This document discusses conductometry, which is a method of analysis based on measuring the electrolytic conductance of a solution. It begins by classifying different electrochemical methods, including conductometry and electrophoresis which do not involve redox reactions. It then discusses key concepts in conductometry such as conductivity, conductance, equivalent conductance, and how various factors like ion nature, temperature, concentration, and electrode size affect conductance. It also provides examples of calculating conductance and equivalent conductance from experimental measurements. Instrumentation for conductometric determination includes a conductance cell and conductivity bridge.

1. 1
CHAPTER NINE
CONDUCTOMETRY
1

2. Classification of electrochemical
methods
1- Conductometry and
electrophoresis: these methods
depend on the movement of ions in
an electric field leading to
conductance of electricity without
occurrence of redox reaction at the
electrode (inert) surface (i.e. no
electron transfer).
2- potentiometry and polarography:
these methods depend on measuring
of voltage or current between two
electrodes electron transfer occurs
where redox reactions take place at
the electrodes surfaces.
3- Electrogravimetry: in this method the
electrolytes are converted to weighable form
by means of an electric current.
2
4- Coulometry: in this method the
electrolytes determined by measuring the
charge of the solution by controlling current
or potential.

3. CONDUCTOMETRY
This is a method of analysis based on measuring electrolytic
conductance.
 Conductance: is the ability of the medium to carry the electric
current.
 Electric current passes through metallic conductor, e.g. wires of iron or
copper in the form of electron flow.
 But conduction of electricity through an electrolyte solution involves
migration of Positively charged cations towards the cathode and
negatively charged anions towards the anode.
 i.e. current is carried by all ions present in solution.
 It depends on the total number of ions present in the solution
 Conductance (G) is the reciprocal of the electric resistance R: G= 1/R
Conductance unit is Ω−1 (also called Siemens, S). (G= conductance
and R = resistance)
R
1
G 
3

4. From Ohm's law:
(E is the potential difference, I is the current intensity)
Unit of resistance = ohm
Unit of conductance = ohm-1 or seimen or mho
Conductance of aqueous electrolyte solutions is, as rule in the range of
mS or μS.
In order to do conductance measurements, 2 metal electrodes (Pt)
should be immersed in the solution and connected to an alternating
current (AC) or DC voltage source.
 Under the effect of the potential difference b/n electrodes, each ion in
solution moves to the electrode of opposite charge.
According to Ohm’s law, the electric current in this circuit including the so/n is:
G= I/V
4
I
E
R 

5. G depends on both so/n properties i.e. conductivity, k & geometrical
parameter of the cell i.e. electrode area, A, & distance b/n electrodes, l,
Fig.1 according to the ff equation: G=k (A/l)
The ratio (A/l) (in cm) is termed as the cell constant and can be
determined by measuring the conductance (G) of a solution with a
known conductivity.
Conductivity (in Ω−1cm−1 or derivate units) depends on solution
properties, such as ion concentrations, ion charges and ion mobilities1.
It is worth noting that H+ and OH- have a much higher mobility
compared to other ions (TABLE 1).
5
Introduction

6. Fig. 1. Schematics of conductance
measurement setup.
Table 1. Mobility of ions in H2O at 25oC.
6
Cont…

7. Conductivity is widely used for estimating the overall ion content in
various sample of practical interest (Table 2).
But conductivity values cannot indicate the conc of a specific ion in
the sample.
 Ion conc. can be determined by means of conductometric titration.
The rxn b/n the ion of interest & the added reagent (i.e.
neutralization, precipitation, or formation of complex cpd) brings
about a strong modification of solution conductivity.
 The reagent should be added in the form of a standard so/n and the
conductance (conductivity) is measured as a function of the added
volume.
This procedure is a conductometric titration.
 With a properly selected rxn, the end point of the titration is
indicated by a particular point on the titration curve (i.e. conductance
vs. added reagent volume). 7
Cont…

8. Table 2.Typical conductivity values for various kinds of H2O samples.
8
Cont…

9. The conductance of electrolytic solutions depends on:
1- Nature of ions:
The velocity by which ions move towards the electrodes
carrying the electric current varies according to their
nature (i.e. size, molecular weight, number of charge
the ion carries).
Velocity of ions α charge
α 1 / size
α 1 / molecular weight
α 1 / hydration
9

