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Conductometry (Pharmaceutical analysis)
1. DISCOVER . LEARN .
EMPOWER
Mr. Yunes Alsayadi
Assistant Professor
of Pharmaceutical
Analysis
E 10695
UNIVERSITY INSTITUTE OF
PHARMA SCIENCES
B. Pharm
Conductometry
3. Introduction
Conductometry is used to analyze ionic species and to
monitor a chemical reaction by studying the
electrolytic conductivity of the reacting species or the
resultant products.
It has notable applications in analytical chemistry.
Conductivity measurement can be performed directly
by using a conductivity meter or by performing
conductometric titration.
Conductometric analysis of electrolytes is a long-
time practice.
4. Definition
It is an electrochemical method of analysis used for the
determination or measurement of the electrical
conductance of an electrolyte solution by means of a
conductometer.
Electric conductivity of an electrolyte solution depends
on :
Type of ions (cations, anions, singly or doubly charged
Concentration of ions
Temperature
Mobility of ions
5. Principle
The main principle involved in this method is that the
movement of the ions creates the electrical
conductivity. The movement of the ions is mainly
depended on the concentration of the ions.
The electric conductance in accordance with ohms
law which states that the strength of current (i)
passing through conductor is directly proportional to
potential difference & inversely to resistance.
i =V/R
6. Important Definitions and Relations
Conductance (G): ease with which current flows per unit area of
conductor per unit potential applied & is reciprocal to resistance (R)
, its unit is Siemens (ohm-1)
G = 1⁄ R
Resistance (R): is a measure of the conductors opposition to the flow
of electric charge, its unit is ohm.
R =1/G
Specific resistance (ρ): resistance offered by a substance of 1 cm
length (l) and 1 sq.cm surface area (A), its unit is ohm cm
ρ = aR/l
Specific conductivity (kv): conductivity offered by a substance of 1
cm length (l) and 1 sq. cm surface area, its unit is siemens cm-1
kv =1 ⁄ ρ
7. Equivalent conductivity (λv): conductivity of a solution
containing equivalent weight of the solute between
electrodes 1 cm apart and 1 sq. cm surface area, its unit is
siemens cm-1
Equivalent conductivity = specific conductivity (kv) X volume
of solution containing 1 gram equivalent weight of electrolyte
Molar conductivity (µv): conductivity of a solution
containing molecular weight of the solute between
electrodes 1 cm apart and 1 sq. cm surface area
Molar conductivity = specific conductivity (kv) X
volume of solution containing 1 molecular weight of
electrolyte
8. The conductance of the solution
depends on:
Temperature: It is increased by increase of
temperature.
Nature of ions: size, molecular weight, number
of charges the ion carries and other factors
The concentration of ions: As the number of
ions increases the conductance of the solution
increases.
The size of the electrodes.
9. Instrumentation
The instrument used for measurement of conductance are known as
conductometers. It consists of :
Current source
Mechanical high frequency AC generator.
DC current is not employed in conductance measurement because
electrodes becomes polarised leading to high cell resistance.
Conductivity cells
Made of pyrex or quartz and are fitted with two platinum electrodes.
Should be placed in vessel containing water to maintain constant
temperature
Types :
1. Wide mouthed cell
2. Cell for reactions producing precipitation
3. Dip type cells Instrumentation
10. Electrodes:
Platinum sheets, each of 1 cm2 are fixed at
distance of 1 cm
The surface is coated with platinum
to avoid polarization effects and
increase effective surface area.
Platinisation of electrodes is done by coating solution of 3%
chlorplatinic acid and lead acetate on it to get uniform coating.
Electrodes usage depends on conductivity and concentration.
If concentration is low then electrodes should be largely and
closely packed.
11. Measurment
The instrument used to measure conductance is
called conductance bridge or conductometer.
Classical circuit employed for measurement is
wheatstone bridge.
Various types are:
– Kohlrausch conductance bridge
– Direct reading conductance bridge
– Phillips conductance bridge
– Mullard’s conductance bridge
– Pye’s conductance bridge Measurement
12. The measured conductivity is not always equal to the
specific conductivity of solution, because the physical
configuration of platinum electrode i.e, length and
area of electrodes varies from one another
Hence conductivity of solution is obtained by
calculating a factor called “cell constant”.
Cell constant: Defined as ratio of distance between
the two electrodes(l) to the area of electrodes(A)
Therefore, θ =1 ⁄ A
13. Conductometric Titrations
Principle
The determination of end point of a titration by
means of conductivity measurements is known as
conductometric titration.
During the course of titration, the conductivity of the
solution changes, since there is change in the number
and the mobility of ions.
At the end point of the titration, there is a sharp
change in the conductivity of a solution shown by the
intersection of the lines in the graph of conductivity
Vs volume of titrant added.
14. Types of conductometric titrations
Acid –base or neutral titrations
Strong Acid with a Strong Base, e.g. HCl with NaOH
When the base is added in HCl solution,
the conductance falls due to the
replacement of hydrogen ions by the
added cation as H+ ions react with OH −
ions to form undissociated water.
After the equivalence point, the
conductance increases due to the large
conductivity of OH- ions
15. Weak Acid with a Strong Base, e.g. acetic acid with NaOH
Initially the conductance is low due to
the feeble ionization of acetic acid,
followed by increase due to NaOH
Beyond the equivalence point,
steep rise due to excess of NaOH.
16. Strong Acid with a Weak Base, e.g. sulphuric acid with dilute ammonia
Initially the conductance is high and then
it decreases due to the replacement of H+.
But after the endpoint has been reached
the graph becomes almost horizontal,
since the excess aqueous ammonia is not
appreciably ionised in the presence of
ammonium sulphate
17. Weak Acid with a Weak Base, e.g. sulphuric acid with dilute ammonia
The nature of curve before
the equivalence point is similar to the curve
obtained by titrating weak acid against strong base.
After the equivalence point, conductance
virtually remains same as the weak base
which is being added is feebly ionized
and, therefore, is not much conducting
18. Precipitation Titrations
eg Potassium chloride Vs Silver nitrate
KCl + AgNO3 AgCl + KNO3
When silver nitrate added, the first part of the curve
shows no increase in conductivity as there is only
replacement of chloride ions with nitrate ions.
After end point conductivity increases because of
increase in the concentration of silver as well as
nitrate ions.
19. Application
Check water pollution in rivers and lakes.
Solubility of sparingly soluble salts like AgCl, BaSO4 can be
detected
Determination of atmosphericSO2,etimation of vanillin in vanilla
flavour Applications
Alkalinity of fresh water.
Salinity of sea water (oceanography)
Used to trace antibiotics.
Deuterium ion concentration in water- deuterium mixture Food
microbiology- for tracing micro organisms
Tracing antibiotics
Estimate ash content in sugar juices
Purity of distilled and de -ionised water can determined
20. Advantages
Determination of specific conductivity is not
required.
No indicator is necessary.
Suitable for coloured or dilute solutions.
Since end point is determined by graphical
means accurate results are obtained with
minimum error.
Used for analysis of turbid suspensions, weak
acids, weak bases, mix of weak & strong acids.
21. Disadvantages
Increased level of salts in solution masks the
conductivity changes , in such cases it does not
give accurate results.
Application of conductometric titrations to
redox systems is limited because, high
concentrations of hydronium ions in the
solution tends to mask the changes in
conductance
