Pharmaceutical Analysis Unit -V Conductometry

Introduction:

-Conductometric titration is an electro-analytic method of analysis which is very useful when no suitable colour indicators are available for the detection of end point or when the indicator methods are not suitable for analysis.

-Conductometric titration is based on measurement of conductance of ions in a solution.

-The Conductance is due to the migration of ions

Ohm's Law:

 Ohm's law states that the Strength of current(I) flowing through a conductor is directly proportional to the potential difference(E) applied the conductor and inversely proportional to the resistance(R)of the conductor.

               I=E/R

Conductance:

 -Conductance, G, is the reciprocal of resistance (R)

       G=1/R

Specific resistance:

The resistance (R) of the conductor is directly proportional to its length(l) and inversely proportional to its area(a)

             R∝l and R∝1/a

Specific Conductance

The specific conductance of any conductor is the reciprocal of specific resistance and is denoted by 'k'

Electrical Conductivity

It is defined as the conductivity in ohms of a solution containing one gm. equivalent of solute when placed between two sufficiently large electrodes, which are one cm apart.

It is denoted by λ.

Equivalent conductance

This is defined as "the conductance of a solution containing 1g equivalent of electrolyte when placed between two sufficiently large electrodes which are 1cm apart".

It  is denoted by λv

Molecular conductance

This may be defined as "the conductance of a solution containing 1g mole of the electrolyte when placed between two sufficiently large electrodes placed 1cm apart".

It is denoted by μv

Conductivity cell:

 -The solution whose conductance is to be measure is taken in a cell known as conductivity cell. These are generally made up of high quality glass, Quartz and sometimes fitted with platinum electrodes.

-The electrodes usually consist of platinum plates sealed into glass tubes.

-there are different forms of conductivity cells, some common type are as follow:-

a) Conductivity cell having wide mouth bottle with cork having holes for passing two platinum wire.

b)Conductivity cell consists of electrodes which are fixed in the lid which is having opening for the stirrer and the burette.

c)Conductivity cell having a wide bore glass tube, the tip of which has two platinum plates .

   Glass tube contains two fixed copper wires, the terminals of which are taken out for connection.

Conductometric Titrations curve:

• Various types of titrations can be performed using Conductometry.
• Major titration which can be performed with Conductometry can be classified under:-

A)Acid Base Titration:

It can be classified into following types:-

1.Strong acid with Strong base,, e.g. HCl with NaOH:

• Before NaOH is added, the conductance is high due to the presence of highly mobile hydrogen ions.
• When the base is added, 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
• This decrease in the conductance continues till the equivalence point.
• At the equivalence point, the solution contains only NaCl.
• After the equivalence point, the conductance increases due to the large conductivity of OH- ions  

     HCl + NaOH → NaCl + H2O
 

2.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.
• On the addition of base, there is decrease in conductance not only due to the replacement of H+ by Na+ but also suppresses the dissociation of acetic acid due to common ion acetate
• . But very soon, the conductance increases on adding NaOH as NaOH neutralizes the un-dissociated CH3COOH to CH3COONa which is the strong electrolyte.  
• This increase in conductance continues raise up to the equivalence point. The graph near the equivalence point is curved due the hydrolysis of salt CH3COONa. Beyond the equivalence point, conductance increases more rapidly with the addition of NaOH due to the highly conducting OH− ions

             HCl + NH4OH → H2O + NH4Cl

3. Strong Acid with a Weak Base, e.g. Sulfuric 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 ionized in the presence of ammonium sulphate

                                        NaOH + CH3COOH → H2O + CH3COONa

4. Weak Acid with a Weak Base: 

• 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

    NH4OH + CH3COOH → CH3COONH4 + H2O

Fig. 6.5: Conductometric titration of a weak acid (acetic acid) vs. a weak base (NH4OH)

B)Displacement titration

• Displacement titration involves displacement of one ion by another.
• Displacement titration of following type:-

1. Titration of salt of Strong acid and weak base v/s strong base: 

• Ammonium chloride is a salt of strong acid and weak base while sodium hydroxide is a strong base. .
•  Initially there is plateau because only displacement of ammonium and chloride ions with sodium and chloride ions takes place till the end point.
•  After end point, further addition of NaOH Cause share increase. 

