- Its sensitivity is sufficient for the determination
trace elements and toxic impurities.
6- Samples of natural origin
7- high speed analysis which is important for QC
RECENT DEVLOPEMNTS
In refinary industries.
Estimation of polyelectrolytic solution.
Biotechnology.
Microbiosensors for enviromental monitoring.
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References
 Gurdeep.R. chatwal,sham k.anand,instrumental method
of
chemical
analysis,himalaya
publishing
house,2008,p.no.2.482-2.497.
 Hovert H.willard,lynne L.merritt,john A.dean,frank
A.settle,jr.,instrumental method
of analysis CBS
publishers 1986,p.no.732-750.
 Kenneth A. connors,e textbook of pharmaceutical
analysis,third edition,wiley india,p.no. 334.
 Danniel christein,analytical chemistry,2nd edition,wiley
india,p.no. 274.
 www.pharmapaedia.com
49
Continued….
 www.authorstream.com
 Kissinger, P. T., AND W. W. Heineman, eds.,
Laboratory Techniques in Electroanalytical Chemistry,
Dekker, New York, 1984.
 A.H.beckett ,J.B. stenlake,practical pharmaceutical
chemistry,fourth edition ,part –two,p.no-91.
 Lingane, J.J., Electroanalytical Chemistry, 2nd ed.,
Wiley- Interscience, New York, 1958
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