Conductometric Titrations

Submitted to:

Dr Nasir Uddin Khan

Subject:

Physical chemistry lab

Submitted by:

FARAZ ANJUM (ROLL NO 09) M.SC (previous)

Date: 21 September 2010

Department of chemistry

University of Karachi

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History

The word "titration" comes from the Latin word “titulus”, meaning inscription or title.

The French word titre, also from this origin, means rank.

TITRATION The process, operation, or method of determining the concentration of a substance in solution by adding to it a standard reagent of known concentration in carefully measured amounts until a reaction of definite and known proportion is completed, as shown by a color change or by electrical measurement, and then calculating the unknown concentration. OR Titration, by definition, is the determination of rank or concentration of a solution with respect to water with a pH of 7 (which is the pH of pure H2O under standard conditions).

The origins of volumetric analysis are in late-18th-century French chemistry.

Types of titrations

Titrations can be classified by the type of reaction. Different types of titration reaction include:

Acid-base titrations are based on the neutralization reaction between the analyte and an acidic or basic titrant. These most commonly use a pH indicator, a pH meter, or a conductance meter to determine the endpoint.

Redox titrations are based on an oxidation-reduction reaction between the analyte and titrant. These most commonly use a potentiometer or a redox indicator to determine the endpoint.

Complexometric titrations are based on the formation of a complex between the analyte and the titrant. The chelating agent EDTA is very commonly used to titrate metal ions in solution.

Measuring the Endpoint of A Titration

There are many method to measure the end point of a titration such as by

Volumetric titration Conductometric titration Potentiometer PH meter etc

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CONDUCTOMETRIC TITRATION:

Titration in which the end point is determined by measuring the resistance of the solution to an electric current that is passed through it.

The conductance method can be employed to follow the course of a titration, provided that there is a significant difference in conductance between the original solution and the reagent of the product of reaction.

Conductance:

 The conductance is the property of the conductor (metallic as well as electrolytic) which facilitates the flow of electricity through it. It is equal to the reciprocal of resistance i.e.,

                Conductance = 1/Resistance = 1/R                        

It is expressed on the unit called reciprocal ohm (ohm-1 or mho) or siemens

G = 1/RIt is a measure of the ability of a solution to conduct electricity. The conductanceof a solution is the sum of the conductances of all of the ions that are in thesolution. G = Gi

The conductance of a particular ion in solution depends upon the concentration ofthe ion, the charge on the ion, and the size of the ion. As the concentration or thecharge of the ion increases, the conductance of the solution increases. In generalas the size of the solvated ion decreases, its mobility through the solutionincreases and consequently the conductance of the solution increases. In waterthe ion which has the greatest conductance is H+. Of the common, negative ions,OH– has the greatest conductance. Molecular species (uncharged substances) do not contribute to the conductance ofa solution. Electrolyte solutions obey Ohm's law just as metallic conductors do. The current I passing through a given body of solution is proportional to the applied potential difference V. The resistance R of the body of solution in ohms () is given by: R = V/I

where the potential difference is expressed in volts and the current in amperes. The conductance, defined as the reciprocal of the resistance, of a homogeneous body of uniform cross section is proportional to the cross-sectional area A and inversely proportional to the length l, as shown in the Figure on the right: 

G = 1/R = A/l                                  (3)

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where is the conductivity with units -1m-1:

= l/AR = C/R                                  (4)

(By international agreement, the reciprocal ohm, -1, is now called a siemens, 1 S = 1 -1). One

can think of the conductivity as the conductance of a cube of material, 1 m on each edge. Since it

is difficult to build a cell with well defined geometrical parameters A and l, any cell should be

calibrated with a solution of exactly known specific conductance. From Eq.(5) we can determine

the cell constant C :

Cl/A                                  (5)

The conductivity of a solution depends on the ions that are present in it. During many titrations, the conductivity changes significantly

