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Module 9 : Experiments in Chemistry

Lecture 38 : Titrations : Acid-Base, Redox and Complexometric

38.4 Experiment 3: Acid-base titration by conductometric measurement
Theory :
Solution of electrolytes conduct electricity via cautions and anions. The conductivity of the solution depends upon velocity and concentration (no. of ions) of the current carrying species. Ionic conductance is reciprocal to the resistance offered by the solution of the electrolyte. By measuring the variation in the conductance, the end point of acid base titration can be determined. The variation in conductance of the solution occurs due to the change in the number of current carrying ions on adding the titrant. This can be explained by taking an example of an electrolyte A⁺ B^- being titrated by another electrolyte C⁺ D ^- . The conductance of the solution on adding C⁺ D^- will depend upon the number and nature of the products formed on reacting C⁺ D^- with A⁺ B^- .

	(38.3)
Since the ions B^- and C⁺ combine to give a neutral molecule, and charge carrying species are decreased in the solution, hence the conductance will show a decreasing trend. The more conducting ion B^- could also be replaced by the less conducting ion D^-.
Thus decreasing trend will continue till the end point is reached. After the end point, the conductance will start increasing due to the excess number of C⁺ and D^- ions. Thus by recording the variation in conductance, the end point can be determined. This method of analysis is applicable to any titration in which there is a sharp change in the conductivity at the end point. The method has the advantage that it can be used with a coloured solution as well and it will work where no indicator is found suitable. In order to get the accurate end point, it is necessary to keep one of the constituents (preferably titrant) fairly concentrated to avoid volume change of the solution.

Procedure :
Operation of the conductivity bridge: Switch on the instrument five minutes before the start of the exercise. Bring the ‘function switch' to calibration position (cell constant position) and adjust the display reading with the cell constant knob. Keep the range selector at the maximum. Rinse the cell with the solution whose conductance is to be measured. After taking measurements, clean the cell with distilled water.
Titration :
To start the experiments, take 25mL of the acid (HCl) in a beaker and keep the conductivity cell in it in such a position that the electrodes of the cell are inside the acid solution and the body of the cell would not touch the bottom of the beaker. Note down the initial conductance of the solution. Now run NaOH in small volumes (0.5 mL) from the burette and gently stir the solution with a glass rod without striking the cell.
Determine the conductance of this solution. Repeat the determination of conductance of the solution after each addition of base volume of 0.5 mL. Carry out at least three – four more measurements after getting the minima of the conductance. Plot a graph between conductance and the volume of base added. extrapolate the straight line portions of the titration curve to obtain the correct end point.
Titration curve:
Figure shows the progress of titration of HCl with NaOH. The curve AC show the specific conductance of a mixture of acid and salt plotted against volume of NaOH added, while the curve BD corresponds to a mixture of salt and excess NaOH after the end point ( the point at which the two lines cross) has reached.

Figure 38.2 Conductometric titration: Progress of titration of HCl with NaOH
Precautions:
There should be no significant effect in conductivity due to dilution, so the titrant concentration used must be at least 10–100 times that of the solution being titrated.

38.4 Experiment 3: Acid-base titration by conductometric measurement

Theory :

Titration curve: