Conductometry: Principal, Instrumentation and Application

 

Conductance: flow of electricity through a electrolyte solution due to migration of ions by applying potential difference between two electrode.

-         Cathode (-ve charged) will attract cations (+ve charged)

-         Anode (+ve charged) will attract anions (-ve charged)

It is denoted by G. and is reciprocal of resistance R.

So, G= 1/R

Units of conductance is siemens (S).

Specific Conductance:

Resistance is directly proportional to the length (l) and inversely proportional to the cross-section area (a) of the conductor.

R ∝ l/a

R= ρl/a where, ρ= specific resistance

R= ρ= Specific resistance (having 1 m length and 1 m² area)

·        Specific conductance (k): reciprocal of specific resistance

(k)= 1/ ρ,

k= l/a *1/R,

K=l/a * G

Unit of specific conductance is siemens.m^-1

Equivalent conductance:

·        Conductance one gram equivalent solutions. If l=1 m, volume= 1m³ and 1 gm equivalent solution is taken then,

·        G=k

·        It is denoted by the symbol “Λ”. And its unit is siemens.m²/gram equivalent.

 

Molar conductivity:

·        Molar conductivity can also be defined as the conducting power of all the ions that are formed by dissolving a mole of electrolyte in a solution. 

Λ_m = K / C

·        Where K is the specific conductivity and c is the concentration in mole per litre.

·        The unit of molar conductivity is Siemens⋅m²⋅mol^-1.

 

Principal:

Conductometry is based in the principal of determination of changes in conductivity. Change in conductance is due to replacement of ions with each other.

Ionic conductivity is different for different ions.

 

Instrumentation:

 

Current Source:

-         Alternative current source is used.

-         High frequency alternating current generator is employed.

-         Electrical potential is applied, ions will be transferred and ultimately conductance takes place.

 

Conductivity meter:

-         Digital display

-         Calibrator

-         Power switch

 

Conductivity Cell:

-         It is made up of pyrex or quartz

-         Fitted with 2 electrodes

: cathode

: anode

 

Electrodes:

-         Electrodes are made up of platinum

^-          Area of electrode surface = 1 cm²

-         Distance between two electrode is 1 cm

-         Electrodes are plated with platinum black: to avoid polarization

: to increase the surface area

-         Coating is done through 3% chloroplatinic acid and lead acetate

 

Conductometric titration

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 can be located graphically by plotting the changes of conductance as a function of the volume of the added titrant.

The most important advantages of this method are that it can be used for determination of:

– turbid and highly coloured solutions,

– very dilute solutions,

– reaction which is not complete and where there is no suitable indicator, e.g. reaction between weak acid and weak base.

 

Precaution to be considered in conductometric titrations:

1. Upon carrying on titration the titrant used should be at least 10 times concentrated as the solution to be determined, e.g. on determination of 0.01 M hydrochloric acid the titrant sodium hydroxide should be at least 0.1 M. By this way dilution that takes place during titration is minimum and this is necessary because conductivity decreases with dilution.

2. Avoid the presence of ions which will not take part in the reaction, such as the presence of buffer or concentrated acids. These ions will increase the initial conductivity and its change during the titration will be comparatively small and can not be accurately observed.

3. The method is suitable for detection of end point in neutralization, complexation and precipitation reactions but not redox reaction, as there is no electron transfer at the electrode surface.

 

Some typical conductometric titration curves are:

1. Strong Acid with a 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 decreases 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 high conductivity of OH-ions.

Fig: Conductometric titration of strong acid (HCl) vs. strong base (NaOH).

2. Strong Acid with a Weak Base, e.g. HCl 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 chloride (Fig. 2).

Conductometric titration of strong acid (HCl) vs. weak base (NH4OH).

3. Weak Acid with a Strong Base: e.g. CH3COOH with NaOH

Initially a slight decrease in the conductance is caused by binding a small amount of hydrogen ions, originating from dissociation of acetic acid, into water molecules. Next, the gradual conductance increase is connected with the substitution of the weakly dissociated acetic acid by the well dissociated sodium acetate. After the equivalence point has been reached, the conductance increases significantly due to the increasing concentration of OH ions (Fig. 3).

Conductometric titration of weak acid (CH3COOH) vs. strong base (NaOH).

4. Weak Acid with a Weak Base: e.g. CH3COOH with dilute ammonia

Initially a slight decrease in the conductance is caused by binding a small amount of hydrogen ions originating from dissociation of acetic acid and next an increase is observed because of well dissociated salt - ammonium acetate formation. After the equivalence point the conductance increases but much less (Fig. 4).

Conductometric titration of weak acid (CH3COOH) vs. weak base (NH4OH).

Application of Conductance measurement

• They are used to determine the purity of water by checking the pollution levels of various water bodies.
• Conductometric titration is also used to examine the alkalinity of freshwater bodies and the salinity of seawater.
• Conductometry is also used in the food industry by microbiologists in order to trace microorganisms. 
• The application of conductometric titration is also found in the pharmaceutical industry. It is used to check the basicity of organic acids and to detect antibiotics.
• It is used to determine the purity of distilled water or its freshness by examining the chemical equilibrium in ionic reactions.

Advantages of conductometric titration

Some of the main advantages of such titration are given:

1.     It is quite difficult to select suitable indicators for the titration of colored solutions. In such cases, conductometric titration can be carried out.

2.     Titration of very diluted solution and weak acids versus weak bases can not be carried out in volumetric titration but titration of such acid bases can be carried out quite easily with the help of conductometric titration.

3.     In a volumetric titration, special attention is needed near the endpoint to detect the color change but in this titration, no such special care is required near the endpoint.