FACTORS AFFECTING ELECTROLYTIC CONDUCTANCE

 In general, conductance of an electrolyte depends upon the following factors, (1) Nature of electrolyte : The conductance of an electrolyte depends upon the number of ions present in the solution. Therefore, the greater the number of ions in the solution the greater is the conductance. The number of ions produced by an electrolyte depends upon its nature. The strong electrolytes dissociate almost completely into ions in solutions and, therefore, their solutions have high conductance. On the other hand, weak electrolytes, dissociate to only small extents and give lesser number of ions. Therefore, the solutions of weak electrolytes have low conductance.  (2) Concentration of the solution : The molar conductance of electrolytic solution varies with the concentration of the electrolyte. In general, the molar conductance of an electrolyte increases with decrease in concentration or increase in dilution.  The molar conductance of strong electrolyte (HCl, KCl, KNO3)  as well as weak electrolytes ( CH3COOH.NH4OH) increase with decrease in concentration or increase in dilution. The variation is however different for strong and weak electrolytes. The variation of molar conductance with concentration can be explained on the basis of conducting ability of ions for weak and strong electrolytes. For weak electrolytes the variation of Λ with dilution can be explained on the bases of number of ions in solution. The number of ions furnished by an electrolyte in solution depends upon the degree of dissociation with dilution. With the increase in dilution, the degree of dissociation increases and as a result molar conductance increases. The limiting value of molar conductance (Λ0)  corresponds to degree of dissociation equal to 1 i.e., the whole of the electrolyte dissociates. Thus, the degree of dissociation can be calculated at any concentration as,  α = Λc/Λ0

where α  is the degree of dissociation,

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 Λc is the molar conductance at concentration C and  Λ0 is the molar conductance at infinite dilution. For strong electrolytes, there is no increase in the number of ions with dilution because strong electrolytes are completely ionised in solution at all concentrations (By definition). However, in concentrated solutions of strong electrolytes there are strong forces of attraction between the ions of opposite charges called inter-ionic forces. Due to these inter-ionic forces the conducting ability of the ions is less in concentrated solutions. With dilution, the ions become far apart from one another and inter-ionic forces decrease. As a result, molar conductivity increases with dilution. When the concentration of the solution becomes very-very low, the inter-ionic attractions become negligible and the molar conductance approaches the limiting value called molar conductance at infinite dilution. This value is characteristic of each electrolyte. 

1. (3) Temperature : The conductivity of an electrolyte depends upon the temperature. With increase in temperature, the

conductivity of an electrolyte increases. By increasing the temperature, the mobility of the ions in the solution will increase. So temperature has a direct effect on conductance of solution. E.g. by increasing the temperature the conductance will increase.

2. Size of the ions The conductivity of the solution is inversely proportional to the size of the ions .if the size of the ions is increasing then the conductivity of the solution will decrease because the mobility of the ions will decrease by increasing the size of the ions.

e.g. The mobility of the hydrogen ions will be greater than the sodium ions so the conductance of the solution containing sodium ions will be less than the solution containing hydrogen ions. same principal is used here in this experiment ,initially solution contain the hydrogen ions when this solution is titrated against the base solution ,the base reacts with the acid and the number of the hydrogen ions go on decreasing. Finally a stage reaches when there is no hydrogen ion in the solution so the conductivity decreased and remains constant. If further base is added then the number of negative ions increases and the conductivity go on increasing. The point at which the conductivity becomes constant is the equivalence point. The volume of base used at equivalence point is used to calculate the morality of acid and then the strength of acid. Consider a solution of a strong acid, hydrochloric acid, HCl for instance, to which a solution of a strong base, sodium hydroxide NaOH, is added. The reaction occurs. For each amount of NaOH added equivalent amount of hydrogen ions is removed. Effectively, the faster moving H+ cation

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is replaced by the slower moving Na+ ion, and the conductivity of the titrated solution as well as the measured conductance of the cell fall. This continues until the equivalence point is reached, at which we have a solution of sodium chloride, NaCl. If more base is added an increase in conductivity or conductance is observed, since more ions are being added and the neutralization reaction no longer removes an appreciable number any of them. Consequently, in the titration of a strong acid with a strong base, the conductance has a minimum at the equivalence point. This minimum can be used instead of an indicator dye to determine the endpoint of the titration. Conductometric titration curve that is a plot of the measured conductance or conductivity values against the number of milliliters of NaOH solution

 Migration of ions Electricity is carried out through the solution of an electrolyte by migration of ions. Therefore, (1) Ions move toward oppositely charged electrodes at different speeds.  (2) During electrolysis, ions are discharged or liberated in equivalent amounts at the two electrodes, no matter what their relative speed is. (3) Concentration of the electrolyte changes around the electrode due to difference in the speed of the ions. (4) Loss of concentration around any electrode is proportional to the speed of the ion that moves away from the electrode, so    Loss around anode/Loss around cathode = Speed of cation/Speed of anion The relation is valid only when the discharged ions do not react with atoms of the electrodes. But when the ions combine with the material of the electrode, the concentration around the electrode shows an increase.  

