Engineering Chemistry II - Conductometric titration

                                      Conductometric titration

Principle

Conductometric titration is a volumetric method based on the measurement of conductance of the solution during the titration.

The conductance of the solution depends on

i) The number and charge on the free ions, and

ii) the mobility of the ions.

Types of conductometric titrations

a) Acid base titrations.

b) Precipitation titrations.

c) Replacement titrations.

d) Redox titrations.

e) Complexometric titrations.

a) Acid – base titration

i) Strong acid Vs Strong base (HCL Vs NaOH)

Known amount of acid (HCL) is taken in the conductivity cell and the alkali (NaOH) in the burette. Initially the conductivity of the Hcl is high, this is due to the presence of fast moving H⁺ ions. As the NaOH is added gradually, conductance will be going on decreasing until the acid has been completely neutralized. This is due to the replacement of fast moving H⁺ ions by slow moving Na⁺  ions. The point ‘B’ indicates complete neutralization of all H⁺ ions.

          H⁺Cl^-  +  Na⁺OH^-  →      Na⁺ Cl^-  +  H₂O

Further addition of NaOH will introduce the fast moving OH^- ions. Therefore the conductance, after reaching a certain minimum value, will begin to increase.

ii) Weak acid Vs Strong base (CH₃COOH Vs NaOH)

Known amount of weak acid (CH₃COOH) is taken in the conductivity cell and the alkali in the burette. Intitially the conductivity of CH₃COOH is low. This is due to the poor dissociation of CH₃COOH. NaOH is added gradually, conductance will be going on slowly increasing. This is due to the formation of highly ionised CH₃COONa.

CH₃COO^-H⁺        +  Na⁺OH^- →      CH₃COO^- Na⁺  +  H₂O

The point ‘B’ indicates completer neutralization of CH₃COOH. Further addition of NaOH introduces excess of fast moving OH^- ions. Therefore the conductance of the solution begins to increase even more sharply than before.

iii) Strong acid Vs Weak base (Hcl Vs NH₄OH)

Known amount of Hcl is taken in the conductivity cell and NH₄OH is added from the burette. Initially the conductivity of the Hcl is high. As the NH₄OH is added gradually the conductance will be going on decreasing until all the acid has been completely neutralized. This is due to the replacement of H⁺ ions by slow moving NH⁺₄ ions.

H⁺Cl^-  +  NH₄⁺OH^-                   →      NH₄⁺ Cl^-  +  H₂O

Further addition of poorly ionised NH₄OH does not cause any appreciable change in the conductance. The point of intersection ‘B’ is the end point.

iv) Weak acid Vs Weak base (CH₃COOH Vs NH₄OH)

When CH₃COOH is titrated against NH₄OH, conductivity is going on increasing. This is due to the formation of highly ionised salt CH₃COO^- NH⁺_4.

CH₃COO^-H⁺    +  NH₄⁺OH^-        →      CH₃COO^- NH₄⁺  + H₂O

After the neutralization of CH₃COOH, further addition of poorly ionised NH₄OH does not cause any appreciable change in the conductance. The point of intersection ‘B’ is the end point.

Advantages of conductometric titration

i) It gives more accurate end point.

ii) It is also used for the analysis of dilute solutions and weak acids.

iii) Since the end point is detected graphically, no keen observation is necessary near the end point.

Disadvantages of conductometric titration

i) Only limited number of redox titration can be carried out.

ii) It becomes less accurate and less satisfactory, when the total electrolytic concentration is high.

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