Conductometric titration, Specfic conductance and Molar Conductance, Direct conductometry, Applicaiton

1. CONDUCTOMERY
U.A. Deokate

2. 11/1/201826-Feb-08 10:16:37
AM Deokate U.A. 2
Introduction
 Determination of conductance of an electrolyte
solution are called conductometry.
 It is based on measurement of electrical
conductivity of a solution.
 Electrical conductivity is due to movement of
ions under influence of electric filed.
 Two factors are involved speed of ion and their
concentration.
 When two electrodes are immersed in a
solution and a potential is applied across them,
a current will be produced in the external circuit
that connects the two electrodes.

3. 11/1/201826-Feb-08 10:16:37
AM Deokate U.A. 3
A Review of Parameters
 Like metallic conductor the electrolyte solution obeys
Ohms Law.
 It state that current (i) flowing in a conductor is direly
proportional to the applied e.m.f. (E) and inversely
proportional to resistance (R) of the conductor.
 Ohm’s Law: I = E/R or R= E/i
 Where E is the applied potential, i is the current measured, and R is the
resistance of the solution between the two electrodes.
 R depends on size of conductor as well as upon
composition.
 It is direly proportional to length (l) & inversely
proportional to cross section area (A)

4. 11/1/201826-Feb-08 10:16:37
AM Deokate U.A. 4
A Review of Parameters
 R α l and R α l/A
 R α l/A
 R = ρ l/A where ρ constant called resistivity or specific
resistance
 It is defined as the resistance in ohm of a specimen 1
cm in length and 1 cm in cross section
 ρ = RA/ l = ohm x cm2/ cm
 i.e. ohm cm
 For chemical purposes it is more convenient to discuss
the inverse property called the conductance (1/R)
expressed in Ω-1 or Siemens (S),
 where 1S = 1Ω-1 and 1 μ Ω -1 = 1 μS.

5.  The specific resistance is, thus defined as the resistance
of one centimeter cube of a conductor.
 The reciprocal of specific resistance is termed the
specific conductance or it is the conductance of one
centimeter cube of a conductor.
 When a gram equivalent of solute is preset in a solution
placed between two large parallel electrodes at 1cm
apart the conductance is referred as Equivalent
conductance.
 Molar conductivity is defined as the conductivity of one
mole per liter of solution.
 Conductivity at infinite dilution: at infinite dilution the ions
are independent of each other and they individually
contribute to total conductance.
11/1/201826-Feb-08 10:16:37
AM Deokate U.A. 5

6. 11/1/201826-Feb-08 10:16:37
AM Deokate U.A. 6

7. 11/1/201826-Feb-08 10:16:37
AM Deokate U.A. 7
Conductivity measurement
 Conductivity is measured with a cell consisting of two
platinum electrodes at fixed distance apart.
 These electrodes are coated with finely divided
electrodeposited platinum black.
 The ratio l/A is a property of the cell and is called the
cell constant. It is the resistance that is measured with
an alternating current to minimize electrolysis.
 The catalytic properties of the platinum black allow
what ever electrolysis there is to occur so rapidly in
both directions that there is no buidup (depletion of
anions and cations) of products.
 The cell constant is not measured geometrically but by
measuring the resistance of a sample solution of
known conductivity, usually potassium chloride

8.  Conductometric measurements are usually
carried out in a conductance cell, which
consists of two parallel sheets of platinum (pt.)
as inert electrodes.
 The pt. electrodes must be platinized, i.e.
Covered by pt.black this will give large surface
area which will absorb tiny quantities of
electrode reaction products produced during
passage of current.
 The conductance (G) is directly proportional to
the surface area (A) and inversely proportional
to the distance between the two electrodes (L ).

9. Instrumentation:
11/1/201826-Feb-08 10:16:37
AM Deokate U.A. 9

10. 1. Conductance cell

11. 11/1/201826-Feb-08 10:16:37
AM Deokate U.A. 11
Conductivity Cell
 A conductivity measuring cell is formed by two 1-cm
square surfaces spaced 1-cm apart.
 Cells of different physical configuration are
characterized by their cell constant, K. This cell
constant (K) is a function of the electrode areas, the
distance between the electrodes and the electrical
field pattern between the electrodes.
 Since K (cell constant) is a "factor“ which reflects a
particular cell's physical configuration, it must be
multiplied by the observed conductance to obtain
the actual conductivity reading.