.

Experiment: Conductometric Titration of Hydrochloric Acid and Acetic Acid with Sodium Hydroxide

During the titration of hydrochloric acid with sodium hydroxide, the reaction thattakes place in the titration vessel is

H+ + Cl- + Na+ OH- H2O + Cl- + Na+

Before the end point, H+ is removed from the solution by reaction with OH–, andNa+ is added to the solution. Since the relative conductance of H+ is about seventimes that of Na+, the conductance of the solution decreases prior to the end point.

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After the end point, no H+ is available to react, and the conductance of the solutionincreases as a result of the addition of Na+ and OH–. Consequently the titrationcurve has a V-shape as shown in the figure. The end point of the titrationcorresponds to the intersection of the extrapolated linear portions of the titrationcurve.Diagrams of the conductometric titration curves. A, the titration of HCl withNaOH theconductance of the solution after the end point increases more rapidly than it didbefore the end point. The end point corresponds to the intersection of theextrapolated linear portions of the curve.Conductance is usually measured with an alternating current between twoidentical, platinized platinum electrodes. Use of an alternating current preventsthe buildup of reaction products around either electrode and consequentlyprevents polarization of the solution. The electrodes must be rigidly held at a fixeddistance apart during the titrations in order to prevent changes in conductancethat result from an altered solution volume between the electrodes.

Principle:Basic principle is the changes in conductivity of a solution when no. of ions thatare the responsible for the conduction of electricity is changing

2.Temperature:

By increasing the temperature, the mobility of the ions in the solution will increase. So temperature has a direct effect on conductance of solution. E.g. by increasing the temperature the conductance will increase and vice versa.

Procedure:1. 100 Ml of acid was taken in beaker.2. Solution was kept at room temperature for at least 10 minutes.3. Then Dip the electrode of conductometer in solution.4. Measure the initial conductance of solution.

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5. First reading of conductometer was noted.6. Then this solution was titrated against the Base solution of known strength using burette.7. Observe the change in conductance of acid solution after every 5 ml addition of base.8. Plot these values to find the equivalence point.9. Volume of base used till the equivalence point is the volume which is required to fully neutralize

the acid.

Calculations:

Acid Base

M 1V 1=M2 V 2

M=molarity

v=volume used

M 1=M 2V 2

V 1

Volume of Base used (mL) Conductance0 28.25 28

10 27.715 27.220 26.625 21.830 20.635 19.440 18.645 17.655 16.665 13.675 1285 10.695 9

105 7.8110 7.2115 6.6

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120 6130 5135 5.1140 5.3150 5.9160 6.3170 6.8185 7.4

M 1=M 2V 2

V 1

M 1=0.1× 130

100M

M 1=0.13 M

Strength of acid = molarity x molecular wt

=0.13 X 36.5= 4.745 g/mL

GRAPHICAL REPRESENTATION:

0 20 40 60 80 100 120 140 160 180 2000

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Chart Title

Series2Series4

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APPLICATIONS OF CONDUCTOMETRIC TITRATIONS

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Acid-base titrations redox titrations are known to us in which commonly indicators are used to locate the end point e.g., methyl orange, phenolphlthalene for acid base titrations. However electricalconductance measurement can be used as a tool to locate the end point.1. This method can be used with much diluted solutions 2. This method can be used with colored or turbid solutions in which end point can not Be seen by eye 3. This method can be used in which there is no suitable indicator 4. has many applications, i.e. it can be used for acid base, redox, precipitation, or complex titrations5. Determination of sulphur dioxide in air pollution studies6.Determination of soap in oil7.Determination of accelerators in rubber8.Determination of total soap in latex9.Specific conductance of water

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