Conductometry

Conductometry


 

Introduction

♦Conduction of an electric current through an electrolyte the solution involves the migration of positively charged species towards the cathode and negatively charged species towards the anode

♦The conductance is a measure of the current that results from the application of given electrical forces and depend directly on the number of charged particles in the solution

♦Since all ions present in the solution contribute to the conduction process, conductance is an additive property of a solution depending on all the ions present.

♦Solution conductance measurement is for non-specific & knowing the conductance of a given The solution gives very little Idea about its composition.

♦However, the fraction of current carried by giver species is determined by its relative concentration and its inherent mobility in solution.

♦Analysis based on the measurement of conductance in solution is called ‘conductometric Analysis’

♦The migration of ions is responsible conductance of electricity in solution hence two factors involved are the speed of ions and their concentration

♦Because of the non-selective nature of conductance,
the application of direct conductance measurement is very limited.

♦The most imp user of direct conductance measurement has been confirmed to the analysis of binary water-electrolyte concentration.

♦The conductorless titration, in which conductance measurement is used for endpoint detection, can however be applied to the determination of numerous substances.

Conductometry

Advantages

1) Coloured solution where no indicator has been found to be satisfactory,
can be successfully titrated conductometrically.

2) More accurate results are obtained because the endpoint is determined graphically.

3) It is used for titration of a mixture of weak and strong acids.

Disadvantages

less accurate technique
less satisfactory as the total electrolyte concentration increase.
the conductance of a solution can be expressed
by

1/R=x (C1λ1 + C2λ2+ C3λ3 +…..+Ciλi)

where
Ci= concentration of various ions lengths in solution
X = numerical constant characteristics of ions
x= proportionality factor

Conductivity Cell

♦The solution whose conductivity is to be measured is taken in a cell called conductivity cell.
♦Different types of conductivity cells are used for different purposes.

♦The three types of cell are generally used

Type A

The cell consists of smaller electrodes separated by larger distances and is useful for the measurement of high conductance accurately.

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Type B

It is a simple dipping cell and it is used in the conductometric titration.

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Type C

A cell consists of larger electrodes separated by a smaller distance and is useful for the measurement of the flow of conductance.

Exp. No……….

Fig. Is shown in

Conductometry
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♦The fundamental requirement is for a part of electrodes that are firmly located in a constant geometry with respect to one another.

♦The cells are usually made up of pyrex glass or quartz and are fitted with the platinum electrode.
♦The electrodes are platinum plates are coated with finely divided platinum to reduce the polarisation effect.

Conductometric titration

♦These can be carried out in that titration in which there is a sharp change in conductivity at the endpoint.

♦Determination of endpoint of titration with the help of conductivity measurement is termed conductometric titration.

♦In this titration, the titrant is added from the burette and
conductivities are followed during the course of
titration.

♦values of conductivities are plotted against the volume of titrant in the graph.

♦Since the measured conductivity is a linear function of the concentration of ions
present, two lines will be obtained which interest each other at a point known as
“endpoints” or equivalence point.

♦When the reaction is not quantitative, there may be some curvature in the curve or in the line near the endpoint, which may be due to hydrolysis, a dissociation of the product, or appreciable solubility, in case of precipitation.

♦In this case, the endpoint can be determined by extrapolating these portions
of the line.

♦These can be carried out in that titration in which there is a sharp change in conductivity at the endpoint.

♦Determination of endpoint of titration with the help of conductivity measurement is termed conductometric titration.

♦In this titration, the titrant is added from the burette, and conductivities are followed during the course of titration.

♦values of conductivities are plotted against the volume of titrant in the graph.

♦Since the measured conductivity is a linear function of the concentration of ions
present, two lines will be obtained which interest each other at a point known as
“endpoints) or equivalence point.

♦When the reaction is not quantitative, there may be some curvature in the curve or in the line near the endpoint, which may be due to hydrolysis, a dissociation of the product, or appreciable solubility, in case of precipitation.

♦In this case, the endpoint can be determined by extrapolating these portions
of the line.
♦the conductivity of a solution is affected by changes in temperature the resistance decrease by about 1 to 2% for each degree increase in temperature hence required to maintain the desired temperature (constant temp).

♦A number of titration cells have been fused in conductometric titration
The simplest cell consists of dipping electrodes in a beaker of suitable size and a mechanical stirring device.

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♦In the case of precipitation titration, the electrode is mounted vertically to
prevent excessive amounts of precipitate from collecting on electrode surfaces.

♦”It is known that a large effective surface area minimizes polarization, most of the electrodes used in conductometric titration, therefore, are coated with platinum black.
♦the platinization of the electrode also helps in the establishment of a sharp balance point.
♦Required to maintain constant temperature hence thermostat is used.
♦in order to get a straight-line graph, it is essential that the total volume of the solution remains constant during its titration.

actual conductivity =[v + V / V] × observed conductivity

where v= volume of titrant
V =initial volume
v+V = final volume

Types of conductometric titration

A) Acid-base titration- well adapted to the conductometric endpoint because of very high conductance.

B)Strong acid with strong base –
let us consider the titration of HCl with NaOH

[H+ +CL] + [Na+ +CL] → [Na+ +CL] + H2O

  • At the start of the titration, the acid solution has high conductivity due to highly mobile ‘H’ ions. The share due to chloride ions will be smaller because of less mobility as compared to ‘H’.
  •  When NaOH is added to HCI the highly mobile ‘H’ ions are replaced by less mobile sodium ions. This will result in a decrease in conductivity rapidly. The solution at neutralization i.e., at the endpoint containing only sodium and chloride. ions will have minimum conductivity.
  • Tempt is constant.
  •  The graph is plotted as follows.
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b) Strong acid with a weak base:

♦Consider strong acid and as he; and a weak base is NH4OH( ammonium hydroxide ).
♦When NH4OH is added is HCI then conductivity decreases because of the replacement of for moving H+ ions by slow-moving NH4+ ions.