12. 11/1/201826-Feb-08 10:16:37
AM Deokate U.A. 12
Application of condutometry
 Direct conductivity
 In water treatment plant conductivity of water is
measured, which is due to ionic compounds
dissolved in water.
 Increase impurities shows increase conductivity.
 Conductivity of water@25OC
 Absolute pure water 0.055 µs/cm or 18.3 megohm/cm
 Distilled water 0. 5 µs/cm or 2 megohm/cm
 D.M. water 5 µs/cm or 200 Kiloohm/cm
 TDS in ppm as NaCl = cond. In µs/cm x 0.5
 TDS in ppm = cond. In µs/cm x 0.75

13. 11/1/201826-Feb-08 10:16:37
AM Deokate U.A. 13
Drawback of direct conductivity
 Non specific
 Can not distinguish between different types of
ions
 Conductivity in aq solution is due to motion of
ions and increases with increase in temp.
 A small difference in temp make considerable
change in conductance of solution.

14. 11/1/201826-Feb-08 10:16:37
AM Deokate U.A. 14
Condutometric titrations
 It is the one of the most important application
 Various titration such as acid-base titrations,
replacement titrations, precipitations and
complex formation reaction involving titration
are possible
 The principle involved is; when a soln of one
electrolyte is added to the soln of other the
overall conductance will depend on reaction
occurring in them.

15. 11/1/201826-Feb-08 10:16:37
AM Deokate U.A. 15
Condutometric titrations
 Although water is itself a very poor conductor of
electricity, the presence of ionic species in
solution increases the conductance considerably.
 The conductance depends on the type and
concentration of all ions present in solution, as
well as the temperature and construction of the
conductance-monitoring device.
 Thus, monitoring the conductance can provide a
sensitive measure of the changes taking place in
the ionic composition of a solution, in the course
of chemical reaction occurring during
conductometric titration.

16. 11/1/201826-Feb-08 10:16:37
AM Deokate U.A. 16
Condutometric titration of strong acid and
strong base Change in conductivity during the
course of a titration in which HCl is
the analyte and KOH is the titrant.
 Initially, the measured conductivity will
have a relatively high value, As KOH
titrant is added, the conductivity
decreases, since HCl is converted to
KCl and water.
 At the equivalence point in the
titration, all of the HCl has been
neutralized to form KCl, and the
conductivity will be at its lowest value.
 As KOH is added after reaching the
equivalence point, the conductivity will
begin to rise again, since KCl and
unreacted KOH are both ionized in
solution.
 The shape of the titration curve
obtained is shown

17. 11/1/201826-Feb-08 10:16:37
AM Deokate U.A. 17
Condutometric titration of strong acid and
weak base (HCL with ammonia solution)
 Initially, the measured
conductivity will have a
relatively high value,
 As ammonia soln as titrant is
added, the conductivity
decreases, since HCl is
converted to NH4Cl and water.
 At the equivalence point in the
titration, all of the HCl has been
neutralized to form NH4Cl, and
the conductivity will be at its
lowest value.
 After the end point the graph
become almost horizontal.
Volume of titrant added
C
o
n
d
u
c
t
a
n
c
e

18. 11/1/201826-Feb-08 10:16:37
AM Deokate U.A. 18
Condutometric titration of weak acid and
strong base (acetic acid with NaOH)
 Initially, the conductance is
due to ionization of acetic
acid ,
 The salt form increases
conductance .
 At the equivalence point
rise in conductance is
observed due to hydroxyl
ions
Volume of titrant added
C
o
n
d
u
c
t
a
n
c
e

19. 11/1/201826-Feb-08 10:16:37
AM Deokate U.A. 19
Condutometric titration of weak acid and
weak base (acetic acid and ammonia solution)
 Initially, the conductance is
due to ionization of acetic
acid ,
 The salt form increases
conductance .
 After the end point the
graph become almost
horizontal. Volume of titrant added
C
o
n
d
u
c
t
a
n
c
e

20. 11/1/201826-Feb-08 10:16:37
AM Deokate U.A. 20
Conductometric Titration of mixture of acids
(HCL and acetic acid with strong base)
Amount of NaOH Solution Added
Initially conductance is due
to neutralization of strong
acid and then rises as the
weak acid is converted in
to its salts.
After the complete
neutralization of both acid
conductance finally rises
more steeply as the
excess of alkali (OH) ions
are introduced