10. 2- Temperature:
Conductance is increased by increase of temperature as:
Energy of ions is increased &
viscosity and hydration are decreased.
An increase of temperature by 10C is accompanied by 2%
increase in conductance.
for this reason the measurement must be carried out at
constant temperature using thermostatically controlled
conditions.
10

11. 3- Concentration of ions:
 The electrical conductance of a solution is a summation of
contribution from all ions present.
 It depends on the number of ions/unit volume of solution and their
nature i.e. each ion represents itself independently.
 As the number of ions increases the conductance of the solution
increases.
 The linear relationship between conductance and c is valid only for
diluted solutions. In highly conc solu, there is interionic attraction
forces which decreases conductance.
11

12.  But at infinite dilution, interionic attraction is negligible and
each ion represents itself independently.
 At infinite dilution activityy (a) equals concentration (c)
and activity coefficient a/c = 1, outside the infinite dilution
a/c < 1
 So conductometric determination must be carried out on
dilute solution
 Infinite dilution is a solution that contains so much solvent
that when you add more liquid, there is no change in
concentration. This means that no matter how much
solvent is added to the solution, the properties of the
solute .
12

13. 4- The size of the electrodes:
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 ).
13

14. G α A, G α 1/L, G α A/L
G= K A/L
 K is known as specific conductance or conductivity
it is the conductance when L is unity (1cm) and A is unity (1cm2). OR it
is the conductance of a one cubic cm of liquid (1cm3).
K = G L/A (where L/A is known as cell constant)
 The dimension of K is 1 / ohm.cm or ohm-1cm-1 or seimen/cm.
 K, is a constant value for a specific solution. But it changes with (is
affected by) concentration, temperature and change in cell constant.
 N.B. For solution with high conductance(G) we use cells with low A/L
or high cell constant (L/A) and vice versa.
14

15. Equivalent Conductance
It is the conductance of one gram equivalent of solute contained
between two electrodes spaced one centimetre apart.
Equivalent conductance C = concentration in
So as: the equivalent wt of the substances ↑
their equivalent conductances (λ) ↓
e.g. The λ of HCl (eq.wt. 36.5) is higher than NaCl (eq.wt. 58.5)
Equivalent conductance is used to express the ability of
individual substance to conduct electric current.
 Notice that: as the concentration of ions increases the equivalent conductance
decreases but the total conductivity of the solution increases.
x1000
C
K

gram equivalent / liter.
15

16.  From the previous equation it is clear that the equivalent
conductance increases as the concentration decreases until it
reaches a constant value at infinite dilution which is known as:
Equivalent conductance at infinite dilution or
Limiting equivalent conductance, or Mobility (λ∞).
Which is the conductance of the equivalent weight of the substance at
infinite dilution.
It is characteristic for the solutes.
16
Because specific conductance depends on concentration we
introduce the term molar conductance m
c
Λ

m

17. b- For weak electrolytes
non linear relationship exists
between λ &
their motilities' are determined by
knowledge of the equivalent ionic
conductance of their ions.
λ
λ∞
a- For strong electrolytes
a linear relationship exists between
λ &
Extrapolation of this straight-line
relationship to zero concentration
yields a value for the mobility
c
λ = λ ∞ - a
a = constant
c
c
17

18. Equivalent ionic conductance:
At infinite dilution, inter-ionic attractions become nil; the overall
conductance of the solution consists of the sum of the individual
equivalent ionic conductance.
Overall conductance = λ∞
+ + λ∞
-
λ∞
+ is the equivalent ionic conductance of the cation at infinite
dilution.
λ∞
- is the equivalent ionic conductance of the anion at infinite
dilution.
Equivalent ionic conductance differ according to the nature of
ions (i.e. Charge, size, hydration).
18

19. Cations λ∞ anions λ∞
H+ 350 OH- 198
Na+ 42.6 Cl- 76
K+ 74 NO3
- 71
NH4
+ 73 CH3COO- 41
Ag+ 62 propionate 36
½ Ba2+ 64 ½ SO4
2- 80
19