[H++ Cl] + [NH4+0H] [NH4+Cl] +H2O

♦The addition of NH4OH after endpoint goes not change the condition because, NH4OH a weak base ionized electrolyte, has a very small conductivity compared with that of the acid or its salt.

Conductometry
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C)Weak acid with strong base:

consider weak acid is (CH3 COOH)with strong based (NaOH)
when a small amount of NaOH is added to CH3COO,The conductivity decrease initially and increase with the further addition of NaOH
[CH3COO-+]+[Na++OH-] [CH3COO-+Na+-]+H2O
When the neutralization of and is complete further of alkali produce an excess of OH’ ion .therefore, starts to increase more rapidly.

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D) Weak acid with weak base:

* The conductivity is governed by the extent of ionization and concentration before its endpoint.
* It decreases firstly as the onion formed surprises the ionization, passes through a minimum, and then increases lip to point.
* This is because of the conversion of non-conducting weak and into its conducting salts.

Conductometry
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e) Very weak acid with strong base:

* Very weak like acid-like, bond acid with NaOH solution.
* Initial conductance is very small but increases progressively as neutralization processed. This is because of salt formation which accounts for rising in conductance due to hydrolysis.
* After the equivalence point the sharp rise in conductance is due to excess hydroxide ion added as the titrant.

F)Mixture of HC1 (strong acid) and acetic acid (weak acid) with strong base.

Initially, conductance falls due to the neutralization of strong acid and then rises as the weak acidic is converted into its salt.
After the complete neutralization of both acid conductance finally rises more due to excess ‘OH’ ion.

Conductometry
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B)Replacement TITRATION:

*When a strong acid reacts with sodium or potassium salt of weak acids, the weaker acid will replaced 1st, consider the titration of sodium acetate with hydrochloric acid.
[CH3COO-+NA+]+[H+C1-] [CH3COO-H+]+Ha+C1-]+[Na+C1-]

*In this titration only a slight increase in conductance is obtained lip to the endpoint.
*This is because of the fact that chloride ion has higher conductance than acetic ions.
*After the endpoint, it increases conductivity more rapidly, owing to the addition of excess HC1. The endpoint can be determined by the interaction of two lines.
*Similarly titration of a strong acid (NH4-OH) can be carried out with NaOH.
[NH4+CI-] +[Na +oh-] [nh4-oh]+[Na +CI-]

Application .

In the determination of alkaloids, as alkaloids are weak bases and are often insoluble in better and thus indicator methods are not applicable.

c) Precipitation and complex formation titration.

* Thus titration can be satisfactorily performed of so, provided that.
1. The reaction product is sparingly soluble.
2. form a stable complex.
3. the precipitate does not have strong adsorbent properties.
A slow rate of precipitation prolongs the time of titration. This is seduced by the seeding technique.
* titration of silver nitrate with potassium. chloride / sod. sulfate with barium chloride gives the shape of the graph depicted in the figure.

Conductometry
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d) Redox titration:

* The redox type of titration not possible by conductometer.
*In redox reaction a large excess of acid or base is used for completion of the reaction. This interferes by making the changes in conduction.
* Hence redox titration not performed by conductometer.

# Advantages of conductometric titration:

* used in cases of colored liquid where ordinary indicators cannot work.
* they can be used for the analysis of dilute solution and also for a very weak acid.
* graph is directly obtained calculation is obtained easily.

# Disadvantages:

* less accurate.
* less satisfactory.
* salt formation can occur error.

# Application:

1. Acid – Base titration:

The acid-base titration gives one or more breaks at the endpoint. The unknown solution can be analyzed quantitively with the help of their breaks in terms of standard alkali, or acid added some typical acid-base titrations, wires are shown below.

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2. Measurement of dielectric constant:

* consider a non – conducting liquid that is placed between metal plates of a cell. This system will behave as two capacitors Cg &  Cs in sales.
* The capacitance Cg will vary from a fixed value co, determined by the geometry of cell when with air to aa value ‘n’ dependent upon the dielectric constant of the sample. This can be represented by the equation.
Cg = Co D
It should be mentioned here that, the value Co depends on.
a. The dielectric constant of the container.
b. The thickness of the glass walls.
c. The effective plate area.
* The capacitance of the cell will approach the value of Cg as a limit because it is not a linear function of the dielectric constant.
* Instrument responses of some solutions of various dielectric constants are shown in the figure.

Conductometry
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3. Angle of mixture of organic compound.

* The analysis of binary mixtures can be carried out with the help of calibration were prepared from a known solution.
* This mixture of ortho and paraxylene, hexanes and benzene, methyl alcohol and ethyl alcohol, and many other mixtures have been analyzed accurately.
4. Complexometric titration:
* The method of high-frequency titration has also been used in complexometric titrations. Thus the ions Mg2+, Ca2+, 1a3+ give a sharp and point.
* When titrated with D.T.A. similarly d, methyl glyoxime has been used as a complexion regent in case of titration of Ni2+.

WRITTEN BY: MS. MAYURI MACHHINDRA LENDAVE ( B.PHARMA) ( Ass. professor)

Conductometry

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