                          NaOH + NH4Cl → NaCl + NH4OH

 

2.Titration of salt of strong base and weak acid v/s strong acid:-

• Sodium acetate is a salt of strong base and weak acid while hydrochloric acid is a strong acid.
•  Initially there is displacement of acetate ions by chloride ions and conductivity increased gradually till the end point 
• After end point, the further addition of HCL cause sharp increase in conductivity.

NH4OH + CH3COOH → CH3COONH4 + H2O

c) Precipitation Titration

• The Precipitation titration does not involves H+ and OH ions therefore cannot be carried out much effectively as the acid base titration.
• In Precipitation titration one pair of ions gets substituted for another.
• If a cation to be precipitated, a titrant whose cation has less mobility is selected 
• for e.g.. titration  of silver nitrate with KCL

                     AgNO3 + KCl → AgCl (s) + KNO3 

D) Redox titration:

• In redox titration, there is decrease in conductivity due to decrease in hydrogen ion concentration at the point. 
• initial conductance is kept at low level because it is difficult to detect small change in conductivity due to change in H+ ion concentration .
• Redox titration manually performed in acidic medium.
• Titration of ferrous ions with dichromate ions is an example of redox titration using Conductometry titration

e)Non-aqueous titration:

Non-aqueous titrations of weak acid and weak bases can be performed by Conductometry 

Example : titration of perchorlic acid in dioxan-formic acid with weak bases.

Application:

1.Determination of ionic of water

The product of ionic concentration of H+ and OH in gm. moles per liter is constant at a constant temperature which is known as ionic product of water.

2.Determination of solubility sparingly soluble salts:

Substances like AgCl, BaSO4 etc. are regarded as sparingly soluble in water and their solubility cannot be determined easily by other chemical methods Conductometric titration helps in determining the solubility of these sparingly soluble salts.

3.Determination of basicity of an organic acid

Basicity of organic acid, can be found by using the empirical formula given by Ostwald as,

                                     B=λ1024-λ32/10.8

4. Kinetic Studies

   Rate of a reaction can be calculated with help of Conductometric titrations. It is based on measurement of conductivity before, during and at end of chemical reaction.

Principle

The principle of Conductometric titration is based on the fact that during the titration, one of the ions is replaced by the other and invariably these two ions differ in the ionic conductivity with the result that conductivity of the solution varies during the course of titration. The equivalence point may be located graphically by plotting the change in conductance as a function of the volume of titrant added. In order to reduce the influence of errors in the Conductometric titration to a minimum, the angle between the two branches of the titration curve should be as small as possible . If the angle is very obtuse, a small error in the conductance data can cause a large deviation. The following approximate rules will be found useful. 

•  The smaller the conductivity of the ion which replaces the reacting ion, the more accurate will be the result. Thus it is preferable to titrate a silver salt with lithium chloride rather than with HCl. Generally, cations should be titrated with lithium salts and anions with acetates as these ions have low conductivity  
• The larger the conductivity of the anion of the reagent which reacts with the cation to be determined, or vice versa, the more acute is the angle of titration curve.
•  The titration of a slightly ionized salt does not give good results, since the conductivity increases continuously from the commencement. Hence, the salt present in the cell should be virtually completely dissociated; for a similar reason; the added reagent should also be as strong electrolyte. 
•  Throughout a titration the volume of the solution is always increasing, unless the conductance is corrected for this effect, non linear titration curves result. The correction can be accomplished by multiplying the observed conductance either by total volume (V+V´) or by the factor (V+ V´)/V, where V is the initial volume of solution and V´ is the total volume of the reagent added. The correction presupposes that the conductivity is a linear function of dilution, this is true only to a first approximation.

-:Thank You:-