20. Problem:1
 The measured conductance of o.1 N solu. of a drug is
0.0563 ohm-1 at 250c. The cell constant is 0.52 cm-1,
what is the specific conductance? And what is the
equivalent conductance at that concentration?
(0.0293 ohm-1cm-1)
(293 ohm-1 cm2 /eq)
20

21. Problem
 A certain cell was filled with 0.01M KCl, (K= 0.001409
ohm-1cm-1), it had a resistance of 161.8 ohm at 25ₒ C, and
when filled with 0.005M NaOH it had a resistance of 190
ohm. What is the equivalent conductance of the NaOH
solution?
(240 ohm-1cm2/ eq.)
21

22. INSTRUMENT USED IN CONDUCTOMETRIC
DETERMINATION
To carry out a conductometric measurement it is necessary to measure
the resistance (R) of the solution and the conductance G= l/R
The instrument consists of two parts:
1- Conductance cell: It is the cell in which the
solution to be measured will be filled.
2- Conductivity bridge (Kohlrausch bridge): A mean to measure the
resistance and then converts it to conductivity unit and It is formed of:
a- Wheatstone bridge.
b- An oscillator (to produce A.C. From D.C.).
22

23. The bridge is formed of:
A standard constant resistance R.
Unknown resistance (conductance cell) Rx.
R2 and R3 are formed of a uniform cross
section wire (AB) which is divided to 100
equal parts it is intersected by the sliding
contact C which by changing its position will
change both R2 and R3 (AC and CB).
G is a galvanometer which acts as a
current detector.
G
Rx
A B
C
However, when applying a DC potential between
anode and cathode, electrochemical rxns would
occur at both electrodes (e.g. electrolysis of
water, Electrolysis of H2O is the decomposition
of H2O into oxygen (O2) and hydrogen gas (H2)
due to an electric current being passed through
the water.
reduction of oxygen).
These reactions are not desired, as they disturb
the measurement of conductivity.
N.B: The bridge is supplied by high
voltage attached to an oscillator which
change the direct current (DC) to
alternative current (AC).
WHY ?
to prevent electrolysis in the cell and
polarization of the electrodes.
23

24. Now upon using the bridge:
the unknown cell is attached, the
position of C will be changed
rapidly and automatically till the
balance point , where no current is
detected.
At balance OR
OR
SO
32
1
R
R
R
R x

2
31
R
RR
Rx 
AC
BC
RRx 1
Rx
G
1

BCR
AC1
G
1

X)(100R
X.1
G
1 

AB =100 part
AC = X
CB = 100 – x
Rx
A B
G
C
24

25. 25

26. Application of Conductometry
1- Direct or Absolute Measurements:
1. This is used in industry for checking the purity of distilled water
or other chemicals.
2. Determination of physical constant such as ionization constant.
3. Determination of unknown concentration of pure substance:
where, a series of standard solutions of exact known
concentrations is prepared from pure grade of the substance to
be determined.
 The conductance of the solutions
is measured and a calibration
curve is plotted representing the
conductance against the concentration.
 A solution of exact known
concentration of the substance to
be determined is prepared and
checked from the curve. Concentration
Conductance
26

27. 2- Indirect Conductometry (Conductometric Titrations):
 A conductometric titration involves measurements of the conductance
after successive addition of the titrant.
 The end point is obtained from a plot of the conductance against
ml of titrant.
 The most important advantages of this method are that it can be used
for determination of:
1-Turbid and highly colored solutions.
2-very dilute solutions
3-Reactions which are not complete and where there is no suitable
indicator e.g. reaction between weak acid and weak base.
27

28. 2. application of direct conductometry
a. Determination of high purity water
Conductivity is used to determine the purity of drink water ,
natural water, deionized water and wastewater
Conductivity of high purity water is about 510-8 S· cm-1
b. Determination of the total concentration of strong electrolyte
solutions
salinity of soil and sea water
c. Determination of gaseous environmental contaminant in air
eg. SO3 NO2, acidic rain
determine the change of conductivity after absorption
1. conductometric titration
Applications of Conductometric Analysis
conductometric titration is usually used in the determination of dilute
acids, weak acids and mixed acids