Conductometric titrations

Aug 26, 2022, 16:45 IST

About Conductometric titrations

Conductance measurements are frequently employed to find the end points of acid-alkali and other titrations. The principle involved is that electrical conductance depends upon the number and mobility of ions.Consider,

Strong Acid Base Conductometric titrations

in the first instance, the titration of a strong acid, like hydrochloric acid, with a strong base, like sodium hydroxide. The acid is taken in the conductivity vessel and the alkali in the burette. The conductance of hydrochloric acid is due to the presence of hydrogen and chloride ions. As alkali is added gradually, the hydrogen ions are replaced by slow moving sodium ions, as represented below :

H+(aq) + Cl–(aq) + [Na+(aq) + OH–(aq)] → Na+(aq) + Cl–(aq) + H2O(l) (unionised)

Hence, on continued addition of sodium hydroxide, the conductance will go on decreasing until the acid has been completely neutralized. Any subsequent addition of alkali will result in introducing fast moving hydroxyl ions. The conductance, therefore, after reaching a certain minimum value, will begin to increase. On plotting the conductance against the volume of alkali added, the points will lie on two straight lines AB and CD in figure.mentioned in the pdf. The point of intersection X of these two lines gives the volume of alkali required for the neutralization.

weak Acid Base Conductometric titrations

Suppose, it is required to titrate a weak acid like acetic acid, against a strong alkali sodium hydroxide. The conductance of the acid will be low on account of its poor dissociation. On adding the alkali, highly ionized sodium acetate is formed and hence the conductance begins to increase.

CH3COOH(aq) + [Na+(aq) + OH–(aq)] →CH3COO–(aq) + Na+(aq) + H2O(l) (unionised)

When the acid is completely neutralized, further addition of alkali introduces excess of fast moving hydroxyl ions. The conductance of the solution, therefore, begins to increase even more sharply than before. On plotting the conductance against the volume of the alkali added, the two lines obtained will be as shown in fig. The point of intersection X gives the end point.

When a mixture of a strong and a weak acid is to be titrated against a strong alkali a combination of curves shown in above figuresis obtained. Suppose a mixture of HCl and CH3COOH is to be titrated against NaOH. Hydrochloric acid, being a much stronger acid, will get titrated first. The titration of acetic acid will commence only after hydrochloric acid has been completely neutralized. Hence, a combination of curves as mentioned above is obtained. While the point B corresponds to the neutralization of HCl, the point C corresponds to the neutralization of CH3COOH.

Finally, suppose it is required to titrate a strong acid like hydrochloric acid, against a weak base, like ammonium hydroxide. The conductance will fall at first due to the replacement of fast moving H+ ions by slow moving NH4+ ions.

H+(aq) + Cl–(aq) + [NH4OH] → NH4+(aq) + Cl–(aq) + H2O(l)

After neutralization of the acid, further addition of weakly ionized ammonium hydroxide will not cause any appreciable change in the conductance. The curves obtained will be shown in figure.

Precipitation Titrations

The titration of silver nitrate against potassium chloride can also be carried out by this method. The reaction involved may be represented as

Ag+(aq) + NO3–(aq) + [K+ + Cl–] → K+(aq) + NO3–(aq) + AgCl(s)

Since the mobility of potassium ion is nearly the same as that of silver ion which it replaces, the conductance will remain more or less constant and will begin to increase only after the end point. The curves obtained will be as shown in figure.

In order to get accurate results, the change of volume during the titration should be as small as possible. For this purpose the titration solution in the burette is usually five to ten times stronger than the solution taken in the conductivity vessel.

Use of Conductometric titrations

Conductometric titrations have several advantages. Coloured solutions, which cannot be titrated by ordinary volumetric methods with the help of indicators, can be successfully titrated conductometrically. The method can also be employed in the case of very dilute solutions and also for weak acids and bases. Further, no special care is necessary near the end point as it is determined graphically.